The Facts On File

DICTIONARYof

INORGANICCHEMISTRY

The Facts On File

DICTIONARYof

INORGANICCHEMISTRY

Edited byJohn Daintith

®

The Facts On File Dictionary of Inorganic Chemistry

Copyright © 2004 by Market House Books Ltd

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The Facts on File dictionary of inorganic chemistry / edited by John Daintith.p. cm.

Includes bibliographical references.ISBN 0-8160-4926-2 (alk. paper).1. Chemistry—Dictionaries. I. Title: Dictionary of inorganic chemistry. II. Daintith,

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CONTENTS

Preface vii

Entries A to Z 1

Appendixes

I. The Periodic Table 244

II. The Chemical Elements 245

III. The Greek Alphabet 247

IV. Fundamental Constants 247

V. Webpages 248

Bibliography 248

vii

PREFACE

This dictionary is one of a series covering the terminology and concepts usedin important branches of science. The Facts on File Dictionary of InorganicChemistry has been designed as an additional source of information for stu-dents taking Advanced Placement (AP) Science courses in high schools. Itwill also be helpful to older students taking introductory college courses.

This volume covers inorganic chemistry and includes basic concepts in phys-ical chemistry, classes of compound, reaction mechanisms, and many im-portant named inorganic compounds. The entries on individual chemicalelements are designed to give a basic survey of the chemistry of the element.The definitions are intended to be clear and informative and, where possi-ble, we have illustrations of chemical structures. The book also has a selec-tion of short biographical entries for people who have made importantcontributions to the field. There are appendixes providing a list of all thechemical elements and a periodic table. A short list of useful webpages anda bibliography are also included.

The book will be a helpful additional source of information for anyonestudying the AP Chemistry course, especially the section on DescriptiveChemistry. It will also be useful to students of metallurgy and other relatedfields.

ACKNOWLEDGMENTS

Contributors

John O. E. Clark B.Sc.Richard Rennie B.Sc., Ph.D.Tom Shields B.Sc.

AAS See atomic absorption spec-troscopy.

absolute temperature Symbol: T Atemperature defined by the relationship:

T = θ + 273.15where θ is the Celsius temperature. The ab-solute scale of temperature was a funda-mental scale based on Charles’ law appliedto an ideal gas:

V = V0(1 + αθ)where V is the volume at temperature θ, V0the volume at 0, and α the thermal expan-sivity of the gas. At low pressures, whenreal gases show ideal behavior, α has thevalue 1/273.15. Therefore, at θ = –273.15the volume of the gas theoretically be-comes zero. In practice, of course, sub-stances become solids at thesetemperatures. Nevertheless, the extrapola-tion can be used to create a scale of tem-perature on which –273.15 degrees Celsius(°C) corresponds to zero (0°). This scalewas also known as the ideal-gas scale; on ittemperature interval units were called de-grees absolute (°A) or degrees Kelvin (°K),and were equal in size to the Celsius de-gree. It can be shown that the absolute tem-perature scale is identical to theTHERMODYNAMIC TEMPERATURE scale, onwhich the temperature interval unit is thekelvin.

absolute zero The zero value of ther-modynamic temperature; 0 kelvin or–273.15 degrees Celsius.

absorption A process in which a gas istaken up by a liquid or solid, or in which aliquid is taken up by a solid. In absorption,the substance absorbed goes into the bulkof the absorping material. Solids that ab-

sorb gases or liquids often have a porousstructure. The absorption of gases in solidsis sometimes called sorption. Compare ad-sorption.

absorption indicator (adsorption indica-tor) An indicator used for titrations thatinvolve a precipitation reaction. Themethod depends upon the fact that at theequivalence point there is a change in thenature of the ions absorbed by the precipi-tate particles. Fluorescein – a fluorescentcompound – is commonly used. For exam-ple, in the titration of sodium chloride so-lution with added silver nitrate, silverchloride is precipitated. Sodium ions andchloride ions are absorbed in the precipi-tate. At the end point, silver ions and ni-trate ions are in slight excess and silver ionsare then absorbed. If fluorescein is present,negative fluorescein ions absorb in prefer-ence to nitrate ions, producing a pink com-plex.

absorption spectrum See spectrum.

abundance 1. The relative amount of agiven element among others; for example,the abundance of oxygen in the Earth’scrust is approximately 50% by mass.2. The amount of a nuclide (stable orradioactive) relative to other nuclides ofthe same element in a given sample. Thenatural abundance is the abundance of anuclide as it occurs in nature. For instance,chlorine has two stable isotopes of masses35 and 37. The abundance of 35Cl is75.5% and that of 37Cl is 24.5%. For someelements the abundance of a particular nu-clide depends on the source.

acac Abbreviation for the bidentate

1

A

accelerator

2

acetyacetonato ligand, derived from acetyl-acetone (CH3COCH2COCH3).

accelerator A CATALYST added to in-crease the rate at which a chemical reactionoccurs.

acceptor The atom or group to which apair of electrons is donated in a coordinatebond. Pi-acceptors are compounds orgroups that accept electrons into pi, p or dorbitals.

accumulator (secondary cell; storage bat-tery) An electric cell or battery that canbe charged by passing an electric currentthrough it. Because the chemical reactionin the cell is reversible, current passedthrough it in the opposite direction towhich it supplies current will convert thereaction products back into their originalforms. The most common example is thelead-acid battery used in automobiles andother vehicles powered by internal com-bustion engines.

acetate See ethanoate.

acetic acid See ethanoic acid.

acetyacetonato See acac.

acetylene See ethyne.

Acheson process See carbon.

achiral Describing a molecule that doesnot exhibit optical activity. See chirality.

acid A substance than contains hydro-gen and dissociates in solution to give hy-drogen ions:

HA ˆ H+ + A–

More accurately, the hydrogen ion is sol-vated (a hydroxonium ion):

HA + H2O ˆ H3O+ + A–

Strong acids are completely dissociated inwater. Examples are sulfuric acid and tri-choloroethanoic acid. Weak acids are onlypartially dissociated. Most organic car-boxylic acids are weak acids. In distinctionto an acid, a base is a compound that pro-duces hydroxide ions in water. Bases are ei-ther ionic hydroxides (e.g. NaOH) orcompounds that form hydroxide ions inwater. These may be metal oxides, for ex-ample:

Na2O + H2O → 2Na+ + 2OH–

Ammonia, amines, and other nitrogenouscompounds can also form OH– ions inwater:

NH3 + H2O ˆ NH4+ + OH–

As with acids, strong bases are completelydissociated; weak bases are partially disso-ciated.

This idea of acids and bases is known asthe Arrhenius theory (named for theSwedish physical chemist Svante AugustArrhenius (1859–1927).

In 1923 the Arrhenius idea of acids andbases was extended by the British chemistThomas Martin Lowry (1874–1936) and,independently, by the Danish physicalchemist Johannes Nicolaus Brønsted(1879–1947). In the Lowry–Brønstedtheory an acid is a compound that can do-nate a proton and a base is a compoundthat can accept a proton. Proton donatorsare called Brønsted acids (or protic acids)and proton acceptors are called Brønstedbases. For example, in the reaction:

CH3COOH + H2O ˆ CH3COO– +H3O+

the CH3COOH is the acid, donating a pro-ton H+ to the water molecule. The water isthe base because it accepts the proton. Inthe reverse reaction, the H3O+ ion is theacid, donating a proton to the base

C

O

C

O

CH3 C CH3

H2

C

O

C

O

CH3 C CH3

H

_

Acac: the bidentate acetylacetonato ligandformed from a diketone

CH3COO–. If two species are related byloss or gain or a proton they are describedas conjugate. So, in this example,CH3COO– is the conjugate base of the acidCH3COOH and CH3COOH is the conju-gate acid of the base CH3COO–.

In a reaction of an amine in water, forexample:

R3N + H2O ˆ R3NH+ + OH–

The amine R3N accepts a proton fromwater and is therefore acting as a base.R3NH+ is its conjugate acid. Water donatesthe proton to the R3N and, in this case,water is acting as an acid (H3O+ is its con-jugate base). Note that water can act asboth an acid and a base depending on thecircumstances. It can accept a proton (fromCH3COOH) and donate a proton (toR3N). Compounds of this type are de-scribed as amphiprotic.

One important aspect of the Lowry–Brønsted theory is that, because it involvesproton transfers, it does not necessarilyhave to involve water. It is possible to de-scribe reactions in nonaqueous solvents,such as liquid ammonia, in terms ofacid–base reactions.

A further generalization of the idea ofacids and bases was the Lewis theory putforward, also in 1923, by the US physicalchemist Gilbert Newton Lewis (1875–1946). In this, an acid (a Lewis acid) is acompound that can accept a pair of elec-trons and a base (a Lewis base) is one thatdonates a pair of electrons.

acid-base indicator An indicator that iseither a weak base or a weak acid andwhose dissociated and undissociated formsdiffer markedly in color. The color changemust occur within a narrow pH range. Ex-amples are METHYL ORANGE and PHENOLPH-THALEIN.

acidic Having a tendency to release aproton or to accept an electron pair from adonor. In aqueous solutions the pH is ameasure of the acidity, i.e. an acidic solu-tion is one in which the concentration ofH3O+ exceeds that in pure water at thesame temperature; i.e. the pH is lower than7. A pH of 7 is regarded as being neutral.

acidic hydrogen A hydrogen atom in amolecule that enters into a dissociationequilibrium when the molecule is dissolvedin a solvent. For example, in ethanoic acid(CH3COOH) the acidic hydrogen is theone on the carboxyl group, –COOH.

acidic oxide An oxide of a nonmetalthat reacts with water to produce an acidor with a base to produce a salt and water.For example, sulfur(VI) oxide (sulfur triox-ide) reacts with water to form sulfuric acid:

SO3 + H2O → H2SO4and with sodium hydroxide to producesodium sulfate and water:

SO3 + NaOH → Na2SO4 + H2OSee also amphoteric; basic oxide.

acidic salt See acid salt.

acidimetry A volumetric analysis oracid-base titration in which a standard so-lution of an acid is gradually added to theunknown (base) solution containing an in-dicator. In the converse procedure, alka-limetry, the standard solution is of a baseand the unknown solution is acidic.

acidity constant See dissociation con-stant.

acid rain See pollution.

acid salt (acidic salt) A salt in whichthere is only partial replacement of theacidic hydrogen of an acid by metal orother cations. For polybasic acids the for-mulae for such salts are of the typeNaHSO4 (sodium hydrogensulfate) andNa3H(CO3)2.2H2O (sodium sesquicarbon-ate). For monobasic acids such as HF theacid salts are of the form KHF2 (potassiumhydrogen difluoride). Although monobasicacid salts were at one time formulated asnormal salts plus excess acid (i.e. KF.HF),it is preferable to treat them as hydrogen-bonded systems of the type K+(F–H–F)–.

actinic radiation Radiation that cancause a chemical reaction; for example, ul-traviolet radiation is actinic. See also pho-tochemistry.

3

actinic radiation

actinides See actinoids.

actinium A soft, silvery-white, highlyradioactive metallic element of group 3(formerly IIIB) of the periodic table. It isusually considered to be the first memberof the ACTINOID series. It occurs in minutequantities in uranium ores as a result of thenatural radioactive decay of 235U. Themetal can be obtained by reducing AcF3with lithium or it can be produced by bom-barding radium with neutrons. It is used asa source of alpha particles and has alsobeen used to generate thermoelectricpower. The metal glows in the dark; it re-acts with water to produce hydrogen.

Symbol: Ac; m.p. 1050±50°C; b.p.3200±300°C; r.d. 10.06 (20°C); p.n. 89;most stable isotope 227Ac (half-life 21.77years); other isotopes have very short half-lives.

actinoid contraction The decrease inthe atomic or ionic radius that occurs in theactinoids as the atomic number increasesfrom actinium through nobelium. The in-crease in atomic number in the actinoids isassociated with the filling of the inner 5fsubshell. It is similar to the LANTHANOID

CONTRACTION.

actinoids (actinides) A group of 15radioactive elements whose electronic con-figurations display filling of the 5f level. Aswith the lanthanoids, the first member, ac-tinium, has no f electrons (Ac [Rn]6d17s2)but other members also show deviationsfrom the smooth trend of f-electron fillingexpected from simple considerations, e.g.thorium Th [Rn]6d27s2, berkelium Bk[Rn]5f86d17s2. The actinoids are all radio-active and their chemistry is often ex-tremely difficult to study. The first eight,actinium, thorium, protactinium, uranium,neptunium, plutonium, americium, andcurium occur naturally, although with theexception of thorium and uranium only intrace amounts. The others are generated byartificial methods using high-energy bom-bardment. See also transuranic elements.

activated charcoal See charcoal.

activated complex See transition state.

activated complex theory A theory ofchemical reactions in which the rate atwhich chemical reactions take place is re-lated to the rate at which the transitionstate (activated complex) is converted intoproducts. Activated complex theory issometimes known as TRANSITION STATE

THEORY. It was put forward by HenryEyring in 1935.

activation energy Symbol: Ea The min-imum energy that a particle, molecule, ion,etc. must acquire before it can react; i.e. theenergy required to initiate a reaction re-gardless of whether the reaction is exother-mic or endothermic. Activation energy isoften represented as an energy barrier thatmust be overcome if a reaction is to takeplace. See Arrhenius equation.

activator See promoter.

active mass See mass action.

activity 1. Symbol: a A corrective con-centration or pressure factor introducedinto equations that describe real solvatedsystems. Certain thermodynamic proper-ties of a solvated substance are dependenton its concentration (e.g. its tendency toreact with other substances). Real sub-stances show departures from ideal behav-ior and thus require such correctionfactors.2. Symbol: A The average number of atomsdisintegrating per unit time in a radioactivesubstance.

actinides

4

ener

gy

A ... B ... C

AB C

AB C

H

ti di tActivation energy

activity coefficient Symbol: f A meas-ure of the degree of deviation from idealityof a solvated substance, defined as:

a = fcwhere a is the activity and c the concentra-tion. For an ideal solute f = 1; for real sys-tems f can be less or greater than unity.

acyclic Describing a compound that isnot cyclic (i.e. a compound that does notcontain a ring in its molecules).

addition reaction A reaction in whichadditional atoms or groups of atoms are in-troduced into an unsaturated organic com-pound, such as an alkene or ketone. Asimple example is the addition of bromineacross the double bond in ethene:

H2C:CH2 + Br2 → BrH2CCH2BrAddition reactions can be induced ei-

ther by electrophiles, which are ions ormolecules that are electron deficient andcan therefore accept electrons, or by nucle-ophiles, which are ions or molecules thatcan donate electrons.

adduct See coordinate bond.

adiabatic change A change duringwhich no heat enters or leaves the system.

In an adiabatic expansion of a gas, me-chanical work is done by the gas as its vol-ume (V) increases, its pressure (p)decreases, and its temperature (T) falls. Foran ideal gas undergoing a reversible adia-batic change it can be shown that

pVγ = K1Tγp1–γ = K2

and TVγ–1 = K3where K1, K2, and K3 are constants and γ isthe ratio of the principal specific heat ca-pacities. Compare isothermal change.

adsorbate A substance that is adsorbedon a surface. See adsorption.

adsorbent The substance on whose sur-face adsorption takes place. See adsorp-tion.

adsorption A process in which a layerof atoms or molecules of one substanceforms on the surface of a solid or liquid. All

solid surfaces take up layers of gas from thesurrounding atmosphere. The adsorbedlayer may be held by chemical bonds(chemisorption) or by weaker van derWaals forces (physisorption).

Compare absorption.

adsorption indicator See absorptionindicator.

aerosol See sol.

AES See atomic emission spectroscopy.

affinity The extent to which one sub-stance is attracted to or reacts with an-other.

afterdamp See firedamp.

agate A hard microcrystalline form ofthe mineral chalcedony, which is a varietyof quartz. Typically agate has greenish orbrownish bands of coloration, and is usedfor making jewelry and ornaments. Mossagate is not banded, but has mosslike pat-terns resulting from the presence of ironand manganese oxides. Agate is also usedin instrument bearings, because of its resis-tance to wear.

air The mixture of gases that surroundsthe Earth. At sea level the composition ofdry air, by volume, is nitrogen 78.08%,oxygen 20.95%, argon 0.93%, carbondioxide 0.03%, neon 0.0018%, helium0.0005%, krypton 0.0001%, and xenon0.00001%.

Air also contains a variable amount ofwater vapor, as well as particulate matter(e.g. dust and pollen) and small amounts ofother gases.

air gas See producer gas.

alabaster A dense, translucent, fine-grained mineral form of dihydrate CALCIUM

SULFATE (CaSO4.2H2O). Due to its softnessit is often carved and polished to make or-naments or works of art.

alchemy An ancient pseudoscience thatwas the precursor of chemistry, dating

5

alchemy

from early Christian times until the 17thcentury. It combined mysticism and exper-imental techniques. Many ancient al-chemists searched for the philosopher’sstone – a substance that could transmutebase metals into gold and produce theelixir of life, a universal remedy for all ills.

alcohol A type of organic compound ofthe general formula ROH, where R is a hy-drocarbon group. Examples of simple alco-hols are methanol (CH3OH) and ETHANOL

(C2H5OH).By definition alcohols have one or more

–OH groups attached to a carbon atomthat is not part of an aromatic ring. Thus,C6H5OH, in which the –OH group is at-tached to the ring, is a phenol whereasphenylmethanol (C6H5CH2OH) is an alco-hol.

Common alcohols are used as solvents,denaturing agents, chemical feedstocks,and in antifreeze preparations. Ethanol isthe intoxicating ingredient in alcoholicbeverages. Propane-1,2,3,-triol (glycerol) isused to make polymers, cosmetic emol-lients, and sweeteners.

alkali A water-soluble strong base.Strictly the term refers to the hydroxides ofthe alkali metals (group 1, formerly sub-group IA) only, but in common usage itrefers to any soluble base. Thus borax so-lution may be described as mildly alkaline.

alkali metals (group 1 elements) Agroup of soft reactive metals, each repre-senting the start of a new period in the pe-riodic table and having an electronicconfiguration consisting of a rare-gasstructure plus one outer electron. The al-kali metals are lithium (Li), sodium (Na),potassium (K), rubidium (Rb), cesium (Cs),and francium (Fr). They formerly wereclassified in subgroup IA of the periodictable.

The elements all easily form positiveions M+ and consequently are highly reac-tive (particularly with any substrate that isoxidizing). As the group is descended thereis a gradual decrease in ionization potentialand an increase in the size of the atoms; thegroup shows several smooth trends that

follow from these facts. For example,lithium reacts in a fairly controlled waywith water, sodium ignites, and potassiumexplodes. There is also a general decreasein the following: melting points, heats ofsublimation, lattice energy of salts, hydra-tion energy of M+, ease of decompositionof nitrates and carbonates, and heat of for-mation of the ‘-ide’ compounds (fluoride,hydride, oxide, carbide, chloride).

Lithium has the smallest ion and there-fore has the highest charge/size ratio and ispolarizing with a tendency towards cova-lent character in its bonding; the remainingelements form typical ionic compounds inwhich ionization, M+X– is regarded ascomplete. The slightly anomalous positionof lithium is illustrated by the similarity ofits chemistry to that of magnesium, in ac-cordance with their diagonal relationshipin the periodic table. For example, lithiumhydroxide is much less soluble than the hy-droxides of the other group 1 elements;lithium perchlorate is soluble in several or-ganic solvents. Because of the higher latticeenergies associated with smaller ionslithium hydride and nitride are fairly stablecompared to NaH, which decomposes at345°C. Na2N, K3N etc., are not obtainedpure and decompose below room tempera-ture.

The oxides also display the trend inproperties as lithium forms M2O with onlytraces of M2O2, sodium forms M2O2 andat high temperatures and pressures MO2,potassium, rubidium, and cesium formM2O2 if oxygen is restricted but MO2 ifburnt in air. Hydrolysis of the oxides or di-rect reaction of the metal with water leadsto the formation of the hydroxide ion.

Salts of the bases MOH are known forall acids and these are generally white crys-talline solids. The ions M+ are hydrated inwater and remain unchanged in most reac-tions of alkali metal salts.

Because of the ease of formation of theions M+ there are very few complexes ofthe type MLn

+ apart from solvated speciesof very low correlation times.

Francium is formed only by radioactivedecay and in nuclear reactions; all the iso-topes of francium have short half-lives, thelongest of which (223Fr) is 21 minutes. The

alcohol

6

few chemical studies that have been carriedout on francium indicate that it has similarproperties to those of the other alkali met-als.

alkalimetry See acidimetry.

alkaline earth See alkaline-earth met-als.

alkaline-earth metals (group 2 el-ements) A group of moderately reactivemetals, harder and less volatile than the al-kali metals. They were formerly classifiedin subgroup IIA of the periodic table. Theterm alkaline earth strictly refers to the ox-ides, but is often used loosely for the el-ements themselves. The electronicconfigurations are all those of a rare-gasstructure with an additional two electronsin the outer s orbital. The elements areberyllium (Be), magnesium (Mg), calcium(Ca), strontium (Sr), barium (Ba), and ra-dium (Ra). The group shows an increasingtendency to ionize to the divalent stateM2+. The first member, beryllium, has amuch higher ionization potential than theothers and the smallest atomic radius.Thus it has a high charge/size ratio andconsequently the bonding in berylliumcompounds is largely covalent. The chem-istry of the heavier members of the group islargely that of divalent ions.

The group displays a typical trend to-ward metallic character as the group is de-scended. For example, berylliumhydroxide is amphoteric; magnesium hy-droxide is almost insoluble in water and isslightly basic; calcium hydroxide is spar-ingly soluble and distinctly basic; andstrontium and barium hydroxides are in-creasingly soluble in water and stronglybasic. The group also displays a smoothtrend in the solubilities of the sulfates(MgSO4 is soluble, CaSO4 sparingly solu-ble, and BaSO4 very insoluble). The trendto increasing metallic character is alsoshown by the increase in thermal stabilitiesof the carbonates and nitrates with increas-ing relative atomic mass.

The elements all burn in air (berylliummust be finely powdered) to give the oxideMO (covalent in the case of beryllium) and

for barium the peroxide BaO2 in additionto BaO. The heavier oxides, CaO, SrO, andBaO, react with water to form hydroxides,M(OH)2; magnesium oxide reacts only athigh temperatures and beryllium oxide notat all. The metals Ca, Sr, and Ba all reactreadily with water to give the hydroxide:

M + 2H2O → M2+ + 2OH– + H2In contrast, magnesium requires diluteacids in order to react (to the salt plus hy-drogen), and beryllium is resistant to acidattack. A similar trend is seen in the directreaction of hydrogen: under mild condi-tions calcium, strontium, and barium giveionic hydrides, high pressures are requiredto form magnesium hydride, and berylliumhydride can not be prepared by direct com-bination.

Because of its higher polarizing power,beryllium forms a range of complexes inwhich the beryllium atom should betreated as an electron acceptor (i.e. the va-cant p orbitals are being used). Complexessuch as etherates, acetylethanoates, and thetetrafluoride (BeF4

2–) are formed, all ofwhich are tetrahedral. In contrast Mg2+,Ca2+, Sr2+, and Ba2+ have poor acceptorproperties and form only weak complexes,even with donors such as ammonia or edta.

All isotopes of radium are radioactiveand radium was once widely used forradiotherapy. The half-life of 226Ra(formed by decay of 238U) is 1600 years.

allotropy The ability of certain elementsto exist in more than one physical form (al-lotrope). Carbon, sulfur, and phosphorusare the most common examples. Allotropyis more common in groups 14, 15, and 16of the periodic table than in other groups.See also enantiotropy; monotropy. Com-pare polymorphism.

alloy A mixture of two or more metals(e.g. BRONZE or BRASS) or a metal withsmall amounts of nonmetals (e.g. STEEL).Alloys may be completely homogeneousmixtures or may contain small particles ofone PHASE in the other phase. Alloys arestronger, harder, and often more corrosionresistant than their components, but ex-hibit reduced ductility and lower electrical

7

alloy

conductivity. Most metals encountered ineveryday life are actually alloys.

Alnico (Trademark) Any of a group ofvery hard brittle alloys used to make pow-erful permanent magnets. They containnickel, aluminum, cobalt, and copper invarious proportions. Iron, titanium, andniobium can also be present. They magne-tize strongly when exposed to an excitingmagnetic field and resist demagnitizationeven when exposed to a reverse magnetiz-ing force.

alpha particle A He2+ ion emitted withhigh kinetic energy by a radioactive sub-stance undergoing alpha decay. Alpha par-ticles are emitted at high velocity, and areused to cause nuclear disintegration reac-tions.

alum A type of double salt. Alums aredouble sulfates obtained by crystallizingmixtures in the correct proportions. Theyhave the general formula:

M2SO4.M′2(SO4)3 .24H2Owhere M is a univalent metal or ion, andM′ is a trivalent metal. Thus, ALUMINUM

POTASSIUM SULFATE (called potash alum, orsimply alum) is

K2SO4.Al2(SO4)3.24H2Oand aluminum ammonium sulfate (calledammonium alum) is

(NH4)2SO4.Al2(SO4)3.24H2OThe name alum originally came from

the presence of Al3+ as the trivalent ion, butis now also applied to other double saltscontaining trivalent ions, thus,chromium(III) potassium sulfate (chromealum) is

K2SO4.Cr2(SO4)3.24H2O

alumina See aluminum oxide.

aluminate See aluminum hydroxide.

aluminosilicate See silicates.

aluminum A soft moderately reactivemetal; the second element in group 13 (for-merly IIIA) of the periodic table. Alu-minum has the electronic structure of neonplus three additional outer electrons. There

are numerous minerals of aluminum; it isthe most common metallic element in theEarth’s crust (8.1% by mass) and the thirdin order of abundance. Commercially im-portant minerals are bauxite (hydratedAl2O3), corundum (anhydrous Al2O3),cryolite (sodium hexafluroaluminateNa3AlF6), and clays and mica (aluminosil-icates).

The metal is produced on a massivescale by the Hall-Heroult method in whichaluminum oxide, a nonelectrolyte, is dis-solved in molten cryolite and electrolyzedin a large cell. The bauxite contains ironoxide and other impurities, which wouldcontaminate the product, so the bauxite isdissolved in hot alkali, the impurities areremoved by filtration, and the pure alu-minum oxide then precipitated by acidifi-cation. In the cell, molten aluminum istapped off from the base and carbon diox-ide evolved at the graphite anodes, whichare consumed in the process. The alu-minum atom is much bigger than boron(the first member of group 13) and its ion-ization potential is not particularly high.Consequently aluminum forms positiveAl3+ ions. However, aluminum also hasnonmetallic chemical properties. Thus, it isamphoteric and also forms a number ofcovalently bonded compounds.

Unlike boron, aluminum does not forma vast range of hydrides – AlH3 and Al2H6may exist at low pressures, and the onlystable hydride, (AlH3)n, must be preparedby reduction of aluminum trichloride. Theion AlH4

– is widely used in the form ofLiAlH4 as a powerful reducing agent.

The reaction of aluminum metal withoxygen is very exothermic but at ordinarytemperatures an impervious film of theoxide protects the bulk metal from furtherattack. This oxide film also protects alu-minum from oxidizing acids. There is onlyone oxide, Al2O3, but a variety of poly-morphs and hydrates are known. It is rela-tively inert and has a high melting point,and for this reason is widely used as a fur-nace lining and for general refractorybrick. Aluminum metal will react with al-kalis, releasing hydrogen to initially pro-duce Al(OH)3, then Al(OH)4

–.

Alnico

8

Aluminum reacts readily with the halo-gens; in the case of chlorine thin sheets ofthe metal will burst into flame. Aluminumfluoride has a high melting point (1290°C)and is ionic. The other halides are dimers inthe vapor phase (two halogen bridges).Aluminum also forms a sulfide (Al2S3), ni-tride (AlN), and carbide (Al4C), the lattertwo at extremely high temperatures.

Because of aluminum’s ability to ex-pand its coordination number and ten-dency towards covalence it forms a varietyof complexes such as AlF6

2– and AlCl4–.Symbol: Al; m.p. 660.37°C; b.p.

2470°C; r.d. 2.698 (20°C); p.n. 13; r.a.m.26.981539.

aluminum acetate See aluminumethanoate.

aluminum bromide (AlBr3) A whitesolid soluble in water and many organicsolvents.

aluminum chloride (AlCl3) A white co-valent solid that fumes in moist air and re-acts violently with water according to theequation:

AlCl3 + 3H2O → Al(OH)3 + 3HClIt is prepared by heating aluminum in

dry chlorine or dry hydrogen chloride orindustrially by heating aluminum oxideand carbon in the presence of chlorine.Vapor-density measurements show that itsstructure is a dimer; it consists of Al2Cl6molecules in the vapor. Aluminum chlorideis used as a catalyst in various organic re-actions, and in the cracking of petroleum.

aluminum ethanoate (aluminum acetate;Al(OOCCH3)3) A white solid soluble inwater. It is usually obtained as the dibasicsalt, basic aluminum ethanoate, Al(OH)-

(CH3COO)2. It is prepared by dissolvingaluminum hydroxide in ethanoic acid andis used extensively as a mordant in dyeing,as a size for paper and cardboard products,and in tanning. The solution is hydrolyzedand contains various complex aluminum-hydroxyl species and colloidal aluminumhydroxide.

aluminum fluoride (AlF3) A whitecrystalline solid that is slightly soluble inwater but insoluble in most organic sol-vents. Its primary use is as an additive tothe cryolite (Na3AlF6) electrolyte in theproduction of aluminum.

aluminum hydroxide (aluminate;Al(OH)3) A white powder prepared as acolorless gelatinous precipitate by addingammonia solution or a small amount ofsodium hydroxide solution to a solution ofan aluminum salt. It is an amphoteric hy-droxide and is used as a foaming agent infire extinguishers and as a mordant in dye-ing.

Its amphoteric nature causes it to dis-solve in excess sodium hydroxide solutionto form the aluminate (tetrahydroxoalumi-nate(III) ion):

Al(OH)3 + OH– → Al(OH)4– + H2O

When precipitating from solution, alu-minum hydroxide readily absorbs coloredmatter from dyes to form lakes.

aluminum nitrate (Al(NO3)3.9H2O) Ahydrated white crystalline solid preparedby dissolving freshly prepared aluminumhydroxide in nitric acid. It is used as amordant. It cannot be prepared by the ac-tion of dilute nitric acid on aluminum be-cause the metal is rendered passive by athin surface layer of oxide.

aluminum oxide (alumina; Al2O3) Awhite crystalline powder that is almost in-soluble in water, occurring in two mainforms, one of which is weakly acidic, andthe other amphoteric. It occurs naturally asbauxite, corundum, and emery, and withminute amounts of chromium and cobaltas ruby and sapphire, respectively. It ismanufactured by heating aluminum hy-droxide. It is used in the extraction by elec-

9

aluminum oxide

Al

Cl

ClCl

Cl

Al

Cl

Cl

Aluminum chloride: the dimer Al2Cl6

trolysis of aluminum, as an abrasive(corundum), in furnace linings (because ofits refractory properties), and as a catalyst(e.g. in the dehydration of alcohols).

aluminum potassium sulfate (potashalum; Al2(SO4)3.K2SO4.24H2O) A whitecrystalline solid, soluble in water but insol-uble in alcohol, prepared by mixing solu-tions of ammonium and aluminum sulfatesfollowed by crystallization. It is used as amordant for dyes, as a waterproofingagent, and as a tanning additive.

aluminum sulfate (Al2(SO4)3.18H2O,A12504) A white crystalline solid. Both thehydrated and anhydrous forms are solublein water, but only the anhydrous form issoluble in ethanol, and to only a slight de-gree. It is used as a size for paper, a precip-itating agent in sewage and watertreatment, a foaming agent in fire control,and as a fireproofing agent. Its solutionsare acidic by hydrolysis, containing suchspecies as Al(H2O)5(OH)2+. It is preparedby dissolving Al(OH)3 in sulfuric acid.

aluminum trimethyl See trimethylalu-minum.

amalgam An alloy of mercury with atleast one other metal. Amalgams may beliquid or solid. An amalgam of sodium(Na/Hg) with water is used as a source ofNASCENT HYDROGEN:

2Na/Hg + 2H2O (l) → 2NaOH(aq) +H2(g) + Hg

An amalgam of Ha/Ag/Sn was used in den-tistry.

amatol A high explosive that consists ofa mixture of ammonium nitrate and TNT(trinitrotoluene).

ambidentate ligand See isomerism.

americium A highly toxic radioactivesilvery element of the actinoid series ofmetals. A transuranic element, it is foundnaturally on Earth in trace amounts in ura-nium ore. It can also be synthesized bybombarding 239Pu with neutrons. Themetal can be obtained by reducing the tri-fluoride with barium. It reacts with oxy-gen, steam, and acids. 241Am has been usedin gamma-ray radiography and in smokealarms.

Symbol: Am; m.p. 1172°C; b.p.2607°C; r.d. 13.67 (20°C); p.n. 95; moststable isotope 243Am (half-life 7.37 × 103

years).

amethyst A purple form of the mineralquartz (silicon(IV) oxide, SiO2) used as asemiprecious gemstone. The color comesfrom impurities such as oxides of iron.

ammine A complex in which ammoniamolecules are coordinated to a metal ion;e.g. [Cu(NH3)4]2+.

ammonia (NH3) A colorless gas with acharacteristic pungent odor. On coolingand compression it forms a colorless liq-uid, which becomes a white solid on fur-ther cooling. Ammonia is very soluble inwater (a saturated solution at 0°C contains36.9% of ammonia): the aqueous solutionis alkaline and contains a proportion offree ammonia. Ammonia is also soluble in

aluminum potassium sulfate

10

N

H

H

ˆ

H

H

H

N ˆ N

H H

H

H

Ammonia: umbrella inversion of the molecule

ethanol. It occurs naturally to a small ex-tent in the atmosphere, and is usually pro-duced in the laboratory by heating anammonium salt with a strong alkali. Am-monia is synthesized industrially from hy-drogen and atmospheric nitrogen by theHABER PROCESS.

The compound does not burn readily inair, but ignites – giving a yellowish-brownflame – in oxygen. It will react with atmos-pheric oxygen in the presence of platinumor a heavy metal catalyst – a reaction usedas the basis of the commercial manufactureof nitric acid, which involves the oxidationof ammonia to nitrogen monoxide andthen to nitrogen dioxide. Ammonia coordi-nates readily to form ammines and reactswith sodium or potassium to form inor-ganic amides and with acids to form am-monium salts; for example, it reacts withhydrogen chloride to form ammoniumchloride:

NH3(g) + HCl(g) → NH4Cl(g)Ammonia is used commercially in the

manufacture of fertilizers, mainly ammo-nium nitrate, urea, and ammonium sulfate.It is also used to make explosives, resins,and dyes. As a liquefied gas it is used in therefrigeration industry. Liquid ammonia isan excellent solvent for certain substances,which ionize in the solutions to give ionicreactions similar to those occurring inaqueous solutions. Ammonia is marketedas the liquid, compressed in cylinders (‘an-hydrous ammonia’), or as aqueous solu-tions of various strengths. See alsoammonium hydroxide.

ammoniacal Describing a solution inaqueous ammonia.

ammonia-soda process See Solvayprocess.

ammonia solution See ammonium hy-droxide.

ammonium alum See alum.

ammonium carbonate (sal volatile;(NH4)2CO3) A white solid that crystal-lizes as plates or prisms. It is very soluble inwater and readily decomposes on heating

to ammonia, carbon dioxide, and water.The white solid sold commercially as am-monium carbonate is actually a double saltof both ammonium hydrogencarbonate(NH4HCO3) and ammonium amino-methanoate (NH2CO2NH4). This salt ismanufactured from ammonium chlorideand calcium carbonate. It decomposes onexposure to damp air into ammonium hy-drogencarbonate and ammonia, and it re-acts with ammonia to give the trueammonium carbonate. Commercial am-monium carbonate is used in baking pow-ders, smelling salts, in the dyeing andwool-scouring industries, and in coughmedicines.

ammonium chloride (sal ammoniac;NH4Cl) A white crystalline solid with acharacteristic saline taste. It is very solublein water. Ammonium chloride can be man-ufactured by the action of ammonia on hy-drochloric acid. It sublimes on heatingaccording to the equilibrium reaction:

NH4Cl(s) ˆ NH3(g) + HCl(g)Ammonium chloride is used in galva-

nizing, as a flux for soldering, as a mordantin dyeing and calico printing, and in themanufacture of Leclanché and ‘dry’ cells.

ammonium hydroxide (ammonia solu-tion; NH4OH) An alkali that is formedwhen ammonia dissolves in water. It prob-ably contains hydrated ammonia mol-ecules as well as some NH4

+ and OH– ions.It is a useful reagent and cleansing agent.

ammonium ion The ion NH4+, formed

by coordination of NH3 to H+. It has tetra-hedral symmetry.

ammonium nitrate (NH4NO3) A col-orless crystalline solid that is very solublein water and also soluble in ethanol. It isusually manufactured by the action of am-monia gas on nitric acid. It is used in fertil-izers because of its high nitrogen content,and in the manufacture of explosives androcket propellants.

ammonium phosphate (triammoniumphosphate(V); (NH4)3PO4) A colorlesscrystalline salt made from ammonia and

11

ammonium phosphate

phosphoric(V) acid, used as a fertilizer toadd both nitrogen and phosphorus to thesoil.

ammonium sulfate ((NH4)2SO4) A col-orless crystalline solid that is soluble inwater but not in ethanol. When heatedcarefully it gives ammonium hydrogensul-fate, which on stronger heating yields ni-trogen, ammonia, sulfur(IV) oxide (sulfurdioxide), and water. Ammonium sulfate ismanufactured by the action of ammoniaon sulfuric acid. It is an important ammo-nium salt because of its widespread use asa fertilizer. Its only drawback as a fertilizeris that it tends to leave an acidic residue inthe soil.

amorphous Describing a solid sub-stance that has no ‘long-range’ regulararrangement of atoms; i.e. is not crys-talline. Amorphous materials can consistof minute particles that possess order oververy short distances. Glasses are amor-phous, because the atoms in the solid havea random arrangement. X-ray diffractionanalysis has shown that many substancesthat were once described as amorphous arein fact composed of very small crystals. Forexample, charcoal, coke, and soot (allforms of carbon) are made up of smallgraphitelike crystals.

amount of substance Symbol: n Ameasure of the number of elementary enti-ties present in a substance. It is measured inMOLES. See also Avogadro constant.

ampere Symbol: A The SI base unit ofelectric current, defined as the constantcurrent that, maintained in two straightparallel infinite conductors of negligiblecircular cross section placed one meterapart in vacuum, would produce a forcebetween the conductors of 2 × 10–7 newtonper meter.

amphiprotic See acid; solvent.

ampholyte ion See zwitterion.

amphoteric Describing material thatcan display both acidic and basic proper-

ties. The term is most commonly applied tothe oxides and hydroxides of metals thatcan form both cations and complex anions.For example, zinc oxide dissolves in acidsto form zinc salts and also dissolves in al-kalis to form zincates, [Zn(OH)4]2–. Theamino acids are also considered to be am-photeric because they contain both acidicand basic groups.

amu See atomic mass unit.

analysis The process of determining theconstituents or components of a sample.There are two broad major classes ofanalysis, QUALITATIVE ANALYSIS – essentiallyanswering the question ‘what is it?’ – andQUANTITATIVE ANALYSIS – answering thequestion ‘how much of such and such acomponent is present?’ There is a vastnumber of analytical methods that can beapplied, depending on the nature of thesample and the purpose of the analy-sis. These include GRAVIMETRIC ANALYSIS,VOLUMETRIC, and systematic qualitativeanalysis (classical wet methods); and in-strumental methods, such as CHROMATOG-RAPHY, SPECTROSCOPY, NUCLEAR MAGNETIC

RESONANCE, POLAROGRAPHY, and fluores-cence techniques.

ångstrom Symbol: Å A unit of lengthdefined as 10–10 meter, formerly used tomeasure wavelengths of radiation, includ-ing those of visible light, and inter-molecu-lar distances. The preferred SI unit for suchmeasurements is the nanometer. Oneångstrom equals 0.1 nanometer. Theångstrom was named for Anders JonasÅngstrom (1814–74), a Swedish physicistand astronomer.

anhydride A compound formed by re-moving water from an acid or, less com-monly, a base. Many nonmetal oxides areanhydrides of acids: for example CO2 is theanhydride of H2CO3 and SO3 is the anhy-dride of H2SO4.

anhydrite See calcium sulfate.

anhydrous Describing a substance thatlacks moisture, or a salt lacking water of

ammonium sulfate

12

crystallization. For example, on strongheating, blue crystals of copper(II) sulfatepentahydrate, CuSO4.5H2O, form whiteanhydrous copper(II) sulfate, CuSO4.

anion A negatively charged ion, formedby the addition of electrons to atoms ormolecules. In electrolysis anions are at-tracted to the positive electrode or anode.Compare cation.

anionic detergent See detergents.

anionic resin An ION-EXCHANGE ma-terial that can exchange anions, such as Cl–

and OH–, for anions in the surroundingmedium. Such resins are often produced bythe addition of a quaternary ammoniumgroup (–N(CH3)3

+) or a phenolic group(–OH–) to a stable polyphenylethene resin.A typical exchange reaction is:

resin–N(CH3)3+Cl– + KOH ˆ

resin–N(CH3)3+OH– + KCl

Anionic resins can be used to separatemixtures of halide ions. Such mixtures canbe attached to the resin and recovered sep-arately by ELUTION.

anisotropic Describing certain sub-stances that have one or more physicalproperties, such as refractive index, thatdiffer according to direction. Most crystalsare anisotropic.

annealing A type of heat treatment ap-plied to metals to change their physicalproperties. The metal is heated to, and heldat, an appropriate temperature beforebeing cooled at a suitable rate to producethe desired grain structure. Annealing ismost commonly used to remove thestresses that have arisen during rolling, toincrease the softness of the metal, and tomake it easier to machine. Objects made ofglass can also be annealed to removestrains.

anode In electrolysis, the electrode thatis at a positive potential with respect to thecathode, and to which anions are thereforeattracted. In any electrical system, such asa discharge tube or electronic device, the

anode is the terminal from which electronsflow out of the system.

anode sludge See electrolytic refining.

anodizing An industrial process for pro-tecting aluminum with an oxide layer. Thealuminum object is made the anode in anelectrolytic cell containing an oxidizingacid (e.g. sulfuric(VI) acid). The layer ofAl2O3 formed is porous and can be coloredwith certain dyes.

anthracite The highest grade of COAL,with a carbon content of between 92% and98%. It burns with a hot blue flame, givesoff little smoke and leaves hardly any ash.

antibonding orbital See orbital.

anti-isomer See isomerism.

antimonic Designating an antimony(V)compound.

antimonous Designating an antimony(III)compound.

antimony A toxic metalloid element ofgroup 15 (formerly VA) of the periodictable. It exists in three allotropic forms; themost stable is a hard, brittle, silvery-bluemetal. Yellow and black antimony, theother two allotropes, are unstable andnonmetallic. Antimony is found in manyminerals, principally stibnite (Sb2S3), fromwhich it is recovered by reduction withiron or by first roasting it to yield theoxide. It is also a poor conductor of heatand electricity, making it useful in themanufacture of semiconductors. Antimonycompounds are used in pigments, flame re-tardants, medical treatments, ceramics,and glass.

Symbol: Sb; m.p. 630.74°C; b.p.1750°C; r.d. 6.691; p.n. 51; r.a.m. 112.74.

antimony(III) chloride (antimonytrichloride; SbCl3) A white deliquescentsolid, formerly known as butter of anti-mony. It is prepared by direct combinationof antimony and chlorine. It is readily hy-drolyzed by cold water to form a white pre-

13

antimony(III) chloride

cipitate of antimony(III) chloride oxide(antimonyl chloride, SbOCl):

SbCl3 + H2O = SbOCl + 2HCl

antimony(III) chloride oxide See anti-mony(III) chloride.

antimonyl chloride See antimony(III)chloride.

antimony(III) oxide (antimony trioxide;Sb2O3) A white insoluble solid. It is anamphoteric oxide with a strong tendencyto act as a base. It can be prepared by di-rect oxidation by air, oxygen, or steam andis formed when antimony(III) chloride ishydrolyzed by excess boiling water. It isused as a flame retardant in plastics and asan additive in paints to make them moreopaque.

antimony(V) oxide (antimony pentox-ide; Sb2O5) A yellow solid. It is usuallyformed by the action of concentrated nitricacid on antimony or by the hydrolysis ofantimony(V) chloride. Although an acidicoxide, it is only slightly soluble in water. Itis used as a flame retardant.

antimony pentoxide See antimony(V)oxide.

antimony trichloride See antimony(III)chloride.

antimony trioxide See antimony(III)oxide.

antioxidant A substance that inhibitsoxidation. Antioxidants are added to suchproducts as foods, paints, plastics, andrubber to delay their oxidation by atmos-pheric oxygen. Some work by formingCHELATES with metal ions, thus neutralizingthe catalytic effect of the ions in the oxida-tion process. Other types remove interme-diate oxygen FREE RADICALS. Naturallyoccurring antioxidants can limit tissue orcell damage in the body. They include vita-mins C and E, and β-carotene.

antiparallel spins Spins of two neigh-boring electrons in which the magnetic mo-

ments associated with electron spin arealigned in opposite directions.

apatite A common, naturally occurringphosphate of calcium, Ca5(PO4)3(OH,F,Cl),that occurs in several color varieties. Crys-tals are hexagonal and have a greasy luster.Apatite occurs in rocks of igneous andmetamorphic origin and is mined as asource of phosphorus for use in fertilizers.Gemstone quality crystals are known fromseveral locations.

aprotic See solvent.

aqua fortis An old name for nitric acid,HNO3.

aqua regia A mixture of concentratednitric acid and three to four parts of con-centrated hydrochloric acid. With the ex-ception of silver, with which it forms aninsoluble chloride, it dissolves all metals,including gold. The mixture contains chlo-rine and NOCl (nitrosyl chloride). Thename means ‘royal water’.

aqueous Describing a solution in water.

aragonite An anhydrous mineral formof calcium carbonate, CaCO3, which oc-curs associated with limestone and in somemetamorphic rocks. It is also the main in-gredient of pearls. It is not as stable as cal-cite, into which it may change over time.Pure aragonite is colorless or white, but im-purities such as strontium, zinc, or leadmay tint it various colors.

argentic oxide See silver(II) oxide.

argentous oxide See silver(I) oxide.

argon An inert colorless odorlessmonatomic element of the rare-gas group.It forms 0.93% by volume of air, fromwhich it is obtained by fractional distilla-tion. Argon is used to provide an inert at-mosphere in electric and fluorescent lights,in aluminum welding, and in titanium andsilicon extraction. The element forms noknown compounds.

antimony(III) chloride oxide

14

Symbol: Ar; m.p. –189.37°C; b.p.–185.86°C; r.d. 0.0001 784 (0°C); p.n. 18;r.a.m. 39.95.

Arrhenius, Svante August (1859–1927) Swedish physical chemist who firstpostulated that the electrical conductivityof electrolytes is due to the dissolved sub-stance being dissociated into electricallycharged particles (ions). He put forwardthis idea in 1883 and developed it in 1887.Arrhenius also made a major contributionto the theory of chemical reactions in 1889when he suggested that a molecule can takepart in a chemical reaction only if its en-ergy is higher than a certain value. Thisgave rise to the Arrhenius equation relatingthe rate of a chemical reaction to the ab-solute temperature. He also performed cal-culations that led him to the idea of thegreenhouse effect. Arrhenius won the 1903Nobel Prize for chemistry for his theory ofelectrolytes.

Arrhenius equation An equation, pro-posed by Svante Arrhenius in 1889, that re-lates the rate constant of a chemicalreaction to the temperature at which the re-action is taking place:

k = Aexp(–Ea/RT)where A is a constant for the given reac-tion, k the rate constant, T the thermody-namic temperature in kelvins, R the gasconstant, and Ea the activation energy ofthe reaction.

Reactions proceed at different rates atdifferent temperatures, i.e. the magnitudeof the rate constant is temperature depend-ent. The Arrhenius equation is often writ-ten in a logarithmic form, i.e.

logek = logeA – Ea/RTThis equation enables the ACTIVATION

ENERGY for a reaction to be determined.The equation can also be applied to prob-lems dealing with diffusion, viscosity, elec-trolytic conduction, etc.

Arrhenius theory See acid; base.

arsenate(III) (arsenite) A salt of the hy-pothetical arsenic(III) acid, formed by re-acting arsenic(III) oxide, A2O3, withalkalis. Arsenate(III) salts contain the ion

AsO33–. Copper arsenate(III) is used as an

insecticide.

arsenate(V) A salt of arsenic(V) acid,made by reacting arsenic(III) oxide, As2O3,with nitric acid. Arsenate(V) salts containthe ion AsO4

3–. Disodiumhydrogenarsen-ate(V) is used in printing calico.

arsenic A toxic metalloid element ofgroup 15 (formerly VB) of the periodictable. It exists in three allotropic forms; themost stable is a brittle gray metal. Yellowand black arsenic, the other allotropes, arenot metallic. Arsenic is found native and inseveral ores including mispickel or ar-senopyrite (FeSAs), realgar (As4S4), and or-piment (As2S3). The ores are roasted toproduce arsenic(III) oxide, which is thenreduced with carbon or hydrogen to re-cover the element. Arsenic reacts with hotacids and molten sodium hydroxide but isunaffected by water, acids, or alkalis atnormal temperatures. It is used in semicon-ductors, alloys, lasers, and fireworks. Ar-senic and its compounds are poisonous andtherefore also find use in insecticides, ro-dentricides, and herbicides.

Symbol: As; m.p. 817°C (gray) at 3MPa pressure; sublimes at 616°C (gray);r.d. 5.78 (gray at 20°C); p.n. 33; r.a.m.74.92159.

arsenic(III) chloride (arsenious chloride;AsCl3) A poisonous oily liquid. It fumesin moist air due to hydrolysis with watervapor:

AsCl3 + 3H2O = As2O3 + 6HClArsenic(III) chloride is covalent and ex-

hibits nonmetallic properties.

arsenic hydride See arsine.

arsenic(III) oxide (white arsenic; arse-nious oxide; As2O3) A colorless crys-talline solid that is very poisonous (0.1 gwould be a lethal dose). Analysis of thesolid and vapor states suggests a dimerizedstructure of As4O6. An amphoteric oxide,arsenic(III) oxide is sparingly soluble inwater, producing an acidic solution. It isformed when arsenic is burned in air oroxygen. It is used as an insecticide, herbi-

15

arsenic(III) oxide

cide, and defoliant, and in the manufactureof glass and ceramics having a milky iri-descence.

arsenic(V) oxide (arsenic oxide; As2O5)A white amorphous deliquescent solid. It isan acidic oxide prepared by dissolving ar-senic(III) oxide in hot concentrated nitricacid, followed by crystallization then heat-ing to 210°C.

arsenide A compound of arsenic and an-other metal. For example, with iron arsenicforms iron(III) arsenide, FeAs2, while withgallium arsenic forms gallium arsenide,GaAs. Gallium arsenide is an importantsemiconductor.

arsenious chloride See arsenic(III)chloride.

arsenious oxide See arsenic(III) oxide.

arsenite See arsenate(III).

arsine (arsenic hydride; AsH3) A highlypoisonous colorless gas with an unpleasantsmell. It is produced by reacting mineralacids with arsenides or by reducing arseniccompounds with nascent hydrogen. Arsinedecomposes to arsenic and hydrogen at230°C. This phenomenon is put to use inMarsh’s test for arsenic, in which arsinegenerated from a sample is fed through aglass tube. Here the arsine, if present, de-composes to leave a brown deposit of ar-senic, which can be distinguished fromantimony by the fact that antimony willnot dissolve in NaOC1. Arsine is used tomake n-type semiconductors doped withtrace amounts of arsenic.

artificial radioactivity Radioactivityinduced by bombarding stable nuclei withhigh-energy particles. For example:

12

37Al + 0

1n → 12

14Na + 2

4Herepresents the bombardment of aluminumwith neutrons to produce an isotope ofsodium and helium nuclei (alpha particles).All transuranic elements above curium,atomic number 96, are artificially radio-active because they do not occur in nature.Even neptunium, plutonium, americium,

and curium, elements 93 through 96, areonly found in very minute amounts in na-ture and thus are also usually produced ar-tificially.

asbestos Any of several fibrous varietiesof various rock-forming silicate minerals,such as the amphiboles and chrysotile. As-bestos has many uses that employ its prop-erties of exceptional heat-resistance,chemical inertness, and electrical resis-tance. It is, for example, spun and woveninto fabric that is used to make fireproofclothing and brake linings, uses for whichthere are currently no adequate substitutes.However, prolonged exposure to asbestosdust may cause asbestosis – a serious, pro-gressive disease that eventually leads to res-piratory failure. Mesothelioma, amalignant cancer of the membrane enclos-ing the lung, may also result.

aspirator An apparatus for sucking agas or liquid from a vessel or body cavity.

associated liquids See association.

association The combination of mol-ecules of a substance with those of anotherto form more complex species. An exampleis a mixture of water and ethanol (whichare termed associated liquids), the mol-ecules of which combine via hydrogenbonding.

astatine A radioactive halogen elementof group 17 (formerly VIIA) of the periodictable. It occurs in minute quantities in ura-nium ores as the result of radioactivedecay, and can be artifically created bybombarding 200Bi with alpha particles.Many short-lived radioisotopes areknown, all alpha-particle emitters. Due toits rapid decay it is used as a radioactivetracer in medicine.

Symbol: At; m.p. 302°C (est.); b.p.337°C (est.); p.n. 85; most stable isotope210At (half-life 8.1 hours).

Aston, Francis William (1877–1945)British chemist and physicist. Aston’s maincontribution to science was the develop-ment of the mass spectrograph. This en-

arsenic(V) oxide

16

abled atomic masses to be determined anddemonstrated the existence of isotopes.From experiments soon after World War IAston was able to explain Prout’s hypoth-esis and the exceptions to it. He discoveredthat neon consists of two isotopes neon-20and neon-22, with the former being tentimes more common. This gives an averageof 20.2 for the weight of a large collectionof neon atoms. The atomic weights (rela-tive atomic masses) of other elements wereexplained in a similar way. Between themid-1920s and mid-1930s he was able todetermine atomic weights more accuratelyusing a new mass spectrograph. He foundsmall discrepancies from the whole num-bers postulated by Prout due to the bindingenergy of nuclei. He was awarded the 1922Nobel Prize for chemistry.

asymmetric atom See isomerism; opti-cal activity.

atmolysis The separation of gases byusing their different rates of diffusionthrough a porous membrane or partition.

atmosphere A unit of atmospheric pres-sure, equal to 101.325 kilopascals in SIunits. It is used in chemistry only for rough

indications of high pressure; in particular,those of high-pressure industrial processes.

atom The smallest part of an elementthat can exist as a stable entity. Atoms con-sist of a small, dense, positively chargednucleus, made up of neutrons and protons,with electrons in a cloud around this nu-cleus. The chemical reactions of an elementare determined by the number of its elec-trons, which is equal to the number of pro-tons in its nucleus. All atoms of a givenelement have the same number of protons,an identifying quantity known as the ele-ment’s PROTON NUMBER. A given elementmay also have two or more isotopes, whichdiffer in the number of neutrons in theirnuclei.

The electrons surrounding the nucleusare grouped into energy levels traditionallycalled SHELLS – i.e. main orbits around thenucleus. Within these main orbits theremay be subshells corresponding to atomicORBITALS.An electron in an atom is specified by fourquantum numbers:1. The principal quantum number (n),

which specifies the main energy levels.The principal quantum number is a pos-itive integer which can have values 1, 2,

17

atom

Quantumproperty

Integer values Quantum number

Principal, n

Orbital, l

Magnetic, m

1,2,3, ...

0 to n –1

–l, ..., 0, ..., + l

0

1 2 3

10

–1 0 +10 0

10

–1 0 +10 –1 0 +1 +2–2

2

Atom: quantum numbers

3, etc. The corresponding shells are de-noted by letters K, L, M, etc., the K shell (n= 1) being the one nearest to the nucleus.The maximum number of electrons in agiven shell is 2n2.2. The orbital quantum number (l), which

specifies the angular momentum of theelectron and thus the shape of the or-bital. For a given value of n, 1 can havepossible integer values of n–1, n–2, … 2,1, 0. For instance, the M shell (n = 3) hasthree subshells with different values of l(0, 1, and 2). Subshells with angular mo-mentum 0, 1, 2, and 3 are designated byletters s, p, d, and f.

3. The magnetic quantum number (m)which determines the orientation of the electron orbital in a magnetic field.This number can have integer values –l, –(l – 1) … 0 … + (l – l), +l.

4. The spin quantum number (ms), whichspecifies the intrinsic angular momen-tum of the electron. It can have values+½ and –½.According to the exclusion principle

enunciated by Pauli in 1925 (the Pauli ex-clusion principle), no two electrons in anatom can have the same set of four quan-tum numbers. Each electron’s unique set offour quantum numbers therefore specifiesits QUANTUM STATE and helps to explain theelectronic structure of atoms. See also Bohrtheory.

atomic absorption spectroscopy (AAS)A method of chemical analysis in which asample is vaporized and an absorptionspectrum is taken of the vapor. The el-ements present are identified by their char-acteristic absorption lines.

atomic absorption spectroscopy

18

nitrogen atom (ground state)

magnesium atom (ground state)

scandium atom (ground state)

1s 2s 2p

3s1s 2s 2p

1s 2s 2p

1s 2s 2p

1s 2s 2p

1s 2s 2p

3s 3p 4s 3d

carbon atom (ground state)

beryllium atom (ground state)

beryllium atom (excited state)

Atom: examples of box-and-arrow diagrams

atomic emission spectroscopy (AES)A technique of chemical analysis that in-volves vaporizing a sample of material,with atoms in their excited states emittingelectromagnetic radiation at particular fre-quencies characteristic of that type ofatom.

atomic force microscope (AFM) Aninstrument used to investigate surfaces. Asmall probe consisting of a very small chipof diamond is held just above a surface ofa sample by a spring-loaded cantilever. Asthe probe is slowly moved over the surfacethe force between the surface and the tip ismeasured and recorded and the probe isautomatically raised and lowered to keepthis force constant. Scanning the surface inthis way enables a contor map of the sur-face to be generated with the help of a com-puter. An atomic force microscope closelyresembles a SCANNING TUNNELLING MICRO-SCOPE (STM) in some ways, although it usesforces rather than electrical signals to in-vestigate the surface. Like an STM, it canresolve individual molecules. Unlike anSTM, it can be used to investigate noncon-ducting materials, a feature that is useful ininvestigating biological samples.

atomic heat See Dulong and Petit’s law.

atomicity The number of atoms permolecule of an element. Helium (He), forexample, has an atomicity of one, nitrogen(N2) two, and ozone (O3) three.

atomic mass unit (amu) Symbol: u Aunit of mass used to indicate the relativeatomic mass of atoms and molecules, equalto 1/12 of the mass of an atom of carbon-12. It is equal to 1.660540 × 10–27 kilo-gram. In biochemistry it is sometimesknown as the dalton.

atomic number See proton number.

atomic orbital See orbital.

atomic weight See relative atomic mass(r.a.m.).

atto- Symbol: a A prefix used with SI

units denoting 10–18. For example, 1 at-tometer (am) = 10–18 meter (m).

Aufbau principle A principle that gov-erns the order in which the atomic orbitalsare filled in elements of successive protonnumber; i.e. in order of increasing energy.The name is from the German aufbauen,meaning ‘to build up’. The order is as fol-lows:1s2, 2s2, 2p6, 3s2, 3p6, 4s2, 3d10, 4p6, 5s2,4d10, 5p6, 6s2, 4f14, 5d10, 6p6, 7s2, 5f14,6d10

with the superscript indicating the maxi-mum number of electrons for each level.

Note that degenerate orbitals are occu-pied singly before spin pairing occurs.Note also the unexpected position of the dlevels, the filling of which give rise to thefirst, second, and third transition series,and the unusual position of the f levels, thefilling of which give rise to the lanthanoidsand actinoids.

Auger effect An effect in which an ex-cited ion decays by emission of an electronrather than by a photon. For example, if asubstance is bombarded by high-energyelectrons or gamma rays, an electron froman inner shell may be ejected. The result isa positive ion in an excited state. This ionwill decay to its ground state as an outerelectron falls to an inner shell. The energyreleased may result in the emission of aphoton in the x-ray region of the electro-magnetic spectrum (x-ray fluorescence).Alternatively, the energy may be releasedin the form of a second electron ejectedfrom the atom resulting in a doublycharged ion. The emitted electron, knownas an Auger electron, has a characteristicenergy corresponding to the difference inenergy levels in the ion. The Auger effect isa form of autoionization. It is named forFrench physicist Pierre Auger (1899–1994), who discovered it in 1925.

Auger electron See Auger effect.

auric Designating a compound ofgold(III).

auric chloride See gold(III) chloride.

19

auric chloride

aurous Designating a compound ofgold(I).

autocatalysis See catalyst.

autoclave An apparatus consisting of anairtight container whose contents areheated by high-pressure steam; the appara-tus may also have a mechanism by whichto agitate the contents. Autoclaves are usedto react substances under pressure at hightemperature, to sterilize objects and sub-stances, and to carry out industrialprocesses.

autoionization The spontaneous ion-ization of excited atoms, ions, or mol-ecules, as in the AUGER EFFECT.

average atomic mass See relativeatomic mass. Count of Quaregna and Cer-reto

Avogadro, Lorenzo Romano AmedeoCarlo, Count of Quaregna and Cer-reto (1776–1856) Italian scientist. Avo-gadro is mainly remembered for a paper hewrote in 1811 in which he put forwardwhat is now known as AVOGADRO’S LAW.Avogadro was able to use this idea to show

that molecules of hydrogen and oxygen arediatomic and that the formula of water isH2O. He was also able to give an explana-tion of Gay-Lussac’s law. Avogadro’swork made little impact in his lifetime andwas revived later by Stanislao CANNIZ-ZARO.

Avogadro constant (Avogadro’s num-ber) Symbol: NA The number of particlesin one MOLE of a substance. Its value is6.02242 × 1023 mol–1.

Avogadro’s law (Avogadro’s hypothesis)The principle that equal volumes of allgases at the same temperature and pressurecontain equal numbers of molecules. It isoften called Avogadro’s hypothesis be-cause it was first proposed by the Italianchemist and physicist Amedeo Avogadro(1776–1856) in 1811. It is strictly true onlyfor ideal gases.

Avogadro’s number See Avogadroconstant.

azide An inorganic compound contain-ing the ion N3

–, or an organic compoundhaving the general formula RN3. Heavymetal azides are highly explosive.

aurous

20

back e.m.f. An e.m.f. that opposes thenormal flow of electric charge in a circuitor circuit element. In some electrolytic cellsa back e.m.f. is caused by the layer of hy-drogen bubbles that builds up on the cath-ode as hydrogen ions pick up electrons andform gas molecules (i.e. as a result of PO-LARIZATION of the electrode).

baking powder A mixture of sodiumhydrogencarbonate (sodium bicarbonate,baking soda) and a weakly acidic sub-stance, such as tartaric acid or potassiumhydrogentartrate (cream of tartar). The ad-dition of moisture or heat causes a reactionthat produces bubbles of carbon dioxidegas, which make dough or cake mixturerise. It is used as a yeast substitute in bak-ing certain types of bread.

baking soda See sodium hydrogencar-bonate.

ball mill A device commonly used in thechemical industry for grinding solid ma-terial. Ball mills usually have slowly rotat-ing steel-lined drums containing steel balls.The material is crushed by the tumbling ac-tion of the balls in the drum. Comparehammer mill.

Balmer series A series of lines in thespectrum of radiation emitted by excitedhydrogen atoms. The lines correspond tothe atomic electrons falling into the secondlowest energy level, emitting energy as ra-diation. The wavelengths (λ) of the radia-tion in the Balmer series are given by:

1/λ = R(1/22 – 1/n2)where n is an integer and R is the Rydbergconstant. The series is named for J. J.Balmer (1825–98), who discovered a gen-

eral equation for spectral lines in 1885. Seealso Bohr theory; spectral series.

banana bond (bent bond) A multicen-ter bond of the type present in BORON HY-DRIDES. The term is used in a quite separatesense for the bonds in certain strained-ringorganic compounds.

band spectrum A SPECTRUM that ap-pears as a number of bands of emitted orabsorbed radiation. Band spectra are char-acteristic of molecules. Often each bandcan be resolved into a number of closelyspaced lines. The different bands corre-spond to changes of electron orbit in themolecules. The closely spaced lines in eachband, seen under higher resolution, are theresult of different vibrational states of themolecule.

Barff process A process formerly usedfor protecting iron from corrosion by heat-ing it in steam to form a layer of tri-irontetroxide (Fe3O4). It is also known as theBower–Barff process and is now only ofhistorical interest.

barites See barium sulfate.

barium A dense, low-melting reactivemetal; the fifth member of group 2 (for-merly IIA) of the periodic table and a typi-cal alkaline-earth element. The electronicconfiguration is that of xenon with two ad-ditional outer 6s electrons. Barium is oflow abundance; it is found in mineral formas witherite (BaCO3) and barytes (BaSO4).The metal is obtained by the electrolysis ofthe fused chloride using a cooled cathodewhich is slowly withdrawn from the melt.Because of its low melting point barium isreadily purified by vacuum distillation.

21

B

Barium metal is used as a ‘getter’, i.e., acompound added to a vacuum system toremove the last traces of oxygen. It is alsoa constituent of certain bearing alloys.

Barium has a low ionization potentialand a large radius. It is therefore stronglyelectropositive and its properties, andthose of its compounds, are very similar tothose of the other alkaline-earth elementscalcium and strontium. Notable differ-ences in the chemistry of barium from therest of the group are:1. The much higher stability of the carbon-

ate.2. The formation of a peroxide below

800°C. Barium peroxide decomposes onstrong heating to give oxygen and bar-ium oxide:

BaO2 ˆ BaO + OBarium is also notable for the very low

solubility of the sulfate. Barium com-pounds give a characteristic green color toflames, a phenomenon that is used in qual-itative analysis. Barium salts are all highlytoxic with the exception of the most insol-uble materials. Barium sulfate is used as aradiopaque material in making x-rayphotographs of the gastric and intestinaltract.

Symbol: Ba; m.p. 729°C; b.p. 1640°C;r.d. 3.594 (20°C); p.n. 56; r.a.m. 137.327.

barium bicarbonate See barium hydro-gencarbonate.

barium carbonate (BaCO3) A white in-soluble salt that occurs naturally as themineral witherite. It can be precipitated byadding an alkali carbonate to a solution ofa barium salt. On heating it decomposes re-versibly with the formation of the oxideand carbon dioxide:

BaCO3 ˆ BaO + CO2The compound is highly toxic and is usedas a rat poison.

barium chloride (BaCl2) A white solidthat can be prepared by dissolving bariumcarbonate in hydrochloric acid and crystal-lizing out the dihydrate (BaCl2.2H2O).Barium chloride is used as the electrolyte inthe extraction of barium, as a rat poison,and in the leather industry. In the labora-

tory barium chloride solution is used as atest for sulfates, with which it gives a whiteprecipitate.

barium hydrogencarbonate (barium bi-carbonate; Ba(HCO3)2) A compoundthat occurs only in aqueous solution. It isformed by the action of cold water con-taining carbon dioxide on barium carbon-ate.

BaCO3 + CO2 + H2O ˆ Ba(HCO3)2

barium hydroxide (baryta; Ba(OH)2) A white solid usually obtained as the oc-tahydrate, Ba(OH)2.8H2O. It can be madeby adding water to barium oxide. Bariumhydroxide is the most soluble of the group2 hydroxides. It is used in volumetricanalysis for the estimation of weak acidsusing phenolphthalein as an indicator.

barium oxide (BaO) A white powderprepared by heating barium in oxygen orby heating barium carbonate. It has beenused in the manufacture of additives for lu-bricating oils, and in pigments.

barium peroxide (BaO2) A dense off-white powder that can be prepared by care-fully heating barium oxide in oxygen. Thereaction, which is reversible, was the basisof the now obsolete BRIN PROCESS for ob-taining oxygen. Barium peroxide is usedfor bleaching straw and silk and is alsoused in the laboratory preparation of hy-drogen peroxide.

barium sulfate (BaSO4) A white solidthat occurs naturally as the mineral barites,also known as heavy spar. Barium sulfate isvery insoluble in water and can be pre-pared easily as a precipitate by adding sul-furic acid to barium chloride. It is animportant industrial chemical. Under thename of blanc fixe it is used as a pigmentextender in surface coating compositions.It is also used in the glass and rubber in-dustries. Because barium compounds areopaque to x-rays barium sulfate is used inmedicine for taking radiographs of the di-gestive system.

Bartlett, Neil (1932– ) British–

barium bicarbonate

22

American chemist. Bartlett was studyingmetal fluorides and found that the com-pound platinum hexafluoride (PtF6) has anextremely high electron affinity, and reactswith molecular oxygen to form the novelcompound O2

+PtF6–. This was the first ex-

ample of a compound containing the oxy-gen cation. At the time it was anunquestioned assumption of chemistry thatthe rare gases – helium, neon, argon, kryp-ton, and xenon – were completely inert.Bartlett knew that the ionization potentialof xenon was not too much greater thanthe ionization potential of the oxygen mol-ecule and was able to produce xenon hexa-fluoroplatinate (XePtF6) by direct reaction– the first compound of a rare gas. Oncethe first compound had been detectedxenon was soon shown to form other com-pounds, such as xenon fluoride (XeF4) andoxyfluoride (XeOF4). Krypton and radonwere also found to form compounds al-though the lighter rare gases have so far re-mained inactive.

baryta See barium hydroxide.

basalt A dark-colored basic igneousrock derived from solidified volcanic lava.It consists mainly of fine crystals of pyrox-ine and plagioclase feldspar.

base In the Arrhenius theory, a com-pound that releases hydroxide ions, OH–,in aqueous solution. Basic solutions have apH greater than 7. In the Lowry-Brønstedtheory of ACIDS and bases a base is a sub-stance that tends to accept a proton. ThusOH– is basic because it accepts H+ to formwater, but H2O is also a base (althoughsomewhat weaker) because it can accept afurther proton to form H3O+. In this treat-ment the ions of classical mineral acidssuch as SO4

2– and NO3– are weak conju-

gate bases of their respective conjugateacids; i.e. H2SO4 and HNO3, respectively.Ammonia is also a base. In solution it do-nates an electron pair to a proton to forman ammonium ion:

NH3 + H2O ˆ NH4+ + OH–

Here the ammonium ion is the conjugateacid. Other nitrogenous compounds, suchas trimethylamine ((CH3)3N) and pyridine,

are examples of organic bases. See alsoacid; Lewis acid.

base-catalyzed reaction A reactioncatalyzed by bases; i.e. by hydroxide ions.

base metal A common metal such asiron, lead, or copper, as distinguished froma precious metal such as gold, silver, orplatinum.

base unit A unit within a system ofmeasurement from which other units maybe derived by combining it with one ormore other base units. For example thejoule, a derived unit, can be defined interms of base units as one metre (m)squared, kilogram (kg) per second (s)squared, or m2kgs–2. With the current ex-ception of the kilogram, base SI UNITS aredefined in terms of reproducible physicalphenomena.

basic Behaving as a base. Any solutionin which the concentration of OH– ions isgreater than that in pure water at the sametemperature is described as basic; i.e. thepH of a basic solution is greater than 7.

basic aluminum ethanoate See alu-minum ethanoate.

basic oxide An oxide of a metal that re-acts with water to form a base, or with anacid to form a salt and water. Calciumoxide is a typical example. It reacts withwater to form calcium hydroxide:

CaO + H2O → Ca(OH)2and with hydrochloric acid to produce cal-cium chloride and water:

CaO + 2HCl → CaCl2 + H2OCompare acidic oxide.

basic oxygen process See Bessemerprocess.

basic salt A compound intermediate be-tween a normal salt and a hydroxide oroxide. The term is often restricted tohydroxyhalides (such as Pb(OH)Cl,Mg2(OH)3Cl2.4H2O, and Zn(OH)F) andhydroxy-oxy salts (for example, 2PbCO3.-Pb(OH)2, Cu(OH)2.CuCl2, Cu(OH)2.-

23

basic salt

CuCO3). The hydroxyhalides are essen-tially closely packed assemblies of OH–

with metal or halide ions in the octahedralholes. The hydroxy-oxy salts usually havemore complex structures than those im-plied by the formulae.

basic slag See slag.

battery A number of electric cells work-ing together. Many dry ‘batteries’ used inradios, flashlights, etc., are in fact singlecells. If a number of identical cells are con-nected in series, the total e.m.f. of the bat-tery is the sum of the e.m.f.s of theindividual cells. If the cells are in parallel,the e.m.f. of the battery is the same as thatof one cell, but the current drawn fromeach is less (the total current is split amongthe cells).

bauxite A mineral hydrated form of alu-minum hydroxide; the principal ore of alu-minum.

b.c.c. See body-centered cubic crystal.

Beckmann thermometer A type ofmercury thermometer designed to measuresmall differences in temperature ratherthan scale degrees. Beckmann thermome-ters have a larger bulb than common ther-mometers and a stem with a small internaldiameter, so that a range of 5°C coversabout 30 centimeters in the stem. The mer-cury bulb is connected to the stem in sucha way that the bulk of the mercury can beseparated from the stem once a particular5° range has been attained. The thermome-ter can thus be set for any particular range.It is named for the German chemist ErnstBeckmann (1853–1923).

becquerel Symbol: Bq The SI unit of ac-tivity equal to the activity of a radioactivesubstance that has one spontaneous nu-clear change per second; 1 Bq = 1 s–1. It isnamed for the discoverer of radioactivity,the French physicist Antoine Henri Be-querel (1852–1908).

Beer–Lambert law A law that relatesthe intensity of electromagnetic radiation

passing through a material to the length ofthe path of the radiation through the ma-terial. It has the form log I/0I = E c l, whereI0 is the incident intensity of light, I is theintensity of light after it has passed througha sample of the material of length l, c is theconcentration of absorbing species in thematerial and E is a constant known as theabsorption coefficient. The law is namedfor the German mathematician JohannHeinrich Lambert (1728–77) and the Ger-man astronomer Wilhelm Beer (1797–1850). The physical significance of theBeer–Lambert law is that the intensity ofelectromagnetic radiation passing througha sample of material decreases exponen-tially with the thickness and concentrationof the sample (for a given wavelength ofelectromagnetic radiation). If deviationsfrom the Beer–Lambert law occur this isdue to such phenomena as dissociation orthe formation of complexes.

Belousov–Zhabotinskii reaction (B–Zreaction) An oscillating chemical reactionin which there are periodic oscillations inthe color of a mixture of sulfuric acid,potassium bromate, cerium (or iron) sul-fate, and propanedioic acid. The period ofoscillation is about one minute. The colorchanges are caused by repeated oxidationsand reductions of cerium (or iron) ions.The reaction was first observed by theRussian chemist B. P. Belousov in the caseof cerium and modified to iron by A. M.Zhabotinskii in 1963. The mechanism ofthe B–Z reaction is highly complicated andinvolves a large number of individual steps.

bench dilute acid See dilute.

beneficiation The process of separatingan ore into a useful component and wastematerial (known as gangue). The process issometimes described as ore dressing.

bent bond See banana bond.

bentonite A type of clay that is used asan absorbent in making paper and as a pre-cipitating agent to remove proteins frombeer and wine. The gelatinous suspensionit forms with water is also used to bind to-

basic slag

24

gether the sand used for making castings.Chemically bentonite is an aluminosilicateof variable composition.

bent sandwich compound See sand-wich compound.

Bergius, Friedrich Karl Rudolph(1884–1949) German industrial chemist.Bergius worked with Hermann Nernst atBerlin and Fritz Haber at Karlsruhe, wherehe became interested in high-pressurechemical reactions. He is noted for his de-velopment of the BERGIUS PROCESS. Heshared the Nobel Prize for chemistry withCarl Bosch in 1931.

Bergius process A process formerlyused for making hydrocarbon fuels fromcoal. A powdered mixture of coal, heavyoil, and a catalyst was heated with hydro-gen at high pressure. The process was usedby Germany as a source of motor fuel inWorld War II. It was developed byFriedrich Bergius in 1912.

berkelium A silvery radioactivetransuranic element of the actinoid seriesof metals, not found naturally on Earth.The first samples were prepared by bom-barding 291Am with alpha particles. Sev-eral radioisotopes have since beensynthesized. The metal reacts with oxygen,steam, and acids. It tends to collect in boneand thus may one day find use in medicalresearch.

Symbol: Bk; m.p. 1050°C; b.p. un-known; r.d. 14.79 (20°C); p.n. 97; moststable isotope 247Bk (half-life 1400 years).

beryl A mineral, 3BeO.Al2O3.6SiO2,used as a source of beryllium. Crystals arehexagonal. There are several color varietiesincluding the gemstones emerald, which iscolored green by traces of chromiumoxide, and aquamarine, which has a blue-green color.

beryllate See beryllium; beryllium hy-droxide.

beryllium A light metallic element, sim-ilar to aluminum but somewhat harder; the

first element in group 2 (formerly IIA) ofthe periodic table. It has the electronic con-figuration of helium with two additionalouter 2s electrons.

Beryllium occurs in a number of miner-als such as beryllonite (NaBePO4),chrysoberyl (Be(AlO2)2), bertrandite(4BeO.2SiO2), and beryl (3BeO.Al2O3.-6SiO2). The element accounts for only0.0006% by mass of the Earth’s crust. Themetal is obtained by conversion of the oreto the sulfate at high temperature and pres-sure with concentrated sulfuric acid, thento the chloride, followed by electrolysis ofthe fused chloride. Alternatively, it is possi-ble to treat the ore with hydrogen fluoridefollowed by electrolysis of the fused fluo-ride. The metal has a much lower generalreactivity than other elements in group 2. Itis used as an antioxidant and hardener incopper, steel, bronze, and nickel alloys.

Beryllium has the highest ionization po-tential of group 2 and the smallest atomicradius. Consequently it is less electroposi-tive and more polarizing than other mem-bers of the group. Thus, Be2+ ions do notexist as such in either solids or solutions,and there is partial covalent character inthe bonds, even with the most electronega-tive elements. The metal reacts directlywith oxygen, nitrogen, sulfur, and thehalogens at various elevated temperatures,to form the oxide BeO, nitride Be3N2, sul-fide BeS, and halides BeX2, all of which arecovalent. Beryllium does not react directlywith hydrogen but a polymeric hydride(BeH2)n can be prepared by reduction of(CH3)2Be using lithium tetrahydroalumi-nate(III) (LiAlH4).

Beryllium is amphoteric forming beryl-late species, such as [Be(OH)4]2– and[Be(OH)]3

3+. The hydroxide is only weaklybasic. The element does not form a truecarbonate; the basic beryllium carbonate,BeCO3.Be(OH)2 is formed when sodiumcarbonate is added to solutions of beryl-lium compounds.

Beryllium hydride, chloride, and di-methylberyllium form polymeric bridgedspecies but, whereas the bridging in thechloride is via an electron pair on chlorineatoms and can be regarded as an electron-pair donor bond, the bonding in the hy-

25

beryllium

dride and in the methyl compound involvestwo-electron three-center bonds similar tothose found in the BORON HYDRIDES. COM-PLEXES are quite common with beryllium;some examples include [BeCl4]2–, (R2O)2-BeCl2, and [Be(NH3)4]Cl2. Beryllium andits compounds are very toxic and maycause serious respiratory diseases if in-haled.

Symbol: Be; m.p. 1278±5°C; b.p.2970°C (660 kPa); r.d. 1.85 (20°C); p.n. 4;r.a.m. 9.012182.

beryllium bicarbonate See berylliumhydrogencarbonate.

beryllium bronze See bronze.

beryllium carbonate (BeCO3) An un-stable solid prepared by prolonged treat-ment of a suspension of berylliumhydroxide with carbon dioxide. The result-ing solution is evaporated and filtered in anatmosphere of carbon dioxide.

beryllium chloride (BeCl2) A whitesolid obtained by passing chlorine over aheated mixture of beryllium oxide and car-bon. The compound is not ionic: it is apoor conductor in the fused state and in thesolid form consists of a covalent polymericstructure. It is readily soluble in organicsolvents but is hydrolyzed in the presenceof water to give the hydroxide. The anhy-drous salt is used as a catalyst.

beryllium hydrogencarbonate (beryl-lium bicarbonate; Be(HCO3)2) A com-pound formed in solution by the action ofcarbon dioxide on a suspension of the car-bonate, to which it reverts on heating:

BeCO3 + CO2 + H2O ˆ Be(HCO3)2

beryllium hydroxide (Be(OH)2) Awhite solid that can be precipitated fromberyllium-containing solutions by an al-kali. Beryllium hydroxide, like aluminumhydroxide, is amphoteric. In an excess ofalkali, beryllium hydroxide dissolves togive a beryllate, Be(OH)4

2–.

beryllium oxide (BeO) A white solidformed by heating beryllium in oxygen or

by the thermal decomposition of berylliumhydroxide or carbonate. Beryllium oxide isinsoluble in water but it shows basic prop-erties by dissolving in acids to form beryl-lium salts:

BeO + 2H+ → Be2+ + H2OHowever, beryllium oxide also resem-

bles acidic oxides by reacting with alkalisto form beryllates:

BeO + 2OH– + H2O → Be(OH)42–

It is thus an amphoteric oxide. Beryl-lium oxide is used in the production of re-fractory materials, high-output transistors,and printed circuits. Its chemical propertiesof beryllium oxide are similar to those ofaluminum oxide.

beryllium sulfate (BeSO4) An insolublesalt prepared by the reaction of berylliumoxide with concentrated sulfuric acid. Onheating, it breaks down to give the oxide.

Berzelius, Jöns Jacob (1779–1848)Swedish chemist who had a major influ-ence on the development of modern chem-istry. He introduced the symbols now usedto denote chemical elements and coined anumber of words still used in chemistry,such as ‘catalysis’, ‘halogen’, ‘isomerism’,and ‘protein’. He also determined relativeatomic masses (atomic weights) accurately,thereby helping establish Dalton’s atomictheory. Berzelius was also involved in thediscovery of several elements: cerium, sili-con, selenium, and thorium, and the intro-duction of some experimental apparatus,such as rubber tubes and filter paper, intochemistry.

Bessemer process A process for mak-ing steel from PIG IRON. A vertical cylindri-cal steel vessel (converter) is used, linedwith a refractory material. Air is blownover and through the molten iron to oxi-dize and thereby remove impurities such ascarbon, silicon, sulfur, and phosphorus byconverting them to slag. For instance, ironscontaining large amounts of phosphorusare treated in a converter lined with a basicmaterial, so that a phosphate slag isformed. The required amount of carbon isthen added to the iron to produce the de-sired type of steel. The process was in-

beryllium bicarbonate

26

vented in 1856 by the British engineer, SirHenry Bessemer (1813–98). In the basicoxygen process, a modern version of theBessemer process, the air is replaced by amixture of oxygen and steam in order tominimize the amount of nitrogen that is ab-sorbed by the steel.

beta particle An electron or positronemitted by a radioactive substance as it de-cays.

Bethe, Hans Albrecht (1906– ) Ger-man physicist who initiated the subject ofcrystal field theory in 1929 when heanalysed the splitting of atomic energy lev-els by surrounding ligands in terms ofgroup theory. This work led to an under-standing of the optical, magnetic, and spec-troscopic properties of transition-metaland rare-earth complexes. Bethe’s main in-terest was nuclear physics. In particular, heexplained the energy of stars in terms ofnuclear reactions that fuse hydrogen intohelium. Bethe won the 1967 Nobel Prizefor physics for this work.

bi- Prefix used formerly in naming acidsalts. The prefix indicates the presence ofhydrogen; for instance, sodium bisulfate(NaHSO4) is sodium hydrogensulfate, etc.In organic chemistry it is sometimes usedwith the meaning ‘two’; e.g. biphenyl.

bicarbonate See hydrogencarbonate.

bimolecular Describing a reaction orstep that involves two molecules, ions, etc.A common example of this type of reactionis the decomposition of hydrogen iodide:

2HI → H2 + I2This takes place between two moleculesand is therefore a bimolecular reaction.Other common examples are:

H2O2 + I2 → OH– + HIOOH– + H+ → H2O.

binary compound A chemical com-pound formed from two elements; e.g.Fe2O3 or NaCl.

biochemistry The study of chemical

compounds and reactions occurring in liv-ing organisms.

bioinorganic chemistry The study ofbiological molecules that contain metalatoms or ions. Many enzymes have activemetal atoms and bioinorganic compoundsare important in other roles such as proteinfolding, oxygen transport, and electrontransfer. Two important examples ofbioinorganic compounds are hemoglobin(containing iron) and chlorophyll (contain-ing magnesium).

bipy Abbreviation for the bidentate lig-and 2,2′-bipyridine.

bipyridine See bipy.

Birkeland–Eyde process An industrialprocess for fixing nitrogen (as nitrogenmonoxide) by passing air through an elec-tric arc:

N2 + O2 → 2NOThe process was invented by two Norwe-gian chemists, Kristian Birkeland 1867–1913) and Samuel Eyde (1866–1940), whointroduced it in 1903 to exploit the cheapsources of hydroelectricity then availablein Scandinavia. See also nitrogen fixation.

bismuth A brittle pinkish metallic el-ement belonging to group 15 (formerlyVA) of the periodic table. It occurs nativeand in the ores bismuthinite (Bi2S3) andbismite (Bi2O3). The element does not reactwith oxygen or water under normal tem-peratures. It can be dissolved by concen-trated nitric acid. It is the most diamagneticof the metals and has less thermal conduc-tivity than any metal except mercury. Bis-muth is widely used in alloys, especiallylow-melting alloys, which find use in heat-activated sprinkler systems. The element

27

bismuth

N N

Bipyridine: 2,2′-bipyridine

has the unusual property of expandingwhen it solidifies, making it a desirablecomponent of alloys used to make detailedcastings. Compounds of bismuth are usedin cosmetics and medicines. It is also usedas a catalyst in the textile industry.

Symbol: Bi; m.p. 271.35°C; b.p.1560±5°C; r.d. 9.747 (20°C); p.n. 83;r.a.m. 208.980 37.

bismuth(III) carbonate dioxide (bis-muthyl carbonate; Bi2O2CO3) A whitesolid prepared by mixing solutions of bis-muth nitrate and ammonium carbonate. Itcontains the (BiO)+ ion (sometimes knownas the bismuthyl ion).

bismuth(III) chloride (bismuth tri-chloride; bismuth(III) chloride oxide;BiCl3) A white deliquescent solid. It canbe prepared by direct combination of bis-muth and chlorine. Bismuth(III) chloridedissolves in excess dilute hydrochloric acidto form a clear liquid, but if diluted it pro-duces a white precipitate of bismuth(III)chloride oxide (bismuthyl chloride,BiOCl):

BiCl3 + H2O ˆ BiOCl + 2HClThis reaction is often discussed in

chemistry as a typical example of a re-versible reaction. It is also a confirmatorytest for bismuth in analysis. Bismuth(V)chloride does not exist.

bismuth(III) chloride oxide See bis-muth(III) chloride.

bismuth(III) nitrate oxide (bismuthyl ni-trate; bismuth subnitrate; BiONO3) Awhite insoluble solid precipitated whenbismuth(III) nitrate is diluted and containsthe (BiO)+ ion. Bismuth(III) nitrate oxide isused in pharmaceutical preparations.

bismuth subnitrate See bismuth(III) ni-trate oxide.

bismuth trichloride See bismuth(III)chloride.

bismuthyl carbonate See bismuth(III)carbonate dioxide.

bismuthyl chloride See bismuth(III)chloride.

bismuthyl compound A compoundcontaining the (BiO)+ ion or BiO grouping.

bismuthyl ion See bismuth(III) carbon-ate dioxide.

bismuthyl nitrate See bismuth(III) ni-trate oxide.

bistability The ability of a chemicallyreacting system to occur in two steadystates. For an OSCILLATING REACTION tooccur it is necessary for there to be bista-bility. The two steady states are not statesof thermodynamic equilibrium There is asudden jump from one of the bistable statesto the other when a certain critical concen-tration of one of the reactants occurs. Seealso oscillating reaction.

bisulfate See hydrogensulfate.

bisulfite See hydrogensulfite.

bittern The liquid that is left aftersodium chloride has been crystallized fromsea water. It is a source of iodine, bromine,and some magnesium salts.

bitumen A mixture of solid or semisolidhydrocarbons obtained naturally or fromcoal, oil, etc.

bituminous coal A type of second-grade COAL, containing more than 65%carbon but also quantities of gas, coal tar,and water. It is the most abundant type ofcoal. Domestic use of coal is practicallynonexistent in the U.S.A. and Canadatoday.

bivalent See divalent.

Black, Joseph (1728–99) Scottishchemist and physicist, one of the first to usequantitative methods in developing mod-ern chemistry. He discovered carbon diox-ide (which he called ‘fixed air’) when heinvestigated the chemical reactions of lime-stone in a quantitative way, reporting the

bismuth(III) carbonate dioxide

28

work in 1756. The idea of latent heat oc-curred to him in 1757 and in subsequentyears he determined the latent heat in theformation of ice and steam experimentally.He was careful to distinguish between theconcepts of heat and temperature, a dis-tinction that led him to the concept of spe-cific heat.

blackdamp See firedamp.

blanc fixe See barium sulfate.

blast furnace A tall furnace for SMELT-ING iron from various iron ores, especiallyhematite (Fe2)3) and magnetite (Fe3O4). Amixture of the ore with coke and a flux isfed into the top of the furnace and heatedby pre-heated air blown into the bottom ofthe furnace, where temperatures are muchhigher. The flux is often calcium oxide(from limestone). As it descends the fur-nace the ore is first reduced to iron(II)oxide (FeO) and then to molten iron, bothby the action of carbon monoxide (CO)obtained by burning coke in air. MoltenPIG IRON is then run off from the bottom ofthe furnace.

bleach Any substance used to removecolor from materials such as cloth andpaper. All bleaches are oxidizing agents,and include chlorine, sodium chlorate(I)solution (NaClO, sodium hypochlorite),hydrogen peroxide, and sulfur(IV) oxide(SO2, sulfur dioxide). Sunlight also has ableaching effect.

bleaching powder A white solid thatcan be regarded as a mixture of calciumchlorate(I) (calcium hypochlorite(Ca(ClO)2)), calcium chloride, and calciumhydroxide. It is prepared commercially bypassing chlorine through a tilted cylinderdown which is passed calcium hydroxide.Bleaching powder has been used forbleaching paper pulp and fabrics and forsterilizing water. Its activity arises from theformation, in the presence of air containingcarbon dioxide, of the oxidizing agentchloric(I) acid (hypochlorous acid, HClO):

Ca(ClO)2.Ca(OH)2.CaCl2 + 2CO2 →2CaCO3 + CaCl2 + 2HClO

blende A sulfide ore of a metal; e.g. zincblende (ZnS2).

Bloch, Felix (1905–83) Swiss-bornAmerican physicist who invented the tech-nique of nuclear magnetic resonance(NMR) in 1946 (as did Edward Purcell, in-dependently). This led to the extensive useof NMR in investigating the structue ofcomplex molecules. Bloch and Purcellshared the 1952 Nobel Prize for physics fortheir work in this field. Bloch also workedon the quantum theory of solids.

blue vitriol Copper(II) sulfate pentahy-drate (CuSO4.5H2O).

body-centered cubic crystal (b.c.c.) Acrystal structure in which the unit cell hasan atom, ion, or molecule at each corner ofa cube and also at the center of the cube. Inthis type of structure the coordinationnumber is 8. It is less close-packed than theface-centered cubic structure. The alkalimetals form crystals with body-centeredcubic structures. See also cubic crystal.

Bohr, Niels Hendrik David (1885–1962) Danish physicist. Niels Bohr was re-sponsible for a key development in our un-derstanding of atomic structure when heshowed (1913) how the structure of theatom could be explained by imposing‘quantum conditions’ on the orbits of elec-trons, thus allowing only certain orbits.This theory accounted for details of the hy-drogen spectrum. Bohr also contributed tonuclear physics, particularly the theory ofnuclear fission. He was awarded the 1922Nobel Prize for physics for his work onatomic theory.

bohrium A synthetic radioactive el-ement first detected by bombarding a bis-muth target with chromium nuclei. Only asmall number of atoms have ever been pro-duced.

Symbol: Bh; p.n. 107; most stable iso-tope 262Bh (half life 0.1s).

Bohr magneton A unit of magnetic mo-ment that is convenient at the atomic level.It is denoted by µB and given by µB =

29

Bohr magneton

eh/(4πme), where e is the charge of an elec-tron, h is the PLANCK CONSTANT, and me isthe rest mass of an electron. The Bohr mag-neton has a value of 9.274 × 10–24 A m2.

Bohr theory A theory introduced byNiels Bohr (1911) to explain the spectrumof atomic hydrogen. The model he usedwas that of a nucleus with charge +e, or-bited by an electron with charge –e, mov-ing in a circle of radius r. If v is the velocityof the electron, the centripetal force, mv2/ris equal to the force of electrostatic attrac-tion, e2/4πε0r2. Using this, it can be shownthat the total energy of the electron (kineticand potential) is –e2/8πε0r.

If the electron is considered to havewave properties, there must be a wholenumber of wavelengths around the orbit,otherwise the wave would be a progressivewave. For this to occur

nλ = 2πrwhere n is an integer, 1, 2, 3, 4, … Thewavelength, λ, is h/mv, where h is thePlanck constant and mv the momentum.Thus for a given orbit:

nh/2π = mvrThis means that orbits are possible only

when the angular momentum (mvr) is anintegral number of units of h/2π. Angularmomentum is thus quantized. In fact, Bohrin his theory did not use the wave behaviorof the electron to derive this relationship.He assumed from the beginning that angu-lar momentum was quantized in this way.Using the above expressions it can beshown that the electron energy is given by:

E = –me4/8ε02n2 h2

Different values of n (1, 2, 3, etc.) cor-respond to different orbits with differentenergies; n is the principal quantum num-ber. In making a transition from an orbit n1to another orbit n2 the energy difference∆W is given by:

∆W = W1 – W2= me4(1/n2

2 – 1/n12)/8ε0

2h 2

This is equal to hv where v is the fre-quency of radiation emitted or absorbed.Since vλ = c, then

1/λ = me4(1/n12 – 1/n2

2)/8ε02ch3

The theory is in good agreement with ex-periment in predicting the wavelengths oflines in the hydrogen spectrum, although it

is less successful for larger atoms. Differentvalues of n1 and n2 correspond to differentspectral series, with lines given by the ex-pression:

1/λ = R(1/n12 – 1/n2

2)R is the Rydberg constant. Its experi-

mental value is 1.09678 × 107 m–1. Thevalue from Bohr theory (me4/8πε1

2ch2) is1.097 00 × 107 m–1. See also atom.

boiling The process by which a liquid isconverted into a gas or vapor by heating atits boiling point; i.e. the temperature atwhich the vapor pressure of a liquid isequal to atmospheric pressure. This tem-perature is always the same for a particularpure liquid at a given pressure (for refer-ence purposes usually taken as standardpressure).

Boltzmann constant Symbol: k Theconstant 1.38054 J K–1, equal to the gasconstant (R) divided by the Avogadro con-stant (NA). It is named for the Austrianphysicist Ludwig Edward Boltzmann(1844–1906).

Boltzmann formula A fundamental re-sult in statistical mechanics stating that theentropy S of a system is related to the num-ber W of distinguishable ways in which thesystem can be realised by the equation: S =k lnW, where k is the Boltzmann constant.This formula is a quantitative expressionof the idea that the entropy of a system is ameasure of its disorder. It was discoveredby the Austrian physicist Ludwig Boltz-mann in the late 19th century in the courseof his investigations into the foundationsof statistical mechanics.

bomb calorimeter A sealed insulatedcontainer, used for measuring energy re-leased during combustion of substances(e.g. foods and fuels). A known amount ofthe substance is ignited inside the calorime-ter in an atmosphere of pure oxygen, andundergoes complete combustion at con-stant volume. The resultant rise in temper-ature is related to the energy released bythe reaction. Such energy values (calorificvalues) are often quoted in joules per kilo-gram (J kg–1).

Bohr theory

30

bond See chemical bond.

bond dissociation energy See bond en-ergy.

bond energy The energy involved informing a chemical bond. For ammonia,for instance, the energy of the N–H bond isone third of the energy involved for theprocess

NH3 → N + 3HIt is thus one third of the heat of atomiza-tion. This is also known as the mean bondenergy.

The bond dissociation energy is meas-ured in the opposite direction. It is the en-ergy required to break a particular bond ina compound, e.g.:

NH3 → NH2 + HMore formally, it is common to specify thebond enthalpy.

bond enthalpy See bond energy.

bonding orbital See orbital.

bond length The length of a chemicalbond, i.e. the distance between the centersof the nuclei of two atoms joined by achemical bond. Bond lengths may be meas-ured by electron or x-ray diffraction.

boracic acid See boric acid.

borane See boron hydride.

borate See boron.

borax See disodium tetraborate decahy-drate.

borax-bead test A preliminary test inqualitative inorganic analysis that can be aguide to the presence of certain metals. Abead is formed by heating a little disodiumtetraborate decahydrate (borax) on a loopin a platinum wire. A minute sample of thecompound to be tested is introduced intothe bead and the color observed in both theoxidizing and reducing areas of a Bunsen-burner flame. The color is also noted whenthe bead is cold.

boric acid (boracic acid; orthoboric acid;trioxoboric(III) acid; H3BO3) A whitecrystalline solid soluble in water; in solu-tion it is a very weak acid. Boric acid isused as a mild antiseptic eye lotion and wasformerly used as a food preservative. It isused in glazes for enameled objects and is aconstituent of borosilicate glass.

Trioxoboric(III) acid is the full system-atic name for the solid acid and its dilutesolutions; in more concentrated solutionspolymerization occurs to give polydioxo-boric(III) acid.

boric anhydride See boron oxide.

boric(III) oxide See boron oxide.

boride A compound of boron, especiallyone with a more electropositive element.Borides have a wide range of stoichiome-tries, from M4B through to MB6, and canexist in close-packed arrays, chains, andtwo-dimensional nets. Due to their highmelting points and unreactivity withnonoxidizing acids, metal borides are usedin refractory materials. Borides are alsogood abrasives.

31

boride

BORAX-BEAD COLORS (H = hot; C = cold)

Metal Oxidizing flame Reducing flamechromium green H+C green H+Ccobalt blue H+C blue H+Ccopper green H, blue C often opaqueiron brown-red H, yellow C green H+Cmanganese violet H+C colourless H+Cnickel red–brown C gray–black C

Born, Max (1882–1970) Germanphysicist who was one of the founders ofquantum mechanics in the 1920s. In par-ticular, he put forward the ‘Born interpre-tation’ for the wave-function of an electronin terms of probability in 1926. Born alsomade major contributions to the theory ofcrystals and to the quantum theory of mol-ecules. He was awarded a share of the1954 Nobel Prize in physics (together withWalther Bothe) for his work on quantummechanics.

Born–Haber cycle A cycle used in cal-culating the lattice energies of solids. Thesteps involved are:Atomization of sodium:

Na(s) → Na(g) ∆H1Ionization of sodium:

Na(g) → Na+(g) ∆H2Atomization of chlorine:

Cl2(g) → 2Cl(g) ∆H3Ionization of chlorine:

Cl(g) + e– → Cl–(g) ∆H4Formation of solid:

Na+(g) + Cl–(g) → NaCl(s) ∆H5This last step involves the lattice energy

∆H5. The sum of all these enthalpies isequal to the heat of the reaction:

Na(s) + ½Cl2(g) → NaCl(s)See also Hess’s law.

Born–Oppenheimer approximationAn approximation used in the quantumtheory of molecules in which it is assumed

that the motion of atomic nuclei is verymuch slower than the motion of electronsand that it is a good approximation to takethe nuclei to be in fixed positions when dis-cussing electron transitions. The Germanphysicist Max Born and the Americanphysicist Julius Robert Oppenheimer firstused this approximation in 1927.

borohydride ions See boron hydride.

boron A hard, rather brittle metalloidelement of group 3 (formerly IIIA) of theperiodic table. It exists in two forms: as anamorphous yellow-brown powder and as ablack metallic crystal. It has the electronicstructure 1s22s22p1. Boron is of low abun-dance (0.0003%) but occurs in very con-centrated forms in its natural minerals,which include borax (Na2B4O7.10H2O),ulexite (NaCaB5O9.H2O), howlite(Ca2B5SiO9(OH)5), and colemanite(Ca2B6O11). The element is obtained byconversion to boric acid followed by de-hydration to B2O3 then reduction withmagnesium. High-purity boron for semi-conductor applications is obtained by con-version to BORON TRICHLORIDE, which canbe purified by distillation, then reductionusing hydrogen. Only small quantities ofelemental boron are needed commercially;the vast majority of boron is used in theform of disodium tetraborate decahydrate(borax) or boric acid.

Born, Max

32

B

O

B

B

O

O_

O_

O

B

O

O_

B

O

O_

B3O

6

3_

ion (BO2

)n

n _ ion

B

O

O_

O_

Borate: examples of cyclic and linear borate ions

Because boron has a small atom andhas a relatively high ionization potential itscompounds are predominantly covalent;the ion B3+ does not exist. Boron does notreact directly with hydrogen but the hy-drolysis of magnesium boride does pro-duce a range of BORON HYDRIDES. Thespecies BH3 is only a short-lived reactionintermediate.

Finely divided boron burns in oxygenabove 600°C to give BORON OXIDE, B2O3,an acidic oxide which will dissolve slowlyin water to give boric acid (H3BO3) andrapidly in alkalis to give borates such asNa2B4O7. A number of polymeric specieswith B–B and B–O links are known, e.g. alower oxide (BO)x, and a polymeric acid(HBO2)n. Although the parent acid isweak, many salts containing borate anionsare known but their stoichiometry gives lit-tle indication of their structures, many ofwhich are cyclic or linear polymers. Thesepolymers contain both BO3 planar groupsand BO4 tetrahedra. Boric acid and the bo-rates give a range of glassy substances onheating; these contain cross-linked B–O–Bchains and nets. In a procedure known asthe BORAX-BEAD TEST, these materials in themolten state react with metal ions to formborates, which on cooling give characteris-tic colors to the glass.

Boron reacts with nitrogen on strongheating (1000°C) to give BORON NITRIDE.Elemental boron reacts directly with fluo-rine and chlorine but for practical purposes

the halides are obtained via the BF3 routefrom boron oxide:

B2O3 + 3CaF2 + 3H2SO4 → 3CaSO4 +3H2O + 2BF3

BF3 + AlCl3 → AlF3 + BCl3The halides are all covalent molecules,

which are all planar and trigonal in shape.Boron halides are industrially important ascatalysts or promoters in a variety of or-ganic reactions. The decomposition ofboron halides in atmospheres of hydrogenat elevated temperatures is also used to de-posit traces of pure boron in semiconduc-tors.

Boron forms a range of compoundswith elements that are less electronegativethan itself, called BORIDES. Natural boronconsists of two isotopes, 10B (18.83%),used in steel alloys for control rods in nu-clear reactors, and 11B (81.17%). Thesepercentages are sufficiently high for theirdetection by splitting of infrared absorp-tion or by nuclear magnetic resonancespectroscopy.

Both borax and boric acid are used asmild antiseptics and are not generally re-garded as toxic; boron hydrides are, how-ever, highly toxic.

Symbol: B; m.p. 2300°C; b.p. 2658°C;r.d. 2.34 (20°C); p.n. 5; r.a.m. 10.811.

boron hydride (borane) Any of a num-ber of binary compounds of boron and hy-drogen. A mixture of boron hydrides canbe obtained by the action of acid on mag-nesium boride (MgB2); others can be madeby controlled PYROLYSIS of diborane(B2H6), which is the simplest member ofthis class of compounds. Many boraneshave formulae of the type BnHn + 4 andBnHn + 6, where n is an integer. Examplesare B5H9, B6H10, B4H10, and B5H11. Typi-cally, the boron hydrides are volatile,

33

boron hydride

OH

B

O

B OH

O

B

OH

O

BHO

O

O

The ion [B4O5(OH)4]2– present in borax

B

H

HH

B

H

H

H

Boron hydride: the bonding in diborane

highly reactive compounds that oxidize inair, some explosively.

The boron hydrides are examples ofELECTRON-DEFICIENT COMPOUNDS, in whichthere are not enough electrons to form clas-sical electron-pair bonds. For example, di-borane (B2H6) has a formular resemblingthat of ethane (C2H6), but its molecularstructure and bonding are quite different.The two boron atoms are not linked di-rectly but rather are joined through twohydrogen bridges. These B-H-B links can-not be explained in terms of two localizedbonds. Instead, the bonding involves theidea of a MULTICENTER BOND in which threeatoms are bound by two electrons. Theseare known as 3c,2e bonds, i.e. having threecenters and two electrons, in contrast toconventional electron-pair bonds, whichare 2c,2e bonds. In more complex boronhydrides it is possible for three boronatoms to be at the vertices of a trianglewith three orbitals overlapping at the cen-ter, the resulting molecular orbital contain-ing two electrons.

In addition to the neutral boron hy-drides there are numbers of negative boro-hydride ions. Typically these haveformulae of the type BnHn

2–, e.g. B6H62–.

There are various types of boron hydrideand borohydride ion. Those with a closedpolyhedron of boron atoms are designatedcloso compounds. Ones in which there isan incomplete polyhedron (by removal ofone vertex) are called nido compounds(from the Greek word for ‘nest’). Thosewith a more open structure (removal oftwo or more vertices) are arachno com-pounds (from the Greek ‘spider’).

boron nitride (BN) A compoundformed by heating boron in nitrogen in thevicinity of 1000°C. The material has an ex-tremely high melting point and is thermallyvery stable but there is sufficient bond po-larity in the B–N links to permit slow hy-drolysis by water to give ammonia. It hastwo crystalline forms. One is a slipperywhite solid with a layer structure similar tothat of graphite, i.e. hexagonal rings of al-ternating B and N atoms. The other is a ‘di-amond-like’ form of B–N that some claimto be as hard or even harder than diamond.

boron oxide (boric anhydride; boric(III)oxide; diboron trioxide; B2O3) A glassyhygroscopic solid that eventually formsboric acid. It has amphoteric properties.

boron tribromide (BBr3) A colorlessliquid. See boron; boron trichloride.

boron trichloride (BCl3) A fuming liq-uid made by passing dry chlorine overheated boron. It is rapidly hydrolysed bywater:

BCl3 + 3H2O → 3HCl + H3BO3As there are only three pairs of shared

electrons in the outer shell of the boronatom, boron halides form very stable addi-tion compounds with ammonia by the ac-ceptance of a lone electron pair in acoordinate bond to complete a sharedoctet. See also boron.

boron trifluoride (BF3) A colorlessfuming gas made by heating a mixture ofboron oxide, calcium fluoride, and concen-trated sulfuric acid. See boron; borontrichloride.

borosilicates Complex compounds sim-ilar to silicates, but containing BO3 andBO4 units in addition to the SiO4 units.Certain crystalline borosilicate mineralssuch as danburite, CaB2Si2O8, are known.In addition, borosilicate glasses can bemade by using boron oxide in addition tosilicon(IV) oxide. These glasses tend to betougher and more heat resistant than stan-dard silicate glass. Pyrex, for example, iscommonly used in laboratory glasswareand in cookware.

Bosch, Carl (1874–1940) German in-dustrial chemist. Bosch joined the largeGerman dyestuffs company, BadischeAnilin und Soda Fabrik (BASF), in 1899.Following Fritz Haber’s successful small-scale ammonia synthesis in 1909, Boschbegan to develop a high-pressure ammoniaplant at Oppau for BASF. The plant wasopened in 1912 – a successful applicationof the HABER PROCESS on a large scale.Bosch also introduced the use of the water-gas shift reaction as a source of hydrogenfor the process:

boron nitride

34

CO + H2O = CO2 + H2After World War I the large-scale am-

monia fertilizer industry was establishedand the high-pressure technique was ex-tended by BASF to the synthesis ofmethanol from carbon monoxide and hy-drogen in 1923. Bosch shared the NobelPrize for chemistry with Friedrich Bergiusin 1931.

Bosch process The reaction betweencarbon monoxide and steam over a hot cat-alyst:

CO + H2O → CO2 + H2It has been used as a source of hydrogen forthe Haber process. The process was devel-oped by the German chemist Carl Bosch(1874–1940).

bowl classifier A device that separatessolid particles in a mixture of solids andliquid into fractions according to particlesize. Feed enters the center of a shallowbowl, which contains revolving blades.The coarse solids collect on the bottom,fine solids at the edge.

Boyle, Robert (1627–91) English scien-tist who is generally regarded as one of thefounders of modern chemistry. He estab-lished the law known as BOYLE’S LAW relat-ing the pressure and volume of gasesexperimentally and gave an explicit state-ment of it in 1662. In 1661 he published abook entitled The Sceptical Chymist inwhich he put forward the idea that all mat-ter is corpuscular in nature, with the cor-puscules having different sizes and shapes.This idea enabled him to distinguish be-tween compounds and mixtures. Boyle wasalso able to distinguish between acids andbases by using vegetable dyes as indicators.

Boyle’s law At a constant temperature,the pressure of a fixed mass of a gas is in-versely proportional to its volume: i.e.

pV = KHere K is a constant whose value dependson the temperature and on the nature ofthe gas. The law holds strictly only forideal gases. Real gases follow Boyle’s lawat low pressures and high temperatures.See gas laws.

Brackett series See hydrogen atomspectrum.

Bragg, Sir William Henry (1862–1942) and Bragg, Sir William Lawrence(1890–1971) British physicists. The Braggs(father and son) realized very soon after thediscovery of x-ray diffraction in 1912 thatthis phenomenon could be used to deter-mine the structure of crystals. LawrenceBragg formulated the Bragg equation in1912. This initiated the subject of x-raycrystallography. One of their early resultswas to show that sodium chloride consistsof sodium ions and chloride ions ratherthan NaCl molecules. Lawrence Bragg sub-sequently studied silicates and encouragedresearch on complex molecules of biologi-cal interest. The Braggs shared the 1915Nobel Prize for physics.

Bragg equation An equation used todeduce the crystal structure of a materialusing data obtained from x-rays directed atits surface. The conditions under which acrystal will reflect a beam of x-rays withmaximum intensity is:

nλ = 2dsinθwhere θ is the angle of incidence and re-flection (Bragg angle) that the x-rays makewith the crystal planes, n is a small integer,λ is the wavelength of the x-rays, and d isthe distance between the crystal planes.The equation was discovered by LawrenceBragg in 1912.

branched chain See chain.

brass Any of a group of copper–zinc al-loys containing up to 50% of zinc. Thecolor of brass changes from red-gold togolden to silvery-white with increasing zinccontent. Brasses are easy to work and resistcorrosion well. Brasses with up to 35%zinc can be worked cold and are speciallysuited for rolling into sheets, drawing intowire, and making into tubes. Brasses with35–46% zinc are harder and stronger butless ductile; they require hot working (e.g.forging). The properties of brass can be im-proved by the addition of other elements;lead improves its ability to be machined,

35

brass

while aluminum and tin increase its corro-sion resistance. See also bronze.

bremsstrahlung See x-radiation.

brine A concentrated solution of sodiumchloride or calcium chloride. The term isalso applied to naturally occurring solu-tions of other salts.

Brin process An obsolete process forproducing oxygen by first heating bariumoxide in air to produce the peroxide(BaO2); then heating the peroxide at highertemperatures to release oxygen.

bromic(I) acid (hypobromous acid;HBrO) A pale yellow liquid made by re-acting mercury(II) oxide with BROMINE

WATER. Bromic(I) acid is a weak acid inaqueous solution but is a good oxidizingagent with strong bleaching power.

bromic(V) acid (HBrO3) A colorlessliquid made by the addition of dilute sulfu-ric acid to barium bromate. It is a strongacid in aqueous solution.

bromide See halide.

bromination See halogenation.

bromine A deep red, moderately reac-tive element belonging to the halogens; i.e.group 17 (formerly VIIA) of the periodictable. Bromine is a volatile liquid at roomtemperature (mercury being the only otherelement with this property). It occurs insmall amounts in seawater, salt lakes, andsalt deposits but is much less abundantthan chlorine. Bromine reacts with mostmetals but generally with less vigor thanchlorine. It has a lower oxidizing powerthan chlorine and consequently can be re-leased from solutions of bromides by reac-tion with chlorine gas. The laboratorymethod is the more convenient oxidationof bromides using manganese dioxide. In-dustrially most bromine is produced bydisplacement with chlorine from naturallyoccurring brines. Bromine and its com-pounds are used in pharmaceuticals, pho-tography, chemical synthesis, and

fumigants. Bromine compounds are alsoused as flame retardants and refrigerants.

The electropositive elements form elec-trovalent bromides and the nonmetalsform fully covalent bromides. Like chlo-rine, bromine forms oxides, Br2O andBrO2, both of which are unstable. The re-lated oxo-acid anions hypobromite (BrO–)and bromate (BrO3

–) are formed by the re-action of bromine with cold aqueous alkaliand hot aqueous alkali respectively, but thebromine analogs of chlorite and perchlo-rate are not known.

Bromine and the interhalogens, chemi-cal compounds formed between two hao-gens, are highly toxic. Liquid bromine andbromine solutions are also very corrosiveand goggles and gloves should always beworn when handling such compounds.

Symbol: Br; m.p. –7.25°C; b.p.58.78°C; r.d. 3.12 (20°C); p.n. 35; r.a.m.79.904.

bromine trifluoride (BrF3) A colorlessfuming liquid made by direct combinationof fluorine and bromine. It is a very reac-tive compound, its reactions being similarto those of its component halogens.

bromine water A yellow solution ofbromine in water, which containsbromic(I) acid (hypobromous acid, HBrO).It is a weak acid and a strong oxidizingagent, which decomposes to give a mixtureof bromide (Br–) and bromate(V) (BrO3

–)ions.

Brønsted acid See acid.

bronze Any of a group of copper-tin al-loys usually containing 0.5–15% of tin.They are generally harder, stronger in com-pression, and more corrosion resistantthan brass. Zinc is often added, as in gun-metal (2–4% zinc), to increase strengthand corrosion-resistance; bronze coinsoften contain more zinc (2.5%) than tin(0.5%). The presence of lead improves itsmachining qualities.

Some copper-rich alloys containing notin are also called bronzes. Aluminumbronzes, for example, with up to 10% alu-minum, are strong, resistant to corrosion

bremsstrahlung

36

and wear, and can be worked cold or hot;silicon bronzes, with 1–5% silicon, havehigh corrosion-resistance; berylliumbronzes, with about 2% beryllium, arevery hard and strong. See also brass.

brown coal See lignite.

brown-ring test A qualitative test usedfor the detection of an ionic nitrate. Afreshly prepared solution of iron(II) sulfateis mixed with the sample and concentratedsulfuric acid is introduced slowly to thebottom of the tube using a droppingpipette, so that two layers are formed. Abrown ring formed at the interface wherethe liquids meet indicates the presence ofnitrate. The brown color is caused by thepresence of [Fe(NO)]SO4, which breaksdown on shaking.

buckminsterfullerene (fullerene) Anallotrope of carbon discovered in soot in1985, that contains clusters of 60 carbonatoms bound in a highly symmetric poly-hedral structure. The C60 polyhedron has acombination of pentagonal and hexagonalfaces similar to the panels on a soccer ball.The molecule was named after the Ameri-can architect Richard Buckminster Fuller(1895–1983) because its structure resem-bles the geodesic dome, which was in-vented by Fuller. The C60 polyhedra areinformally called bucky balls. The originalmethod of making the allotrope was to firea high-power laser at a graphite target.

This procedure also produces less stablecarbon clusters, such as C70. Buckminster-fullerene can be produced more conve-niently using an electric arc betweengraphite electrodes in an inert gas. The al-lotrope is soluble in benzene, from which itcan be crystallized to give yellow crystals.This solid form is known as fullerite.

The discovery of buckminsterfullereneled to a considerable amount of researchinto its properties and compounds. Partic-ular interest has been shown in trappingmetal ions inside the carbon cage to formenclosure compounds. The term fullerenealso applies to derivatives of buckminster-fullerene and to similar clusters (e.g. C70).Carbon structures similar to that in C60 canalso form small tubes, known as buckytubes.

bucky ball See buckminsterfullerene.

bucky tube See buckminsterfullerene.

buffer A solution in which the pH re-mains reasonably constant when acids oralkalis are added to it; i.e. it acts as a bufferagainst (small) changes in pH. Buffer solu-tions generally contain a weak acid andone of its salts derived from a strong base;e.g. a solution of ethanoic acid and sodiumethanoate. If an acid is added, the H+ reactswith the ethanoate ion (from dissociatedsodium ethanoate) to form undissociatedethanoic acid; if a base is added the OH–

reacts with the ethanoic acid to form waterand the ethanoate ion. The effectiveness ofthe buffering action is determined by theconcentrations of the acid–anion pair:

K = [H+][CH3COO–]/[CH3COOH]where K is the dissociation constant.

Phosphate, oxalate, tartrate, borate,and carbonate systems can also be used forbuffer solutions.

bumping Violent boiling of a liquidcaused when bubbles form at a pressureabove atmospheric pressure.

Bunsen, Robert Wilhelm (1811–99)German chemist. Along with Gustav Kir-choff, Bunsen pioneered the use of spec-troscopy to identify chemical elements.

37

Bunsen, Robert Wilhelm

Buckminsterfullerene

This work was started in 1859 and soonled them to the discovery of rubidium andcesium. The Bunsen burner is named forRobert Bunsen, although it was probablydeveloped by Bunsen’s technician PeterDesdega.

Bunsen burner A gas burner consistingof a vertical metal tube with an adjustableair-inlet hole at the bottom. Gas is admit-ted into the bottom of the tube and thegas–air mixture is burnt at the top. Withtoo little air the flame is yellow and sooty.Correctly adjusted, the burner gives aflame with a pale blue inner cone of in-completely burnt gas, and an almost invis-ible outer flame where the gas is fullyoxidized and reaches a temperature ofabout 1500°C. The inner region is the re-ducing part of the flame and the outer re-gion the oxidizing part. The Bunsen burneris named for Robert Bunsen, who helpedmake it popular, although it was probablyinvented by Michael Faraday and im-proved by Bunsen’s technician Peter Des-dega. See also borax-bead test.

Bunsen cell A type of primary cell inwhich the positive electrode is formed bycarbon plates in a nitric acid solution and

the negative electrode consists of zincplates in sulfuric acid solution.

burette A piece of apparatus used forthe addition of variable volumes of liquidin a controlled and measurable way. Theburette is a long cylindrical graduated tubeof uniform bore fitted with a stopcock anda small-bore exit jet, enabling a drop of liq-uid at a time to be added to a reaction ves-sel. Burettes are widely used for titrationsin volumetric analysis. Standard burettespermit volume measurement to 0.005 cm3

and have a total capacity of 50 cm3; a vari-ety of smaller microburette is available.Similar devices are used to introduce meas-ured volumes of gas at regulated pressurein the investigation of gas reactions.

by-product A substance obtained dur-ing the manufacture of a main chemicalproduct. For example, calcium chloride is aby-product of the Solvay process for mak-ing sodium carbonate. Some metals are ob-tained as by-products in processes toextract other metals. Cadmium, for in-stance, is a by-product in the extraction ofzinc.

B–Z reaction See Belousov–Zhabotin-skii reaction.

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38

cadmium A transition metal, an elementin group 12 (formerly IIB) of the periodictable, obtained as a by-product during theextraction of zinc. It is used to protectother metals from corrosion, as a neutronabsorber in nuclear reactors, in alkali bat-teries, and in certain pigments. It is highlytoxic.

Symbol: Cd; m.p. 320.95°C; b.p.765°C; r.d. 8.65 (20°C); p.n. 48; r.a.m.112.411.

cadmium cell See Weston cadmiumcell.

caesium See cesium.

cage compounds See clathrate.

calamine 1. A mineral consisting of hy-drated zinc silicate (2ZnO.SiO2.H2O). It isalso known as hemimorphite.2. Zinc carbonate (ZnCO3), which occursnaturally as a mineral also known as smith-sonite. The carbonate is used medicinallyin suspension as a soothing lotion for sun-burn and skin complaints. Formerly, thenatural mineral was used but now medici-nal calamine is made by precipitating abasic zinc carbonate from a solution of azinc salt. It is usually colored pink by theaddition of small quantities of iron(III)oxide.

calcination The formation of a calciumcarbonate deposit from hard water.

calcite A mineral form of calcium car-bonate occurring in limestone, chalk, andmarble.

calcium A moderately soft, low-meltingreactive metal; the third element in group 2

(formerly IIA) of the periodic table. Theelectronic configuration is that of argonwith an additional pair of 4s electrons.

Calcium is widely distributed in theEarth’s crust and is the third most abun-dant element. Large deposits occur aschalk or marble, CaCO3; gypsum, CaSO4.-2H2O; anhydrite, CaSO4; fluorspar, CaF;and apatite, CaF2.Ca3(PO4)3. Howeversufficiently large quantities of calciumchloride are available as waste from theSolvay process to satisfy industrial require-ments for the metal, which is produced byelectrolysis of the fused salt. Large quanti-ties of lime, Ca(OH)2, and quicklime,CaO, are produced by decomposition ofthe carbonate for use in both building andagriculture. Several calcium minerals aremined as a source of other substances.Thus, limestone is a cheap source of carbondioxide, gypsum and anhydrite are used inthe manufacture of sulfuric acid, rockphosphate is a source of phosphoric acid,and fluorspar for a range of fluorochemi-cals.

Calcium has a low ionization potentialand a relatively large atomic radius. It istherefore a very electropositive element.The metal is very reactive and the com-pounds contain the divalent ion Ca2+. Cal-cium forms the oxide (CaO), a white ionicsolid, on burning in air, but for practicalpurposes the oxide is best prepared byheating the carbonate, which decomposesat about 800°C. Both the oxide and themetal itself react with water to give thebasic hydroxide (Ca(OH)2). On heatingwith nitrogen, sulfur, or the halogens, cal-cium reacts to form the nitride (Ca3N2),sulfide (CaS), or the halides (CaX2). Cal-cium also reacts directly with hydrogen togive the hydride CaH2 and borides, ar-senides, carbides, and silicides can be pre-

39

C

pared in a similar way. Both the carbonateand sulfate are insoluble. Calcium salts im-part a characteristic brick-red color toflames which is an aid to qualitative analy-sis. At ordinary temperatures calcium hasthe face-centered cubic structure with atransition at 450°C to the close-packedhexagonal structure.

Symbol: Ca; m.p. 839°C; b.p. 1484°C;r.d. 1.55 (20°C); p.n. 20; r.a.m. 40.0878.

calcium acetylide See calcium dicar-bide.

calcium bicarbonate See calcium hy-drogencarbonate.

calcium carbide See calcium dicarbide.

calcium carbonate (CaCO3) A whitesolid that occurs naturally in two crys-talline forms: calcite and aragonite. Theseminerals make up the bulk of such rocks asmarble, limestone, and chalk. Calcium car-bonate also occurs in the mineral dolomite(CaCO3.MgCO3). It is sparingly soluble inwater but dissolves in rainwater containingcarbon dioxide to form calcium hydrogen-carbonate, which causes temporary hard-ness of water. Calcium carbonate is a basicraw material in the Solvay process and isused for making glass, mortar, and cement.

calcium chloride (CaCl2) A white solidthat occurs in a number of hydrated formsand is readily available as a by-product ofthe Solvay process. It is readily soluble inwater and the solution, known as brine, isused in refrigerating plants. Other applica-tions that depend on its water-absorbingproperty and the low freezing point of theaqueous solution include the suppressionof dust on roads and in mines and the melt-ing of snow. Calcium chloride is also usedas the electrolyte in the production of cal-cium.

calcium cyanamide (CaCN2) A solidprepared by heating calcium dicarbide to atemperature in excess of 800°C in anatmosphere of nitrogen. It is used as a fer-tilizer because ammonia and calcium car-

bonate are slowly formed when water isadded:

CaCN2 + 3H2O → CaCO3 + 2NH3Other uses include the defoliation of cot-ton plants and the production ofmelamine.

calcium dicarbide (calcium acetylide; cal-cium carbide; CaC2) A colorless solidwhen pure. In countries where electricity ischeap, calcium dicarbide is produced on alarge scale by heating calcium oxide withcoke at a temperature in excess of 2000°Cin an electric-arc furnace. Water is thenadded to give ethyne (acetylene, C2H2), animportant industrial organic chemical:

CaC2 + 2H2O → C2H2 + Ca(OH)2The structure of calcium dicarbide is in-

teresting because the carbon is present ascarbide ions (C2

2–). See ethyne.

calcium fluoride (CaF2) A white crys-talline compound found naturally as fluo-rite (flourspar).

calcium-fluoride structure (fluoritestructure) A form of crystal structure inwhich each calcium ion is surrounded byeight fluorine ions arranged at the cornersof a cube and each fluorine ion is sur-rounded tetrahedrally by four calciumions.

calcium hydrogencarbonate (calciumbicarbonate, Ca(HCO3)2) A solid formedwhen water containing carbon dioxide dis-solves calcium carbonate:

CaCO3 + H2O + CO2 → Ca(HCO3)2Calcium hydrogencarbonate is a cause oftemporary hard water. The solid is un-known at room temperature. See hardness.

calcium hydroxide (slaked lime; causticlime; Ca(OH)2) A white solid that dis-solves sparingly in water to give the alkaliknown as limewater. Calcium hydroxide ismanufactured by adding water to the oxide(lime or quicklime), a process known asslaking, which evolves much heat. If justsufficient water is added so that the oxideturns to a fine powder, the product isslaked lime. If more water is added, a thicksuspension called milk of lime is formed.

calcium acetylide

40

Calcium hydroxide has many uses. As abase, it is used to neutralize acid soil and inindustrial processes such as the Solvayprocess. It is also used in the manufactureof mortar, whitewash, and bleaching pow-der and for the softening of temporaryhard water.

calcium nitrate (Ca(NO3)2) A deliques-cent salt that is very soluble in water. It isusually crystallized as the tetrahydrateCa(NO3)2.4H2O. When the hydrate isheated, the anhydrous salt is first producedand this subsequently decomposes to givecalcium oxide, nitrogen dioxide, and oxy-gen. Calcium nitrate is used as a nitroge-nous fertilizer.

calcium octadecanoate (calciumstearate; Ca(CH3(CH2)16COO)2) An in-soluble salt of octadecanoic acid. It isformed as ‘scum’ when soap, containingthe soluble salt sodium octadecanoate, ismixed with hard water containing calciumions. See detergents.

calcium oxide (quicklime; CaO) Awhite solid formed by heating calcium inoxygen or, more widely, by the thermal de-composition of calcium carbonate. On alarge scale, limestone (calcium carbonate)is heated in a tall tower called a lime kiln toa temperature of 550°C. The reversible re-action:

CaCO3 ˆ CaO + CO2proceeds in a forward direction as the car-bon dioxide is carried away by the upwardcurrent through the kiln. Calcium oxide isused in extractive metallurgy to produce aslag with the impurities in metal ores; it isalso used as a drying agent and it is an in-termediate for the production of calciumhydroxide.

calcium phosphate (Ca3(PO4)2) Asolid that occurs naturally in the mineralapatite (CaF2.Ca3(PO4)3) and in rock phos-phate. It is the chief constituent of animalbones and is used extensively in fertilizers.

calcium silicate (Ca2SiO4) A white in-soluble crystalline compound found in var-ious cements and minerals. It is also a

component of the slag produced in smelt-ing iron and other metals in a blast furnace.

calcium stearate See calcium octade-canoate.

calcium sulfate (anhydrite; CaSO4) Awhite solid that occurs abundantly as themineral anhydrite and as the dihydrate(CaSO4.2H2O), known as gypsum or al-abaster. When heated, gypsum loses waterto form the hemihydrate (2CaSO4.H2O),which is plaster of Paris. If the water is re-placed, gypsum reforms and sets as a solid.Plaster of Paris is therefore used for takingcasts and for setting broken limbs. Calciumsulfate is sparingly soluble in water and isa cause of permanent hardness in water. Itis used in ceramics, paint, and in papermaking.

Calgon (Trademark) A substance oftenadded to detergents to remove unwantedchemicals that have dissolved in water andwould otherwise react with soap to form ascum. Calgon consists of complicatedsodium polyphosphate molecules, whichabsorb dissolved calcium and magnesiumions. The metal ions become trappedwithin the Calgon molecules.

caliche An impure commercial form ofsodium nitrate.

californium A silvery radioactive trans-uranic element of the actinoid series ofmetals, not found naturally on Earth. Sev-eral radioisotopes have been synthesized,including californium-252, which is usedas an intense source of neutrons in certaintypes of portable detector and in the treat-ment of cancer.

Symbol: Cf; m.p. 900°C; p.n. 98; moststable isotope 251Cf (half-life 900 years).

calixarene See host–guest chemistry.

calomel See mercury(I) chloride.

calomel electrode A half cell having amercury electrode coated with mercury(I)chloride (called calomel), in an electrolyteconsisting of potassium chloride and (satu-

41

calomel electrode

rated) mercury(I) chloride solution. Itsstandard electrode potential against the hy-drogen electrode is accurately known(–0.2415 V at 25°C) and it is a convenientsecondary standard.

calorie Symbol: cal A unit of energy ap-proximately equal to 4.2 joules. It was for-merly defined as the energy needed to raisethe temperature of one gram of water byone degree Celsius. Because the specificthermal capacity of water changes withtemperature, this definition is not precise.The mean or thermochemical calorie(calTH) is defined as 4.184 joules. The in-ternational table calorie (calIT) is defined as4.1868 joules. Formerly the mean caloriewas defined as one hundredth of the heatneeded to raise one gram of water from0°C to 100°C, and the 15°C calorie as theheat needed to raise it from 14.5°C to15.5°C.

calorific value See bomb calorimeter.

calorimeter A device or apparatus formeasuring thermal properties such as spe-cific heat capacity, calorific value, etc. Seebomb calorimeter.

calx A metal oxide obtained by heatingan ore to high temperatures in air.

candela Symbol: cd The SI base unit ofluminous intensity, equal to that, in a givendirection, of a source emitting monochro-matic radiation having a frequency of 540terahertz and a radiant intensity of 1/683watt.

Cannizzaro, Stanislao (1826–1910)Italian chemist. Cannizzaro was responsi-ble for reviving interest in the ideas ofAmedeo AVOGADRO in a pamphlet pub-lished in 1858 and in an address to theChemical Congress at Karlsruhe, Ger-many. His pamphlet clarified the conceptsof atomic and molecular weights and alsoshowed how the molecular weight (nowtermed relative molecular mass) of a com-pound could be determined by measuringits vapor density. He also discovered the

organic reaction known as Cannizzaro’sreaction in 1853.

canonical form See resonance.

carbide A compound of carbon with amore electropositive element. The carbidesof the elements are classified into:1. Ionic carbides, which contain the car-

bide ion C4–. An example is aluminumcarbide, Al4C3. Compounds of this typereact with water to give methane (theywere formerly also called methanides).The dicarbides are also ionic carboncompounds but contain the dicarbideion –C:C–. The best-known example iscalcium dicarbide, CaC2, also known ascalcium carbide, or simply carbide.Compounds of this type give ethyne(acetylene) with water. They were for-merly called acetylides or ethynides.Ionic carbides are formed with very elec-tropositive metals. They are crystalline.

2. Covalent carbides, which have giantmolecular structures, as in SILICON CAR-BIDE (SiC) and boron carbide (B4C3).These are hard solids with high meltingpoints.

3. Interstitial carbides, which are INTERSTI-TIAL COMPOUNDS of carbon with transi-tion metals. Titanium carbide (TiC) is anexample. These compounds are all hardsolids with high melting points andmetallic properties. Some carbides (e.g.nickel carbide Ni3C) have properties in-termediate between those of interstitialand ionic carbides.

carbon The first element of group 14(formerly IVA) of the periodic table. Car-bon is a universal constituent of living mat-ter and the principal deposits of carboncompounds, i.e. chalk and limestone, fromwhich we obtain carbonates, and coal, oil,and gas fields, from which we obtain fossilfuels, are derived from once living organ-isms. Carbon also occurs in the mineraldolomite. The element forms only 0.032%by mass of the Earth’s crust. Minute quan-tities of elemental carbon also occur as theallotropes graphite and diamond. A thirdallotrope, BUCKMINSTERFULLERENE (C60),was discovered in the mid-1980s.

calorie

42

The industrial demand for graphite issuch that it is manufactured in large quan-tities using the Acheson process in whichcoke and small amounts of asphalt or clayare raised to high temperatures. Largequantities of impure carbon are also con-sumed in the reductive extraction of met-als. In addition to the demand for diamondas a gemstone there is a large industrial de-mand for small low-grade diamonds foruse in drilling and grinding machinery.Studies show that graphite and diamondcan be interconverted at 3000°C under ex-tremely high pressures, but that commer-cial exploitation of this process would notbe viable.

Carbon burns in oxygen to form carbondioxide and carbon monoxide. Carbondioxide is soluble in water, forming theweakly acidic carbonic acid, which is theparent acid of the metal carbonates. In con-trast CO is barely soluble in water but willreact with alkali to give the methanoate(formate) ion:

CO + OH– → HCO2–

Carbon will react readily with sulfur atred heat to form carbon disulphide, CS2,but it does not react directly with nitrogen.Cyanogen, (CN)2, must be prepared byheating covalent metal cyanides such asCuCN. Carbon will also react directly withmany metals at elevated temperatures togive CARBIDES. Carbides can also be ob-

tained by heating the metal oxide with car-bon or heating the metal with a hydrocar-bon. There is a bewilderingly wide range ofmetal carbides, both saltlike with elec-tropositive elements (for example, CaC2)and covalent with the metalloids (for ex-ample, SiC), and there are also many inter-stitial carbides formed with metals such asCr, Mn, Fe, Co, and Ni.

Compounds with C–N bonds form asignificant branch of the inorganic chem-istry of carbon: these comprise hydrogencyanide (HCN) and the cyanides, cyanicacid (HNCO) and the cyanates, and thio-cyanic acid (HNCS) and the thiocyanates.

Naturally occurring carbon has the iso-topic composition 12C (98.89%), 13C(1.11%), and radioactive 14C (minutetraces of which are produced in the upperatmosphere as the result of the capture ofhigh-energy neutrons by 14N atoms, whicheach then release a proton to decay into14C). Carbon-14 (14C) is used for radiocar-bon dating because it is taken up in minutequantities by organisms when they arealive and decays over the comparativelylong half-life of 5780 years after theirdeath.

Symbol: C; m.p. 3550°C; b.p. 4830°C(sublimes); C60 sublimes at 530°C; r.d.3.51 (diamond), 2.26 (graphite), 1.65 (C60)(all at 20°C); p.n. 6; r.a.m. 12.011.

43

carbon

Graphite

Diamond

Carbon

carbonate Any salt of carbonic acid (i.e.containing the ion CO3

2–). Many alkaliand alkaline-earth carbonates are impor-tant industrially.

carbonation 1. The solution of carbondioxide in a liquid under pressure, as incarbonated soft drinks.2. The addition of carbon dioxide to com-pounds.

carbon black A finely divided form ofcarbon produced by the incomplete com-bustion of such hydrocarbon fuels as nat-ural gas or oil. It is used as a black pigmentin inks, paints, and plastics, and as a fillerfor rubber in tire manufacture.

carbon dioxide (CO2) A colorlessodorless nonflammable gas formed whencarbon burns in excess oxygen. It is alsoproduced by respiration. Carbon dioxide ispresent in the atmosphere (0.03% by vol-ume) and is converted in plants to carbo-hydrates by photosynthesis. In thelaboratory it is made by the action of diluteacid on metal carbonates. Industrially, it isobtained as a by-product in certainprocesses, such as fermentation and themanufacture of calcium oxide. The mainuses of carbon dioxide are as a refrigerantin its solid state, called dry ice, and in fireextinguishers and carbonated drinks. In-creased levels of carbon dioxide in the at-mosphere from the combustion of fossilfuels are thought to contribute to thegreenhouse effect.

Carbon dioxide is the anhydride of theweak acid carbonic acid, which is formedin water:

CO2 + H2O → H2CO3

carbon disulfide (CS2) A colorless poi-sonous flammable liquid made frommethane (derived from natural gas) andsulfur. It is a solvent for waxes, oils, andrubber, and is used in the manufacture ofviscose rayon. The pure compound is vir-tually odorless, but CS2 usually has a re-volting smell because of the presence ofother sulfur compounds.

carbon fibers Fibers of graphite, which

are used, for instance, to strengthen POLY-MERS. They are made by heating stretchedtextile fibers and have an orientated crystalstructure.

carbonic acid (H2CO3) A DIBASIC ACID

formed in small amounts in solution whencarbon dioxide dissolves in water:

CO2 + H2O ˆ H2CO2It forms two series of salts: hydrogencar-bonates (HCO3

–) and carbonates (CO32–).

The pure acid cannot be isolated.

carbonize (carburize) To convert an or-ganic compound into carbon by incom-plete oxidation at high temperature.

carbon monoxide (CO) A colorlessflammable toxic gas formed by the incom-plete combustion of carbon or carbon-con-taining compounds. Industrially, it isproduced by the oxidation of carbon ormethane, or by the reaction used to pro-duce WATER GAS. It is a powerful reducingagent and is used as such in metallurgy.

Carbon monoxide is neutral and onlysparingly soluble in water. It forms metalcarbonyls with transition metals. Its toxic-ity derives from the fact that hemoglobin(in the blood) has a far stronger affinity forcarbon monoxide than for oxygen, whichis essential for cell metabolism.

carbonyl A complex in which carbonmonoxide ligands are coordinated to ametal atom. A common example istetracarbonyl nickel(0), Ni(CO)4.

carbonyl chloride (phosgene; COCl2) A colorless, toxic gas with a choking smell,made by reacting carbon monoxide andchlorine in the presence of light or a cata-lyst. It is used as a chlorinating agent andto make certain plastics and insecticides; itwas formerly employed as a war gas.

carbonyl group The group =C=O. Itoccurs in organic compounds, and in car-bonyl complexes of transition metals.

carborundum See silicon carbide.

carboxylate ion The ion –COO–, pro-

carbonate

44

duced by ionization of a carboxyl group. Ina carboxylate ion the negative charge isgenerally delocalized over the O–C–Ogrouping and the two C–O bonds have thesame length, intermediate between that ofa double C=O and a single C–O.

carboxyl group The organic group–CO.OH, present in carboxylic acids.

carboxylic acid A type of organic com-pound containing the CARBOXYL GROUP.Simple carboxylic acids have the generalformula RCOOH. Many carboxylic acidsoccur naturally in plants and (in the formof esters) in fats and oils, hence the alter-native name fatty acids.

carburizing See case hardening.

carcinogen An agent that causes or pro-motes the development of cancer in ani-mals, such as tobacco smoke or ionizingradiation.

carnallite A mineral chloride of potas-sium and magnesium, KCl.MgCl2.6H2O.It is used as a source of potassium salts forfertilizers.

Carnot cycle The idealized reversiblecycle of four operations occurring in a per-fect heat engine. These are the successiveadiabatic compression, isothermal expan-sion, adiabatic expansion, and isothermalcompression of the working substance.The cycle returns to its initial pressure, vol-ume, and temperature, and transfers en-ergy to or from mechanical work. Theefficiency of the Carnot cycle is the maxi-mum attainable in a heat engine. It waspublished in 1824 by the French physicistNicolas L. S. Carnot (1796–1832). SeeCarnot’s principle.

Carnot’s principle (Carnot theorem)The efficiency of any heat engine cannot begreater than that of a reversible heat engineoperating over the same temperaturerange. Carnot’s principle follows directlyfrom the second law of thermodynamics,and means that all reversible heat engineshave the same efficiency, independent of

the working substance. If heat is absorbedat temperature T1 and given out at T2, thenthe efficiency according to Carnot’s princi-ple is (T1 – T2)/T1. See Carnot cycle.

Carnot’s theorem See Carnot’s princi-ple.

Caro’s acid See peroxosulfuric(VI) acid.

carrier gas The gas used to carry thesample in gas chromatography.

case hardening Processes for increasingthe hardness of the surface or ‘case’ of thesteel used to make such components asgears and crankshafts. The oldest methodis carburizing, in which the carbon contentof the surface layer is increased by heatingthe component in a carbon-rich environ-ment. Nitriding involves the diffusion ofnitrogen into the surface layer of the steel,thus forming intensely hard NITRIDE parti-cles in the structure. A combination of bothcarburizing and nitriding is sometimes em-ployed.

cast iron Any of various alloys of ironand carbon made by remelting the crudeiron produced in a blast furnace. The car-bon content is usually between 2.4 and4.0% and may be present as iron carbide(FeC) or as graphite. In the former case theproduct is known as white cast iron, and inthe latter as gray cast iron. Other metalsmay be added to improve the properties ofthe alloy. Additional elements such asphosphorus, sulfur, and manganese arealso present as impurities. Cast iron ischeap and has an extensive range of possi-ble properties. It is used on a very largescale.

catalyst A substance that alters the rateof a chemical reaction without itself beingchanged chemically in the reaction. Thecatalyst can, however, undergo physicalchange; for example, large lumps of cata-lyst can, without loss in mass, be convertedinto a powder. Small amounts of catalystare often sufficient to alter the rate of reac-tion considerably. A positive catalyst in-creases the rate of a reaction and a negative

45

catalyst

catalyst reduces it. Homogeneous catalystsare those that act in the same phase as thereactants (i.e. in gaseous and liquid sys-tems). For example, nitrogen(II) oxide gaswill catalyze the reaction betweensulfur(IV) oxide and oxygen in the gaseousphase. Heterogeneous catalysts act in a dif-ferent phase from the reactants. For exam-ple, finely divided nickel (a solid) willcatalyze the hydrogenation of oil (liquid).

The function of a catalyst is to providea new pathway for the reaction, alongwhich the rate-determining step has alower activation energy than in the uncat-alyzed reaction. A catalyst does not changethe products in an equilibrium reactionand their concentration remains identicalto the concentration of the products in anuncatalyzed reaction; i.e. the position ofthe equilibrium remains unchanged. Thecatalyst simply changes the rate at whichequilibrium is attained.

In autocatalysis, one of the products ofthe reaction itself acts as a catalyst. In thistype of reaction the reaction rate increaseswith time up to a maximum value and fi-nally slows down. See also promoter.

catalytic converter A device fitted tothe exhaust system of gasoline-fueled vehi-cles to remove pollutant gases from the ex-haust. It typically consists of a structurehoneycombed to provide maximum areaand coated with platinum, palladium, andrhodium catalysts. Such devices can con-vert carbon monoxide to carbon dioxide,oxides of nitrogen to nitrogen, and un-burned fuel to carbon dioxide and water.

catenation The formation of chains ofatoms in molecules.

cathode In electrolysis, the electrodethat is at a negative potential with respectto the anode, and to which CATIONS are at-tracted. In any electrical system, such as adischarge tube or electronic device, thecathode is the terminal at which electronsenter the system. Compare anode.

cation A positively charged ion, formedby removal of electrons from atoms ormolecules. In electrolysis, cations are at-

tracted to the negatively charged electrodeor cathode. Compare anion.

cationic detergent See detergents.

cationic resin An ION-EXCHANGE ma-terial that can exchange cations, such as H+

and Na+, for cations in the surroundingmedium. Such materials are often pro-duced by adding a sulfonic acid group(–SO3

–H+) to a stable resin. A typical ex-change reaction is:

resin–SO3–H+ + NaCl ˆ

resin–SO3–Na+ + HCl

Cationic resins have been used to greateffect to separate mixtures of cations ofsimilar size having the same charge. Duringthis procedure, the mixtures are attachedto the resins, after which progressive ELU-TION is used to recover the cations in orderof decreasing ionic radius. Promethiumwas first isolated using this technique.

caustic lime See calcium hydroxide.

caustic potash See potassium hydrox-ide.

caustic soda See sodium hydroxide.

Cavendish, Henry (1731–1810) Eng-lish chemist and physicist. Cavendish per-formed pioneering work on gases. Heshowed that ordinary air is a mixture ofoxygen and nitrogen, with nitrogen beingfour times more common. In 1781 he alsodemonstrated that when hydrogen andoxygen are mixed in the proportions of 2:1and the resulting mixture exploded (usingan electric spark) then water is produced,thus showing that water is a compoundand not an element. He also used a spark tocombine nitrogen with oxygen and dis-solved the resulting gas in water, thus gen-erating nitric acid. Cavendish also didimportant work in physics, particularly inelectricity and gravitation.

celestine (celestite) A naturally occur-ring mineral sulfate of strontium, SrSO4,found in sedimentary rocks. It is a sourceof strontium and is also used to make spe-cial glasses.

catalytic converter

46

celestite See celestine.

cell A system having two ELECTRODES ina conducting liquid or paste ELECTROLYTE.An electrolytic cell is used to produce achemical reaction by means of a currentpassing through the electrolyte (i.e. byELECTROLYSIS). Conversely, a voltaic (orgalvanic) cell produces an e.m.f. by meansof chemical reactions that occur at eachelectrode. Electrons are transferred to orfrom the electrodes, giving each a netcharge.

There is a convention for writing cell re-actions in voltaic cells. The DANIELL CELL,for instance, consists of a zinc electrode ina solution of Zn2+ ions connected, via aporous partition, to a solution of Cu2+ ionsin which a copper electrode is placed. Thereactions at the electrodes are

Zn → Zn2+ + 2ei.e. oxidation of the zinc to zinc(II), and

Cu2+ + 2e → Cui.e. reduction of copper(II) to copper. A cellreaction of this type is written:

Zn|Zn2+(aq)|Cu2+(aq)|CuThe e.m.f. is the potential of the electrodeon the right minus the potential on the left.Copper is positive in this case and thee.m.f. of the cell is stated as +1.10 volts. Seealso accumulator.

Celsius scale (centigrade scale) A tem-perature scale in which the temperature ofpure melting ice at standard pressure isfixed at 0° and the temperature of pureboiling water at standard pressure is fixedat 100°. The temperature interval unit onthe Celsius scale is the degree Celsius (°C),which is equal in magnitude to the kelvin.Before 1948 the Celsius scale was knownas the centigrade scale. Celsius’s originalscale had 0° as the water steam fixed pointand 100° as the ice/water fixed point. Seealso temperature scale; Fahrenheit scale.

cement A generic term for chemicalpreparations typically used as bindingagents to bond disparate particles togetherinto a strong, solid mass, e.g. concrete.Such cements commonly consist of a pow-dered mixture of calcium silicates and alu-minates, which is made by heating

limestone (CaCO3) with clay and grindingthe result. When mixed with water, reac-tions occur with the water (hence the namehydraulic cement) and a hard solid alumi-nosilicate is formed.

cementite (Fe3C) A constituent of cer-tain cast irons and steels. The presence ofcementite increases the hardness of thealloy.

centi- Symbol: c A prefix used with SIunits denoting 10–2. For example, 1 cen-timeter (cm) = 10–2 meter (m).

centigrade scale See Celsius scale.

centrifugal pump A device commonlyused for transporting fluids around achemical plant. Centrifugal pumps usuallyhave 6–12 curved vanes that rotate inside afixed circular casing. As the blades rotate,the fluid is impelled out of the pump alonga discharge pipe. Centrifugal pumps do notproduce high pressures but they have theadvantages of being relatively cheap due totheir simple design, have no valves, andwork at high speeds. In addition they arenot damaged if a blockage develops. Com-pare displacement pump.

centrifuge An apparatus for rotating acontainer at high speeds, used to increasethe rate of sedimentation of suspensions orthe separation of two immiscible liquids.See also ultracentrifuge.

ceramics Useful high-melting inorganicmaterials. Ceramics include silicates andaluminosilicates, refractory metal oxides,and metal nitrides, borides, etc. Potteryand porcelain are examples of ceramics.Various types of glass are also sometimesincluded.

cerium A ductile, malleable, gray el-ement of the lanthanoid series of metals. Itoccurs in association with other lan-thanoids in many minerals, including mon-azite and bastnaesite. The metal reactsrapidly with hot water, tarnishes in air, andwill ignite under friction. It is used as a cat-alyst in several alloys (especially for lighter

47

cerium

flints), in tracer bullets and in gas mantles,and in compound form in carbon-arcsearchlights, etc., and in the glass industry.

Symbol: Ce; m.p. 799°C; b.p. 3426°C;r.d. 6.7 (hexagonal structure, 25°C); p.n.58; r.a.m. 140.15.

cermet A synthetic composite materialmade by combining a ceramic and a sin-tered metal. Cermets have better tempera-ture and corrosion resistance than straightceramics. For example, a chromium–alu-mina cermet is used to make blades for gas-turbine engines.

cerussite A naturally occurring form oflead(II) carbonate (PbCO3) that is an im-portant lead ore. It forms orthorhombiccrystals and is often found together withgalena (PbS).

cesium (caesium) A soft, silvery, highlyreactive, low-melting element of the alkali-metal group, the sixth element of group 1(formerly IA) of the periodic table. It isfound in several silicate minerals, includingpollucite (CsAlSi2O6). The metal oxidizesin air and reacts violently with water. Ce-sium is used in photocells, as a catalyst,and in the cesium atomic clock. The radio-active isotopes 134Cs (half life 2.065 years)and 137Cs (half life 30.3 years) are pro-duced in nuclear reactors and are poten-tially dangerous atmospheric pollutants.Cesium-137 is also used as a radiationsource in cancer therapy, and as a medicaltracer.

Symbol: Cs; m.p. 28.4°C; b.p. 678.4°C;r.d. 1.873 (20°C); p.n. 55; r.a.m. 132.91.

cesium-chloride structure A form ofcrystal structure that consists of alternatelayers of cesium ions and chloride ionswith the center of the lattice occupied by acesium ion in contact with eight chlorideions (i.e. four chloride ions in the planeabove and four in the plane below).

c.g.s. system An early metric system ofunits based on the centimeter, the gram,and the second as the fundamental me-chanical units. Much early scientific workwas performed using this system, but it has

now almost been abandoned in favor of SIunits.

chain When two or more atoms formbonds with each other in a molecule, achain of atoms results. This chain may be astraight chain, in which each atom is addedto the end of the chain, or it may be abranched chain, in which the main chain ofatoms has one or more smaller side chainsbranching off it.

chain reaction A self-sustaining chemi-cal reaction consisting of a series of steps,each of which is initiated by the one beforeit. An example is the reaction between hy-drogen and chlorine:

Cl2 → 2Cl•H2 + Cl• → HCl + H•H• + Cl2 → HCl + Cl•

2H• → H22Cl• → Cl2

The first stage, chain initiation, is the dis-sociation of chlorine molecules into atoms;this is followed by two chain propagationreactions. Two molecules of hydrogenchloride are produced and the ejected chlo-rine atom is ready to react with more hy-drogen. The final steps, chain termination,stop the reaction.

Induced nuclear fission reactions alsodepend on chain reactions; the fission reac-tion is maintained by the two or three neu-trons set free in each fission.

chalcogens See group 16 elements.

chalk A soft, natural, fine-grained formof calcium carbonate (CaCO3) formedfrom the skeletal remains of ancient marineorganisms. So-called blackboard chalk ismade from a different compound, calciumsulfate (CaSO4).

chamber process See lead-chamberprocess.

chaotic reaction A chemical reaction inwhich the concentrations of the reactantsshow chaotic behavior, i.e. the evolution ofthe reaction may become unpredictable.An example is the BELOUSOV–ZHABOTINSKII

REACTION. Reactions of this type ususally

cermet

48

involve a large number of complex inter-linked steps. See also bistability; oscillatingreaction.

charcoal An amorphous form of carbonmade by heating wood or other organicmaterial in the absence of air. Activatedcharcoal is charcoal heated to drive off ab-sorbed gas. It is used for absorbing gasesand for removing impurities from liquids.

Charles’ law (Gay-Lussac’s law) For agiven mass of gas at constant pressure, thevolume increases by a constant fraction ofthe volume at 0°C for each Celsius degreerise in temperature. The constant fraction(α) has almost the same value, about1/273, for all gases and Charles’ law canthus be written in the form

V = V0(1 + αvθ)where V is the volume at temperature θ°Cand V0 the volume at 0°C. The constant αvis the thermal expansivity of the gas. For anideal gas its value is 1/273.15.

A similar relationship exists for thepressure of a gas heated at constant vol-ume:

p = p0(1 + αpθ)Here, αp is the pressure coefficient. For anideal gas

αp = αvalthough they differ slightly for real gases.It follows from Charles’ law that for a gasheated at constant pressure,

V/T = Kwhere T is the thermodynamic temperatureand K is a constant. Similarly, at constantvolume, p/T is a constant.

Charles’ law is named for the Frenchchemist and physicist J. A. C. Charles, whodiscovered it by means of experimentsbegun in 1787. The law was published in1802, however, by J. Gay-Lussac, anotherFrench chemist and physicist, is sometimesknown as Gay-Lussac’s law.

chelate A metal coordination COMPLEX

in which one molecule or ion forms a coor-dinate bond at two or more points to thesame metal ion, thus becoming a ligand.The resulting complex contains rings ofatoms that include the metal atom.Chelates may be used to deliver essential

metal ions in fertilizer preparations, whilechelating agents, or compounds capable offorming chelates, may be used to trapheavy metals and thus render them harm-less in the case of metal poisonings.‘Chelate’ comes from the Greek wordmeaning ‘claw’. Ligands forming chelatesare classified according to the number ofsites at which they can coordinate: bi-dentate, tridentate, etc. See also sequestra-tion.

chemical bond A force holding atomstogether in a molecule or in a crystal.Chemical bonds are formed by transfer orsharing of electrons (see electrovalentbond; covalent bond) and typically haveenergies of about 1000 kJ mol–1. Weakerinteractions, such as HYDROGEN BONDS andVAN DER WAALS FORCES, are not regarded aschemical bonds. See also coordinate bond;metallic bond.

chemical combination, laws of Agroup of chemical laws, developed duringthe late 18th and early 19th centuries,which arose from the recognition of the im-portance of quantitative (as opposed toqualitative) study of chemical reactions.The laws are:1. The law of conservation of mass (mat-

ter);2. The law of constant (definite) propor-

tions;3. The law of multiple proportions;4. The law of equivalent (reciprocal) pro-

portions.These laws played a significant part in Dal-ton’s development of his atomic theory in1808.

49

chemical combination, laws of

N

H2C

H2C

N

H2

H2 H2

N

N

H2

CH

CHCu

2+

Chelate

chemical dating A method of usingchemical analysis to find the age of an ar-chaeological specimen in which composi-tional changes have taken place over time.For example, the determination of theamount of fluorine in bone that has beenburied gives an indication of the time thebone has been underground because phos-phate in the bone is gradually replaced byfluoride ions from groundwater. Anotherdating technique depends on the fact that,in living organisms, amino acids are opti-cally active. After death a slow RACEMIZA-TION reaction occurs and a mixture of L-and D-isomers forms. The age of bones canbe accurately determined by measuring therelative amounts of optically active versusoptically inactive acids present.

chemical engineering The branch ofengineering concerned with the design andmaintenance of a chemical plant and itsability to withstand extremes of tempera-ture, pressure, corrosion, and wear. It en-ables laboratory processes producinggrams of material to be converted into alarge-scale plant processes yielding tonnesof material. Chemical engineers plan large-scale chemical processes by linking to-gether the appropriate unit processes andby studying such parameters as heat andmass transfer, separations, and distilla-tions.

chemical equation A method of repre-senting a chemical reaction using chemicalFORMULAS. The formulas of the reactantsare given on the left-hand side of the equa-tion, with the formulas of the productsgiven on the right. The two halves are sep-arated by a directional arrow or arrows, oran equals sign. A number preceding a for-mula, called a stoichiometric coefficient,indicates the number of molecules of thatsubstance involved in the reaction. Theequation must balance – that is, the num-ber of atoms of any one element must bethe same on both sides of the equation. Asimple example is the equation for the re-action between hydrogen and oxygen toform water:

2H2 + O2 → 2H2O

A more complex equation represents thereaction between disodium tetraboratedecahydrate (borax) and aqueous hy-drochloric acid to give boric acid andsodium chloride:

Na2B4O7 + 2HCl + 5H2O → 4H3BO3 +2NaCl

chemical equilbrium See equilibrium.

chemical formula See formula.

chemical potential Symbol: µ. For theith component of a mixture the chemicalpotential µi is defined by the partial deriva-tive of the Gibbs free energy G of the sys-tem with respect to the amount ni of thecomponent, when the temperature, pres-sure, and amounts of other componentsare constant, i.e. µi = ∂G/∂ni. If the chemi-cal potentials of components are equalthen the components are in equilibrium.Also, in a one-component system with twophases it is necessary for the chemical po-tentials to be equal in the two phases forthere to be equilibrium.

chemical reaction A process in whichone or more elements or chemical com-pounds (the reactants) react to produce adifferent substance or substances (theproducts).

chemical shift See nuclear magnetic res-onance.

chemical symbol A letter or pair of let-ters or, in the case of unnamed elements, atriplet of letters that stand for a chemicalelement, as used for example in equationsand chemical formulas. Unlike abbrevia-tions, symbols do not take periods, arewritten exactly the same in all languagesand applications, and are designed to beuniversally understood to represent spe-cific elements, units, quantities, etc.

chemiluminescence The emission oflight during a chemical reaction.

chemisorption See adsorption.

Chile saltpeter See sodium nitrate.

chemical dating

50

China clay (kaolin) A white powderyclay obtained from the natural decomposi-tion of granites. It is used as a filler inpaints and paper-making, in the pottery in-dustries, and in pharmaceuticals. See kaoli-nite.

chiral Having the property of CHIRALITY.

chiral center See isomerism.

chirality The property of existing in left-and right-handed forms; i.e. mirror imagesthat are not superimposable. In chemistrythe term is applied to the existence of opti-cal isomers. ‘Chiral’ comes from the Greekword ‘cheir’, meaning hand. See iso-merism; optical activity.

chlorate A salt of chloric(V) acid.

chloric(I) acid (hypochlorous acid;HClO) A colorless liquid produced whenchlorine dissolves in water. It is a bleachand gives chlorine water its disinfectantproperties. To increase the yield of acid,the chlorine water can be shaken with asmall amount of mercury(II) chloride. TheCl–O bond is broken more easily than theO–H bond in aqueous solution; the acid isconsequently a poor proton donor andhence a weak acid.

chloric(III) acid (chlorous acid; HClO2)A pale yellow liquid produced by reactingchlorine dioxide with water. It is a weakacid and oxidizing agent.

chloric(V) acid (chloric acid; HClO3) Acolorless liquid with a pungent odor,formed by the action of dilute sulfuric acidon barium chlorate. It is a strong acid andhas bleaching properties. Chloric(V) acid isalso a strong oxidizing agent; in concen-trated solution it will ignite organic sub-stances, such as paper and sugar.

chloric(VII) acid (perchloric acid;HClO4) A colorless liquid that fumesstrongly in moist air. It is made by vacuumdistillation of a mixture of potassium per-chlorate and concentrated sulfuric acid. Itis a strong acid and oxidizing agent, and in

contact with organic material it is danger-ously explosive.

The hydrate (HClO4.H2O) of chlo-ric(VII) acid is a white crystalline solid atroom temperature and has an ionic latticestructure of the form (H3O)+(ClO4)–.

chloric acid See chloric(V) acid.

chloride See halide.

chlorination 1. Treatment with chlo-rine; for instance, the use of chlorine to dis-infect water.2. See halogenation.

chlorine A green reactive gaseous el-ement belonging to the halogens; i.e. group17 (formerly VIIA) of the periodic table, ofwhich it is the second element, occurring inseawater, salt lakes, and underground de-posits of the mineral halite as NaCl. It ac-counts for about 0.055% of the Earth’scrust.

Chlorine is strongly oxidizing and canbe liberated from its salts only by strongoxidizing agents, such as manganese(IV)oxide, potassium permanganate(VII), orpotassium dichromate; sulfuric acid is notsufficiently oxidizing to release chlorinefrom chlorides. Industrially, chlorine isprepared by the electrolysis of brine ormagnesium chloride, although in someprocesses chlorine is recovered by the high-temperature oxidation of waste hydrochlo-ric acid. Chlorine is used in largequantities, both as the element to producechlorinated organic solvents, for the pro-duction of polyvinyl chloride (PVC), themajor thermoplastic in use today, and inthe form of hypochlorites for bleaching.

Chlorine reacts directly and often vig-orously with many elements; it reacts ex-plosively with hydrogen in sunlight to formhydrogen chloride, HCl, and combineswith the electropositive elements to formmetal chlorides. The metals of main groups1 and 2 form electrovalent chlorides but anincrease in the metal charge/size ratio leadsto the chlorides becoming increasingly co-valent. For example, CsCl is totally elec-trovalent, AlCl3 has a layer lattice, andTiCl4 is essentially covalent. The elec-

51

chlorine

tronegative elements form volatile molecu-lar chlorides characterized by the single co-valent bond to chlorine. With theexception of Pb2+, Ag+, and Hg2

2+, the elec-trovalent chlorides are soluble in water,dissolving to give the hydrated metal ionand the chloride ion Cl–. Chlorides of met-als other than the most electropositive arehydrolyzed if aqueous solutions are evapo-rated; for example,

ZnCl2 + H2O → Zn(OH)Cl + HClFeCl3 + 3H2O → Fe(OH)3 + 3HCl

Chlorine forms four main oxides,dichlorine oxide, Cl2O; chlorine dioxide,ClO2; dichlorine hexoxide, Cl2O6; anddichlorine heptoxide, Cl2O7, all of whichare highly reactive and explosive. Chlorinedioxide finds commercial application as anactive oxidizing agent, but because of itsexplosive nature it is usually diluted by airor other gases. The chloride ion is able tofunction as a ligand with a large variety ofmetal ions forming such species as [FeCl4]–,[CuCl4]3–, and [Co(NH3)4Cl2]+. The for-mation of anionic chloro-complexes is ap-plied to the separation of metals byion-exchange methods.

Because of the hydrolysis of many metalchlorides when solutions are evaporated,special techniques must be used to prepareanhydrous chlorides. These are:1. Reaction of dry chlorine with the hot

metal;2. For metals having lower valences, reac-

tion of dry hydrogen chloride with thehot metal;

3. Reaction of dry hydrogen chloride onthe hydrated chloride.

The solubility of inorganic metal chloridesis such that they are not an environmentalproblem unless the metal ion itself is toxic.However, chlorine and hydrogen chlorideare both highly toxic. Thus chlorides thathydrolyze to release HCl should also be re-garded as toxic. They should not be han-dled without gloves and precautionsshould be taken against inhalation.

Symbol: Cl; m.p. –100.38°C; b.p.–33.97°C; d. 3.214 kg m–3 (0°C); p.n. 17;r.a.m. 35.4527.

chlorine dioxide (ClO2) An orange gasformed in the laboratory by the action of

concentrated sulfuric acid on potassiumchlorate. It is a powerful oxidizing agent,and its explosive properties in the presenceof a reducing agent were used to make oneof the first combustible matches. It iswidely used in the purification of waterand as a bleach in the flour and wood-pulpindustry. On an industrial scale an aqueoussolution of chlorine dioxide is made bypassing nitrogen dioxide up a towerpacked with a fused mixture of aluminumoxide and clay, down which a solution ofsodium chlorate flows.

chlorine monoxide See dichlorineoxide.

chlorine(I) oxide See dichlorine oxide.

chlorite A a salt of chloric(III) acid(chlorous acid).

chloroplatinic(IV) acid (platinic chlo-ride; H2PtCl6) A reddish compound pre-pared by dissolving platinum in aqua regia.When crystallized from the resulting solu-tion, crystals of the hexahydrate(H2PtCl6.6H2O) are obtained. The crystalsare needle-shaped and deliquesce on expo-sure to moist air. Chloroplatinic acid is arelatively strong acid, giving rise to thefamily of chloroplatinates.

chlorous acid See chloric(III) acid.

chromate Any oxygen-containing deriv-ative of chromium, most particularly thechromate(VI) species.

chromatography A technique used toseparate or analyze complex mixtures. Anumber of related techniques exist; all de-pend on two phases: a mobile phase, whichmay be a liquid or a gas, and a stationaryphase, which is either a solid or a liquidheld by a solid. The sample to be separatedor analyzed is carried by the mobile phasethrough the stationary phase. Differentcomponents of the mixture are absorbed ordissolved to different extents by the sta-tionary phase, and consequently movealong at different rates. In this way thecomponents are separated. There are many

chlorine dioxide

52

different forms of chromatography de-pending on the techniques used and the na-ture of the partition process betweenmobile and stationary phases. The mainclassification is into column chromatogra-phy and planar chromatography.

A simple example of column chro-matography is in the separation of liquidmixtures. A vertical column is packed withan absorbent material, such as alumina(aluminum oxide) or silica gel, represent-ing the stationary phase. The sample is in-troduced into the top of the column andwashed down it (the mobile phase) using asolvent. This process is known as ELUTION.If the components are colored, visiblebands appear down the column as the sam-ple separates out. The components are sep-arated as they emerge from the bottom ofthe column. In this particular example ofchromatography the partition process isadsorption on the particles of alumina orsilica gel. Column chromatography canalso be applied to mixtures of gases, inwhich case it is known as GAS CHROMATOG-RAPHY.

Planar chromatography works in simi-lar fashion, except that the stationaryphase is a flat sheet of absorbent materialsuch as paper. Components of the mixtureare held back by the stationary phase eitherby adsorption (e.g. on the surface of alu-mina) or because they dissolve in it (e.g. inthe moisture within paper). See paper chro-matography; thin-layer chromatography.

chrome alum See alum.

chrome iron ore See chromite.

chromic anhydride See chromium(VI)oxide.

chromic oxide See chromium(III) oxide.

chromite (chrome iron ore; ironchromium oxide; FeCr2O4) A mineralthat consists of mixed oxides of chromiumand iron, the principal ore of chromium. Itoccurs as brownish black masses with ametallic luster in rocks of igneous origin.Large deposits are found in Zimbabwe andthe western United States.

chromium A hard, silver-gray transi-tion metal of group 6 (formerly VIB) of theperiodic table. It occurs naturally asCHROMITE (FeCr2O4). The ore is first con-verted into sodium dichromate(VI) andthen reduced with carbon to chromium(III)oxide, after which it is finally reduced tometallic chromium with aluminum.Chromium is used to make strong alloysteels and stainless steel, and for decorativeelectroplated coatings. It resists corrosionat normal temperatures. It reacts slowlywith dilute hydrochloric and sulfuric acidsto give hydrogen and blue chromium(II)compounds, which quickly oxidize in air togreen chromium(III) ions. The bright col-ors of many chromium compounds makethem useful as pigments. The oxidationstates are +6 in CHROMATES (CrO4

2–) anddichromates (Cr2O7

2–), +3 (the most sta-ble), and +2. In acidic solutions the yellowchromate(VI) ion changes to the orangedichromate(VI) ion. Dichromates arestrong oxidizing agents and are used assuch in the laboratory. For example theyare used as a test for sulfur(IV) oxide (sul-fur dioxide) and to oxidize alcohols. Seepotassium dichromate.

Symbol: Cr; m.p. 1860±20°C; b.p.2672°C; r.d. 7.19 (20°C); p.n. 24; r.a.m.51.9961.

chromium dioxide See chromium(IV)oxide.

chromium(II) oxide (chromous oxide;CrO) A black powder prepared by theoxidation of chromium amalgam with di-lute nitric acid. At high temperatures(around 1000°C) chromium(II) oxide is re-duced by hydrogen.

chromium(III) oxide (chromic oxide;chromium sesquioxide; Cr2O3) A greenpowder that is almost insoluble in water. Ithas the same crystal structure as iron(III)oxide and aluminum(III) oxide. Chro-mium(III) oxide is prepared by gently heat-ing chromium(III) hydroxide or by heatingammonium dichromate. Alternative prepa-rations include the heating of a mixture ofammonium chloride and potassiumdichromate or the decomposition of

53

chromium(III) oxide

chromyl chloride by passing it through ared-hot tube. Chromium(III) oxide is usedas a pigment in the paint and glass indus-tries.

chromium(IV) oxide (chromium diox-ide; CrO2) A black solid prepared byheating chromium(III) hydroxide in oxy-gen at a temperature of 300–350°C.Chromium(IV) oxide is very unstable.

chromium(VI) oxide (chromium triox-ide; chromic anhydride; CrO3) A redcrystalline solid formed when concentratedsulfuric acid is added to a cold saturatedsolution of potassium dichromate. Thelong prismatic needle-shaped crystals thatare produced are extremely deliquescent.Chromium(VI) oxide is readily soluble inwater, forming a solution that containsseveral of the polychromic acids i.e. acidscontaining more than one atom ofchromium. On heating it decomposes togive chromium(III) oxide. Chromium(VI)oxide is used as an oxidizing reagent.

chromium sesquioxide See chro-mium(III) oxide.

chromium trioxide See chromium(VI)oxide.

chromophore A group of atoms in amolecule that is responsible for the color ofthe compound.

chromous oxide See chromium(II)oxide.

chromyl chloride (CrO2Cl2) A darkred covalent liquid prepared either by dis-tilling the vapors evolved when a dry mix-ture of potassium dichromate and sodiumchloride is treated with concentrated sulfu-ric acid or by the action of concentratedsulfuric acid on chromium(VI) oxide dis-solved in concentrated hydrochloric acid.Chromyl chloride is violently hydrolyzedby water, and with solutions of alkalis itundergoes immediate hydrolysis to pro-duce chromate ions. It is used as a potentoxidizing agent.

cinnabar A red mineral form of mer-cury(II) sulfide (HgS), associated withareas of volcanic activity. It is the principalore of mercury.

cis-isomer See isomerism.

cis-trans isomerism See isomerism.

Clark cell A type of cell formerly usedas a standard source of e.m.f. It consists ofa mercury cathode coated with mercurysulfate, and a zinc anode. The electrolyte iszinc sulfate solution. The e.m.f. producedis 1.4345 volts at 15°C. The Clark cell hasbeen superseded as a standard by the WE-STON CADMIUM CELL.

clathrate (cage compound; enclosurecompound) A substance in which small‘guest’ molecules are trapped within thelattice of a crystalline ‘host’ compound.Clathrates are formed when suitable hostcompounds are crystallized in the presenceof molecules of the appropriate size. Al-though the term ‘clathrate compound’ isoften used, clathrates are not true com-pounds; no chemical bonds are formed,and the guest molecules interact by weakvan der Waals forces. The clathrate ismaintained by the cagelike lattice of thehost. The host lattice must be brokendown, for example by heating or dissolu-tion, in order to release the guest. In ZEO-LITES, in comparison, the holes in the hostlattice are large enough to permit entranceor emergence of the guest without breakingbonds in the host lattice. Water (ice), forinstance, forms a clathrate with xenon.

Clausius, Rudolf Julius Emmanuel(1822–88) German physicist. Clausius isbest remembered as one of the founders ofthermodynamics. In papers published inthe 1850s he stated the second law of ther-modynamics and he coined the word en-tropy in 1865. Clausius made othercontributions to thermodynamics and alsoto the development of the kinetic theory ofgases. In electrochemistry he pioneered theidea of dissociation of substances into ionsin solution. He also studied the dielectricproperties of materials.

chromium(IV) oxide

54

clays Naturally occurring aluminosili-cates that form pastes and gels with water.

cleavage The splitting of a crystal alongplanes of atoms, to form smooth surfaces.

close packing The arrangement of par-ticles (usually atoms) in crystalline solids inwhich the UNIT CELL has an atom, ion, ormolecule at each corner and also at the cen-ter of each face of a cube. Each particlethus has 12 nearest neighbors: six in thesame layer (or plane) as itself and threeeach in the layer above and below for a CO-ORDINATION NUMBER of 12. This arrange-ment provides the most economical use ofspace (74% efficiency). The two principaltypes of close packing are FACE-CENTERED

CUBIC CRYSTAL and HEXAGONAL CLOSE PACK-ING.

cluster A three-dimensional structureconsisting of atoms, with the number ofatoms ranging between a few dozen and afew thousand. The atoms in clusters can ei-ther be metal or nonmetal atoms. It isfound that when clusters are made (for ex-ample, by sputtering the surface of a solid),clusters with certain magic numbers ofatoms occur far more abundantly than forother numbers of atoms. These numbers,which occur for several monovalent metal-lic elements including sodium, silver, andgold, are 8, 20, 40, 58, 92 and correspondto the electron shells being filled in an ex-ternal potential. It is predicted that clustersof atoms or cluster compounds in whichthe number of valence electrons is a magicnumber should be stable. For example,Al12C, which has a total of 40 valence elec-trons, is predicted to be a stable clustercompound.

cluster compound A type of compoundin which a cluster of metal atoms are joinedby metal–metal bonds. Cluster compoundsare formed by certain transition elements,such as molybdenum and tungsten.

coagulation The irreversible associa-tion of particles such as colloids into clus-ters. See flocculation.

coal A brown or black sedimentary de-posit that consists mainly of carbon, usedas a fuel and as a source of organic chemi-cals. It is the fossilized remains of decayedplants and algae that chiefly grew in theCarboniferous and Permian periods andwere then buried and subjected to highpressures underground. The various typesof coal, including ANTHRACITE, BITUMINOUS

COAL, and LIGNITE, are classified accordingto their increasing carbon content.

cobalt A lustrous, silvery-blue, hard fer-romagnetic transition metal, the first el-ement of group 9 (formerly subgroupVIIIB) of the periodic table. It occurs in as-sociation with nickel, copper, and arsenicin such minerals as cobaltite [(Co,Fe)AsS],skutterudite [(Co,Ni)As3], and erythrite(Co3(AsO4)2.8H2O). It is used in alloys formagnets, high-temperature cutting tools,and electrical heating elements. Cobaltcompounds are used in catalysts and somepaints. It is an essential dietary mineral,being a component of vitamin B12.

Symbol: Co; m.p. 1495°C; b.p.2870°C; r.d. 8.9 (20°C); p.n. 27; r.a.m.58.93320.

cobaltic oxide See cobalt(III) oxide.

cobaltous oxide See cobalt(II) oxide.

cobalt(II) oxide (cobaltous oxide;CoO) A green powder prepared by theaction of heat on cobalt(II) hydroxide inthe absence of air. Alternatively it may beprepared by the thermal decomposition ofcobalt(II) sulfate, nitrate, or carbonate,also in the absence of air. Cobalt(II) oxideis a basic oxide, reacting with acids to givesolutions of cobalt(II) salts. It is stable inair up to temperatures of around 600°C,after which it absorbs oxygen to form tri-cobalt tetroxide (Co3O4). Cobalt(II) oxidecan be reduced to cobalt by heating in astream of carbon monoxide or hydrogen. Itis used in the pottery industry and in theproduction of vitreous enamels.

cobalt(III) oxide (cobaltic oxide; Co2O3)A dark gray powder formed by the thermaldecomposition of either cobalt(II) nitrate

55

cobalt(III) oxide

or carbonate in air. If heated in air,cobalt(III) oxide undergoes further oxida-tion to give tricobalt tetroxide, Co3O4.

coherent units A system or subset ofUNITS (e.g. SI units) in which the derivedunits are obtained by multiplying or divid-ing together base units, with no numericalfactor involved.

coinage metals A group of malleablemetals forming group 11 (formerly sub-group IB) of the periodic table. They arecopper (Cu), silver (Ag), and gold (Au).These metals all have an outer s1 electronicconfiguration but they differ from the al-kali metals, which also have an outer s1

configuration, in having inner d-electrons.The coinage metals also have much higherIONIZATION POTENTIALS than the alkali met-als and high positive standard ELECTRODE

POTENTIALS, and are therefore much moredifficult to oxidize. A further significantdifference from the alkali metals is the va-riety of oxidation states observed for thecoinage metals. Thus copper in aqueouschemistry is the familiar (hydrated) blue di-valent Cu2+ ion but colorless copper(I)compounds can be prepared with groupssuch as CN– that stabilize low valences,e.g. CuCN. A few compounds ofcopper(III) have also been prepared. Thecommon form of silver is Ag(I), e.g.AgNO3, with a few Ag(II) compounds sta-ble as solids only. The most common oxi-dation state of gold is Au(III), although thecyanide ion again stabilizes Au(I) com-pounds, e.g., [Au(CN)4]3–. The metals allform a large number of coordination com-pounds, again unlike the alkali metals, andare generally described with the other el-ements of the appropriate transition series.

coke A dense substance obtained fromthe carbonization of coal. It is used as afuel and as a reducing agent.

colligative properties A group of prop-erties of solutions that depends on thenumber of particles present, rather thanthe nature of the particles. Colligativeproperties include:1. The lowering of vapor pressure.

2. The elevation of boiling point.3. The lowering of freezing point.4. Osmotic pressure.

Colligative properties are all basedupon empirical observation. The explana-tion of these closely related phenomena de-pends on intermolecular forces and thekinetic behavior of the particles, which isqualitatively similar to those used in deriv-ing the kinetic theory of gases.

collimator An arrangement for produc-ing a parallel beam of radiation for use in aspectrometer or other instrument. A sys-tem of lenses and slits is utilized.

colloid A heterogeneous system inwhich the interfaces between phases,though not visibly apparent, are importantfactors in determining the system’s proper-ties. The three important attributes of col-loids are:1. They contain particles, commonly made

up of large numbers of molecules, form-ing the distinctive unit or dispersedphase.

2. The particles are distributed in a contin-uous medium (the continuous phase).

3. There is a stabilizing agent, which has anaffinity for both the particle and themedium; in many cases the stabilizer is apolar group.

Particles in the dispersed phase typicallyhave diameters in the range 10–6–10–4 mil-limeter. Milk, rubber, and water-basedpaints are typical examples of colloids. Seealso emulsion; gel; sol.

colorimetric analysis Quantitativeanalysis in which the concentration of acolored solute is measured by the intensityof the color. The test solution can be com-pared against standard solutions.

columbium A former name for nio-bium. It is still sometimes used in metal-lurgy and mineralogy.

Symbol: Cb.

column chromatography See chro-matography; gas chromatography.

combustion A reaction with oxygen

coherent units

56

with the production of heat and light. Thecombustion of solids and liquids occurswhen they release flammable vapor, whichreacts with oxygen in the gas phase. Com-bustion reactions usually involve a com-plex sequence of free-radical chainreactions. The light is produced by excitedatoms, molecules, or ions. In highly lumi-nous flames it comes from small incandes-cent particles of carbon.

Sometimes the term is also applied toslow reactions with oxygen, and also to re-actions with other gases (for example, cer-tain metals ‘burn’ in chlorine).

common salt See sodium chloride.

complex (coordination compound) A

type of compound in which molecules orions form COORDINATE BONDS with a metalatom or ion. The coordinating species(called ligands) have lone pairs of elec-trons, which they can donate to the metalatom or ion. They are molecules such asammonia or water, or negative ions such asCl– or CN–. The resulting complex may beneutral or it may be a complex ion. For ex-ample:

Cu2+ + 4NH3 → [Cu(NH3)4]2+

Fe3+ + 6CN– → [Fe(CN)6]3–

Fe2+ + 6CN– → [Fe(CN)6]4–

The formation of such coordinationcomplexes is typical of transition metals.Often the complexes contain unpairedelectrons and are paramagnetic and col-ored. See also chelate.

57

complex

octahedral

square-planar

tetrahedral trigonal-bipyramid

X

X

X

X

X

M

X

X

X

X

M

M

X

X

X

X

X

X

X

X

MX

X

Complex: typical shapes of inorganic complexes

complex ion See complex.

component One of the separate chemi-cal substances in a mixture in which nochemical reactions are taking place. For ex-ample, a mixture of ice and water has onecomponent; a mixture of nitrogen and oxy-gen has two components. When chemicalreactions occur between the substances in amixture, the number of components is de-fined as the number of chemical substancespresent minus the number of equilibriumreactions taking place. Thus, the system:N2 + 3H2 ˆ 2NH3 is a two-componentsystem. See also phase rule.

compound A chemical combination ofatoms of different elements to form a sub-stance in which the ratio of combiningatoms remains fixed and is specific to thatsubstance. The constituent atoms cannotbe separated by physical means; a chemicalreaction is required for the compounds tobe formed or to be changed. The existenceof a compound does not necessarily implythat it is stable. Many compounds havelifetimes of less than a second. Comparemixture.

concentrated Denoting a solution inwhich the amount of solute in the solvent isrelatively high. The term is always relative;for example, whereas concentrated sulfuricacid may contain 96% H2SO4, concen-trated potassium chlorate may contain aslittle as 10% KClO3. Compare dilute.

concentration The amount of sub-stance in a solution per unit quantity of sol-vent. Molar ‘concentration’ (symbol c) isthe amount of substance per cubic meter. Itreplaces molarity, a term formerly used todescribe the amount of substance per cubicdecimeter (liter). DENSITY or mass concen-tration (symbol ρ) is kilograms of soluteper cubic meter. Molality (symbol m) ormolal concentration is the amount of sub-stance per kilogram of solute.

condensation The conversion of a gasor vapor into a liquid or solid by cooling.

conductiometric titration A titration

method in which the electrical conductanceof the reaction mixture is continuouslymeasured throughout the addition of thetitrant and well beyond the equivalencepoint. The operation is carried out in aconductance cell, which is part of a resis-tance bridge circuit. The method dependson the fact that ions have different ionicmobilities, H+ and OH– having particularlyhigh values. The method is used in place oftraditional end-point determination by in-dicators, and is especially useful for weakacid–strong base and strong acid–weakbase titrations for which color-changetitrations are unreliable.

configuration 1. (electron configura-tion) The arrangement of electrons aboutthe nucleus of an atom. Configurations arerepresented by symbols, which contain:1. An integer, which is the value of the

principal quantum number (shell num-ber).

2. A lower-case letter representing thevalue of the orbital quantum number (l),i.e.

s means l = 0, p means l = 1,d means l = 2, f means l = 3.

3. A numerical superscript giving the num-ber of electrons in that particular set; forexample, 1s2, 2p3, 3d5.The ground state electronic configura-

tion (i.e. the most stable or lowest energystate) may then be represented as follows,for example, He, 1s2; N, 1s22s22p5. How-ever, element configurations are commonlyabbreviated by using an inert gas to repre-sent the ‘core’, e.g. Zr has the configura-tion [Kr]4d25s2.2. The arrangement of atoms or groups ina molecule. See also atom.

conjugate acid See acid; base.

conjugate base See acid; base.

conservation of energy, law of Enun-ciated by Helmholtz in 1847, this lawstates that in all processes occurring in anisolated system the energy of the system re-mains constant. The law of course permitsenergy to be converted from one form to

complex ion

58

another (including mass, since energy andmass are equivalent).

conservation of mass (matter), lawof Formulated by Lavoisier in 1774, thislaw states that matter cannot be created ordestroyed. Thus in a chemical reaction thetotal mass of the products equals the totalmass of the reactants, with ‘total mass’ in-cluding any solids, liquids, and gases – suchas air – that participate in the reaction.

constantan A copper-nickel (cupron-ickel) alloy containing 45% nickel. It has ahigh electrical resistivity and very low tem-perature coefficient of resistance and istherefore used in thermocouples and resis-tors.

constant composition, law of Seeconstant proportions; law of.

constant (definite) proportions, law of(constant composition, law of) The prin-ciple that the proportion of each element ina compound is fixed or constant. It followsthat the composition of a pure chemicalcompound is independent of its method ofpreparation. The law was formulated byProust in 1779 after the analysis of a largenumber of compounds.

contact process An industrial processfor the manufacture of sulfuric acid. Sul-fur(IV) oxide and air are passed over aheated catalyst (vanadium(V) oxide orplatinum) and sulfur(VI) oxide is pro-duced:

2SO2 + O2 → 2SO3The sulfur(VI) oxide is dissolved in sulfuricacid:

SO3 + H2SO4 → H2S2O7The resulting OLEUM is then diluted to givesulfuric acid:

H2S2O7 + H2O → 2H2SO4

continuous phase See colloid.

continuous spectrum A spectrum com-posed of a continuous range of emitted orabsorbed radiation. Continuous spectraare produced in the infrared and visible re-

gions by hot solids, liquids, and pressur-ized gases. See also spectrum.

converter See Bessemer process.

coordinate bond (dative bond; dipolarbond; semipolar bond) A COVALENT

bond in which the bonding pair is visual-ized as arising from the donation of a LONE

PAIR from one species to another species,which behaves as an electron acceptor. Thedefinition includes such examples as the‘donation’ of the lone pair of the ammoniamolecule to H+ (an acceptor) to form NH4

+

or to Cu2+ to form [Cu(NH3)4]2+.The donor groups are known as Lewis

bases and the acceptors are either hydro-gen ions or LEWIS ACIDS. Simple combina-tions, such as H3N→BF3, are known asadducts. See also complex.

coordination compound See complex.

coordination number 1. The numberof atoms, molecules, or ions, surroundingany particular atom, ion, or molecule in acrystal.2. The number of coordinate bonds formedto a central atom or ion in a complex.

copper A malleable, ductile, golden-redtransition metal, the first element of group11 (formerly subgroup IB) of the periodictable. It occurs naturally principally in sul-fides such as chalcopyrite (CuFeS2), chal-cocite (Cu2S), and bornite (Cu5FeS4). It isextracted by roasting the ore in a con-trolled air supply and purified by electroly-sis of copper(II) sulfate solution usingimpure copper as the anode and pure cop-per as the cathode. Copper is used exten-sively in electrical conductors and in suchalloys as brass and bronze. It is also an ex-cellent roofing material.

Copper(I) compounds are white (exceptthe oxide, which is red), and copper(II)compounds are blue in solution. Copper isunreactive to dilute acids with the excep-tion of nitric acid. Copper(I) compoundsare unstable in solution and decompose tocopper and copper(II) ions. Both copper(I)and copper(II) ions form complexes, cop-per(II) ions being identified by the dark

59

copper

blue complex [Cu(NH3)4]2+ formed withexcess ammonia solution.

Symbol: Cu; m.p. 1083.5°C; b.p.2567°C; r.d. 8.96 (20°C); p.n. 29; r.a.m.63.546.

copper(II) carbonate (CuCO3) A greencrystalline compound that occurs in min-eral form as the basic salt in azurite andmalachite. See also verdigris.

copper(I) chloride (cuprous chloride;CuCl) A white solid, insoluble in water,prepared by heating copper(II) chloride inconcentrated hydrochloric acid with excesscopper turnings. When the solution is col-orless, it is poured into air-free water (orwater containing sulfur(IV) oxide) and awhite precipitate of copper(I) chloride isobtained. On exposure to air this precipi-tate turns green due to the formation ofbasic copper(II) chloride. Copper(I) chlo-ride is essentially covalent in structure. Itabsorbs carbon monoxide gas and is usedin the rubber industry and in organic chem-istry.

copper(II) chloride (cupric chloride;CuCl2) A compound prepared by dis-solving excess copper(II) oxide orcopper(II) carbonate in dilute hydrochloricacid. On crystallization, emerald greencrystals of the dihydrate (CuCl2.2H2O) areobtained. The anhydrous chloride may beprepared as a brown solid by heating cop-per in excess chlorine. Alternatively the di-hydrate may be dehydrated usingconcentrated sulfuric acid. Dilute solutionsof copper(II) chloride are blue, concen-trated solutions are green, and solutions inthe presence of excess hydrochloric acidare yellow.

copper(II) nitrate (cupric nitrate;Cu(NO3)2) A compound prepared bydissolving either excess copper(II) oxide orcopper(II) carbonate in dilute nitric acid.On crystallization, deep blue crystals of thetrihydrate (Cu(NO3)2.3H2O) are obtained.The crystals are prismatic in shape and areextremely deliquescent. On heating, cop-per(II) nitrate decomposes to givecopper(II) oxide, nitrogen(IV) oxide, and

oxygen. The white anhydrous salt is pre-pared by adding a solution of dinitrogenpentoxide (N2O5) in nitric acid to crystalsof the trihydrate.

copper(I) oxide (cuprous oxide; CuO) An insoluble, red, covalent solid powderprepared by the heating of copper withcopper(II) oxide or the reduction of an al-kaline solution of copper(II) sulfate. Cop-per(I) oxide is easily reduced by hydrogenwhen heated; it is oxidized to copper(II)oxide when heated in air. Copper(I) oxideundergoes DISPROPORTIONATION in acid so-lutions, producing copper(II) ions and cop-per. The oxide dissolves in concentratedhydrochloric acid due to the formation ofthe complex ion [CuCl2]–. It is used in theglass and electronics industries.

copper(II) oxide (cupric oxide; CuO) Ablack solid prepared by the action of heaton copper(II) nitrate, hydroxide, or car-bonate. It is a basic oxide and reacts withdilute acids to form solutions of copper(II)salts. Copper(II) oxide can be reduced tocopper by heating in a stream of hydrogenor carbon monoxide. It can also be reducedby mixing with carbon and heating themixture. Copper(II) oxide is stable up to itsmelting point, after which it decomposes togive oxygen, copper(I) oxide, and eventu-ally copper.

copper(II) sulfate (cupric sulfate; CuSO4)A compound prepared as the hydrate bythe action of dilute sulfuric acid on cop-per(II) oxide or copper(II) carbonate. Oncrystallization, blue triclinic crystals of thepentahydrate (blue vitriol, CuSO4.5H2O)are formed. Industrially copper(II) sulfateis prepared by passing air through a hotmixture of dilute sulfuric acid and scrapcopper. The solution formed is recycleduntil the concentration of the copper(II)sulfate is sufficient. Copper(II) sulfate isreadily soluble in water. The monohydrate(CuSO4.H2O) is formed at 100°C and theanhydrous salt at 250°C. Anhydrous cop-per(II) sulfate is white; it is extremely hy-groscopic and turns blue on absorption ofwater. It decomposes on heating to givecopper(II) oxide and sulfur(VI) oxide.

copper(II) carbonate

60

Copper(II) sulfate is used as a woodpreservative, as a fungicide in Bordeauxmixture, and in the dyeing and electroplat-ing industries.

coral A form of calcium carbonate thatis secreted by various marine animals (suchas Anthozoa) for support and living space.

corrosion Reaction of a metal with anacid, oxygen, or other compound with de-struction of the surface of the metal. Rust-ing is a common form of corrosion.

corundum (Al2O3) A naturally occur-ring mineral form of aluminum oxide thatsometimes contains small amounts of ironand silicon(IV) oxide. It is found in rocksof igneous and metamorphic origin, and inplacer deposits. RUBY and SAPPHIRE are im-pure crystalline forms. It is extremely hard(only second to diamond) and is used invarious polishes, abrasives, and grindingwheels. See also emery.

coulomb Symbol: C The SI derived unitof electric charge, equal to the chargetransported by a steady electric current ofone ampere flowing for one second. In baseunits, 1 C = 1 A s.

coulombmeter (coulometer; voltameter)A device for determining electric charge orelectric current using electrolysis. The massm of material released in time t is meas-ured, after which this value is used to cal-culate the charge (Q) and the current (I)from the electrochemical equivalent (z) ofthe element, using the formula Q = m/z orI = m/zt.

coulometer See coulombmeter.

Coulson, Charles Alfred (1910–74)British theoretical chemist. Coulson wasvery influential in promoting the use ofquantum mechanics in chemistry, particu-larly the use of molecular orbital theory.He used this theory to describe conjugatedsystems such as benzene and to justify theconcept of partial valence. Coulson wasalso able to use molecular orbital theory topredict the properties of many organic

molecules. He wrote several books, includ-ing Valence, the first edition of which waspublished in 1952. Coulson also con-tributed to the theoretical understanding ofelectrons in solids.

coupling A chemical reaction in whichtwo groups or molecules join together.

covalent bond A bond formed by thesharing of an electron pair between twoatoms. The covalent bond is convention-ally represented as a horizontal line be-tween chemical symbols, thus H–Clindicates that between the hydrogen atomand the chlorine atom there is an electronpair formed by electrons of opposite spin,implying that the binding forces arestrongly localized between the two atoms.Molecules are combinations of atomsbound together by covalent bonds; cova-lent bonding energies are of the order103kJmol–1.

Modern bonding theory treats the elec-tron pairing in terms of the interaction ofelectron (atomic) ORBITALS and describesthe covalent bond in terms of both ‘bond-ing’ and ‘anti-bonding’ molecular orbitals.See also coordinate bond; electrovalentbond; polar bond.

covalent carbide See carbide.

covalent crystal A crystal in which theatoms are covalently bonded. They aresometimes referred to as giant lattices orMACROMOLECULAR CRYSTALS. The bestknown completely covalent crystal is dia-mond.

covalent radius The radius an atom isassumed to have when involved in a cova-lent bond. For diatomic molecules com-prised of the same element (e.g. Cl2) this issimply half the measured internuclear dis-tance. For molecules comprised of differentatoms substitutional methods are used. Forexample, the internuclear distance ofbromine fluoride (BrF) is about 180 pi-cometers (pm). Therefore using 71 pm asthe covalent radius of fluorine (from F2) weget 109 pm as the covalent radius of

61

covalent radius

bromine. This is close to the accepted valueof 114 pm.

cream of tartar See potassium hydro-gentartrate.

critical point The conditions of temper-ature and pressure under which a liquidbeing heated in a closed vessel becomes in-distinguishable from its gas or vaporphase. At temperatures below the CRITICAL

TEMPERATURE (Tc) the substance can be liq-uefied by applying pressure; at tempera-tures above Tc this is not possible. For eachsubstance there is one critical point; for ex-ample, for carbon dioxide it is at 31.1°Cand 7397 kilopascals.

critical pressure The lowest pressureneeded to bring about liquefaction of a gasat its CRITICAL TEMPERATURE.

critical temperature The temperature(Tc) below which a gas can be liquefied byapplying pressure and above which noamount of pressure is sufficient to bringabout liquefaction. Some gases have criti-cal temperatures above room temperature(e.g. carbon dioxide 31.1°C and chlorine,144°C) and have thus been known in theliquid state for many years. Liquefactionproved much more difficult for those gases(e.g. oxygen, –118°C, and nitrogen,–146°C) that have very low critical tem-peratures.

critical volume The volume of onemole of a substance at its CRITICAL POINT.

crossed-beam reaction A chemical re-action performed with two molecularbeams intersecting at an angle. It is possi-ble to regard one of the beams as the inci-dent (projectile) beam and the other as thetarget beam. It is possible to control boththe incident and target beams of moleculesand a considerable amount of informationcan be obtained about the mechanism andkinetics of the chemical reaction.

cross linkage An atom or short chainjoining two longer chains in a polymer.

cryohydrate See eutectic.

cryolite See sodium hexafluoroalumi-nate.

cryoscopic constant See depression offreezing point.

crystal A solid substance that has a def-inite geometric shape. A crystal has fixedangles between its faces, which have dis-tinct edges. The crystal will sparkle if thefaces are able to reflect light. The fixed an-gles are caused by the regular arrange-ments of the atoms, ions, or molecules ofthe substance in the crystal. The faces andtheir angles bear a definite relationship tothe arrangement of these particles. If bro-ken, a large crystal will form smaller crys-tals.

crystal-field theory A theory of theproperties of metal complexes. Originallyit was introduced by Hans Bethe in 1929 toaccount for the properties of transition el-ements in ionic crystals. In the theory, theligands surrounding the metal atom or ionare thought of as negative charges, and theeffect of these charges on the energies ofthe d orbitals of the central metal ion isconsidered. Crystal-field theory was suc-cessful in describing the spectroscopic,magnetic, and other properties of com-plexes. It has been superseded by ligand-field theory, in which the overlap of the dorbitals on the metal ion with orbitals onthe ligands is taken into account.

crystal habit The shape of a crystal. Thehabit depends on the way in which thecrystal has grown; i.e. the relative rates ofdevelopment of different faces and theirproportions.

crystalline Describing a substance thathas a regular internal arrangement ofatoms, ions, or molecules, even though itmay not appear visually as geometricallyregular crystals. For instance, lead and therest of the metals are crystalline becausethey are composed of regular accumula-tions of tiny invisible crystals. Compareamorphous.

cream of tartar

62

crystallite A small rudimentary crystal.The term is often used in mineralogy to de-scribe specimens that contain accumula-tions of many minute crystals ofundetermined chemical composition andcrystal structure.

crystallization The process of formingcrystals. When a substance cools from thegaseous or liquid state to the solid state,crystallization occurs. Crystals will alsoform from a solution saturated with asolute.

crystallography The study of the for-mation, structure, and properties of crys-tals. See also x-ray crystallography.

crystalloid A substance that is not a col-loid and which will therefore not passthrough a semipermeable membrane. Seecolloid; semipermeable membrane.

crystal structure The particular repeat-ing arrangement of atoms, molecules, orions, in a crystal. ‘Structure’ refers to theinternal arrangement of particles, not theexternal appearance.

crystal system A classification of crys-tals based on the shapes of their UNIT CELL.If the unit cell is a parallelopiped withlengths a, b, and c and the angles betweenthese edges are α (between b and c), β (be-tween a and c), and γ (between a and b),then crystals can be classified as follows:cubic: a = b = c; α = β = γ = 90°tetragonal: a = b ≠ c; α = β = γ = 90°orthorhombic: a ≠ b ≠ c; α = β = γ = 90°hexagonal: a = b ≠ c; α = β = 90°; γ = 120°trigonal: a = b ≠ c; α = β = γ ≠ 90°monoclinic: a ≠ b ≠ c; α = γ = 90° ≠ βtriclinic: a ≠ b ≠ c; α ≠ β ≠ γThe orthorhombic system is also called therhombic system.

cubic close packing See face-centeredcubic crystal.

cubic crystal Denoting a crystal inwhich the UNIT CELL is a cube. In a simplecubic crystal the particles are arranged atthe corners of a cube. See also body-cen-

tered cubic crystal; face-centered cubiccrystal; crystal system. See illustrationoverleaf.

cupellation A technique used to sepa-rate relatively chemically unreactive metalssuch as silver, gold, platinum, and palla-dium from impurities by the use of an eas-ily oxidizable metal such as lead. Theimpure metal is heated in a furnace and ablast of hot air is directed upon it. The leador other oxidizable metal forms an oxide,leaving the relatively unreactive metal be-hind.

cupric chloride See copper(II) chloride.

cupric nitrate See copper(II) nitrate.

cupric oxide See copper(II) oxide.

cupric sulfate See copper(II) sulfate.

cuprite A red or reddish-black mineralform of copper(I) oxide (Cu2O). It is aprincipal ore of copper.

cupronickel A series of durable copper-nickel alloys containing up to 45% nickel.Those containing 20% and 30% nickel arevery malleable, can be worked cold or hot,and are very corrosion-resistant. They areused, for example, in condenser tubes inpower stations. Cupronickel with 25%nickel is widely used in coinage. See alsoConstantan.

cuprous chloride See copper(I) chlo-ride.

cuprous oxide See copper(I) oxide.

curie Symbol: Ci A unit of radioactivity,equivalent to the amount of a given radio-active substance that produces 3.7 × 1010

disintegrations per second, approximatelyequal to the number of disintegrations pro-duced by one gram of radium-226 per sec-ond. In SI units it is equal to 37 gigabecquerels (gBg).

Curie, Marie (1867– 1934) Polish-bornFrench chemist and physicist. Marie Curie

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Curie, Marie

was, together with her husband Pierre, apioneer in the study of radioactivity. Oneof her first discoveries was that thorium isradioactive. By conducting a very thoroughanalysis of a sample of the uranium min-eral pitchblende the Curies discovered theradioactive elements polonium and radiumin 1898. They shared the 1903 Nobel Prizefor physics with Henri Becquerel for theirwork on radioactivity. By performing ex-periments with different compounds ofuranium under various physical conditionsMarie Curie established that only theamount of uranium determined the ra-dioactivity. This established that radioac-tivity is an atomic property of uranium.Marie Curie was awarded the 1911 NobelPrize for chemistry for her discovery ofpolonium and radium.

curium A highly toxic radioactive sil-very element of the actinoid series of met-als. A transuranic element, it is foundnaturally on Earth only in very traceamounts in uranium ore. Its discovery in1944 was based on its synthesis as the re-

sult of bombarding a plutonium isotopewith helium nuclei. It can accumulate inbone marrow and destroy the body’s ca-pacity to produce red blood cells. Curium-244 and curium-242 have been used inthermoelectric power generators.

Symbol: Cm; m.p. 1340±40°C; b.p. ≅3550°C; r.d. 13.3 (20°C); p.n. 96; moststable isotope 247Cm (half-life 1.56 × 107

years).

Curl, Robert Floyd Jnr (1933– )American chemist. Curl’s initial work wason small clusters of atoms of semiconduc-tors, such as germanium and silicon. In1984, under the influence of Harry KROTO,he became interested in the possibility ofproducing long-chain carbon molecules,and persuaded his colleague Richard SMAL-LEY to deploy the resources of his labora-tory towards this end. Although theyexpected on theoretical grounds to dis-cover linear chain clusters with up to 33carbon atoms, they in fact came across anunexpected molecule with 60 carbonatoms and with a cage-like structure. The

curium

64

face-centered

body-centered simple cubic

Cubic crystal: types of unit cell

discovery of this new allotrope of carbon,later named BUCKMINSTERFULLERENE,opened up a new branch of materials sci-ence. Curl shared the 1996 Nobel Prize forchemistry with Smalley and Kroto.

cyanamide process An industrialprocess for fixing nitrogen by heating cal-cium dicarbide in air.

CaC2 + N2 → CaCN2The product, CALCIUM CYANAMIDE, hy-drolyzes to ammonia and can be used as afertilizer. The process can be expensive ifcheap electrical energy is not available toproduce CALCIUM DICARBIDE. See also nitro-gen fixation.

cyanide A salt of hydrogen cyanide,containing the cyanide ion (CN–).Cyanides are extremely poisonous due totheir ability to form coordinate bonds withthe iron in red blood cells. See hydrocyanicacid.

cyanide process A technique used forthe extraction of gold from its ores. Aftercrushing the gold ore to a fine powder it isagitated with a very dilute solution ofPOTASSIUM CYANIDE. The gold is dissolvedby the cyanide to form potassium di-cyanoaurate(I) (potassium aurocyanide,KAu(CN)2). This complex is reduced withzinc, filtered, and then melted to obtainpure gold.

cyanoferrate A compound containingthe ion [Fe(CN)6]4– (the hexacyanofer-rate(II) ion) or the ion [Fe(CN)6]3– (thehexacyanoferrate(III) ion). These ions areusually encountered as their potassiumsalts. Potassium hexacyanoferrate(II)(K4Fe(CN)6, potassium ferrocyanide) is ayellow crystalline compound. Potassiumhexacyanoferrate(III) (K3Fe(CN)6, potas-sium ferricyanide) is an orange crystallinecompound. Its solution gives a deep blueprecipitate with iron(II) ions, and is used asa test for iron(II) (ferrous) compounds.Prussian blue is a blue pigment containinghexacyanoferrate ions.

cyanogen (C2N2) A highly toxic flam-mable gas prepared by heating covalentmetal hydrides such as mercury(II)cyanide. See also pseudohalogens.

cyclic compound A compound con-taining a ring of atoms. Although the ma-jority of the cyclic compounds are organic,examples are found in inorganic com-pounds such as the silicates, silicones, andpolyphosphates.

cyclization A reaction in which astraight-chain compound is converted intoa cyclic compound.

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cyclization

dalton See atomic mass unit.

Dalton, John (1766–1844) Englishchemist. Dalton is best remembered as lay-ing the foundations for modern atomictheory. One of his first experiments was todetermine the density of water as a func-tion of temperature. In the late 18th andearly 19th centuries Dalton investigatedproperties of gases. This led to the state-ment of Dalton’s law of partial pressures in1801. Dalton first stated his atomic theoryin 1803. He gave a definitive exposition ofhis views in the book A New System ofChemical Philosophy, the first volume ofwhich was published in 1808, with subse-quent volumes being published in 1810and 1827.

Dalton’s atomic theory A theory ex-plaining the formation of compounds byelements, first published by John Dalton in1803. It was the first modern attempt todescribe chemical behavior in terms ofatoms. The theory was based on certainpostulates:1. All elements are composed of small, in-

divisible particles, which Dalton calledatoms.

2. All atoms of the same element are iden-tical and have the same properties.

3. Atoms can neither be created nor de-stroyed.

4. Atoms combine to form ‘compoundatoms’ (i.e. molecules) in simple ratios.Dalton also suggested symbols for the

elements. The theory was used to explainthe law of CONSERVATION OF MASS and thelaws of CHEMICAL COMBINATION.

Dalton’s law (of partial pressures) Thepressure of a mixture of gases in a particu-lar volume is the sum of the partial pres-

sures of each individual constituent gas,with the partial pressure of each gas beingthe pressure that it would exert if it aloneoccupied the same volume. Dalton’s law isstrictly true only for ideal gases.

Daniell cell A type of primary cell in-vented by British chemist John Daniell(1790–1845) in 1836. It consists of twoelectrodes in different electrolytes sepa-rated by a porous pot. The positive elec-trode is copper immersed in copper(II)sulfate solution and the negative zinc–mer-cury amalgam electrode is in either dilutesulfuric acid or a zinc sulfate solution. Theporous pot prevents mixing of the elec-trolytes, but allows ions to pass. With sul-furic acid the e.m.f. is about 1.08 volts;with zinc sulfate it is about 1.10 volts.

At the copper electrode copper ions insolution gain electrons from the metal andare deposited as copper atoms:

Cu2+ + 2e– → CuThe copper electrode thus gains a posi-

tive charge. At the zinc electrode, zincatoms from the electrode lose electrons anddissolve into solution as zinc ions, leavinga net negative charge on the electrode:

Zn → 2e– + Zn2+

darmstadtium A radioactive metallicelement that does not occur naturally onthe Earth. It is made either by bombardinga lead target with nickel nuclei or by bom-barding a plutonium target with sulfur nu-clei. There are several isotopes; the moststable is 269Ds, with a half-life of about 1.7 × 10–4 s. The chemical properties ofdarmstadtium should be similar to those ofplatinum.

Symbol Ds; p.n. 110.

dative bond See coordinate bond.

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D

Davy, Sir Humphry (1778–1829) Eng-lish chemist. Davy is best known for thediscovery of sodium and potassium and forinventing a safety lamp used in mines. Hebegan his researches in electrochemistry inthe early 19th century, soon after the in-vention of the electric cell. In 1807 he dis-covered sodium and potassium byelectrolysis of fused soda and potash. In1810 he showed that chlorine, which henamed, is an element and did not containoxygen, as had been thought. This helpeddestroy the theory that all acids containoxygen and prompted him to suggest thatall acids contain hydrogen.

Davy lamp See safety lamp.

d-block elements The TRANSITION EL-EMENTS of the first, second, and third longperiods of the periodic table, i.e. scandiumto zinc, yttrium to cadmium, and lan-thanum to mercury. They are so called be-cause in general they have inner d-levelswith configurations of the type (n – 1)dxns2

where x = 1–10.

deactivation A process in which the re-activity of a substance is lessened, or eventotally removed . Usually this deactivationis unwanted, as in the poisoning of cata-lysts.

de Broglie wave A wave associatedwith a particle, such as an electron or pro-ton. In 1924, Louis de Broglie (1892–1987) suggested that, since electromag-netic waves can be described as particles(photons), particles of matter could alsohave wave properties. The wavelength (λ)has the same relationship to momentum(p) as in electromagnetic radiation:

λ = h/pwhere h is the Planck constant. See alsoquantum theory.

debye Symbol: D A unit of electric DI-POLE MOMENT equal in SI units to 3.33564× 10–30 coulomb meter. It is used in ex-pressing the dipole moments of molecules.

Debye, Peter Joseph William (1884–1966) Dutch-born American chemist and

physicist. Debye made a number of impor-tant contributions to molecular structureand the theory of solids. Debye is perhapsbest known for his work with Erich HÜCKEL

in 1923 on electrolytes. In his later years hewas concerned with scattering of light andsound by liquids and with polymers. Hewon the 1936 Nobel Prize for chemistryfor his work on molecular structure and di-pole moments.

Debye–Hückel theory A theory ofweak electrolytes. It assumes that the elec-trolytes are fully dissociated and nonidealbehavior arises from electrostatic interac-tions. It is accurate only for very dilute so-lutions.

Debye–Scherrer method A methodused in X-RAY DIFFRACTION in which a crys-tal in powder form is exposed to a beam ofmonochromatic x-rays. Because the crystalis in powder form all possible orientationsof the crystal are presented to the x-raybeam. This has the result that the diffractedx-rays form cones concentric with the orig-inal beam. The Debye–Scherrer method isparticularly useful for determining the lat-tice type of a crystal and the dimensions ofits unit cell. The method was first devel-oped by Peter Debye and Paul Scherrer.

deca- Symbol: da A prefix used with SIunits, denoting 10. For example, 1 de-cameter (dam) = 10 meters (m).

decahydrate A crystalline solid contain-ing ten molecules of water of crystalliza-tion per molecule of compound.

decant To pour off the clear liquidabove a sediment or heavier, immiscibleliquid.

decay 1. The spontaneous breakdown ofa radioactive nuclide. See half-life; radioac-tivity.2. The transition of excited atoms, ions,molecules, etc., to the ground state.

deci- Symbol: d A prefix used with SIunits, denoting 10–1. For example, 1decimeter (dm) = 10–1 meter (m).

67

deci-

decomposition A chemical reaction inwhich a compound is broken down intocompounds with simpler molecules or intoelements.

decrepitation A crackling sound heardwhen certain crystalline solids are heated,usually as a result of loss of water of crys-tallization.

defect An irregularity in the orderedarrangement of particles in a crystal lattice.There are two main kinds of defects incrystals: point defects and dislocations.Point defects occur at single lattice points.There are three types. A vacancy, alsoknown as a Schottky defect, is a missingatom; i.e. a vacant lattice point. An inter-stitial is an atom that is in a position that isnot a normal lattice point. If an atommoves off its normal lattice point to an in-terstitial position the result (vacancy plusinterstitial) is called a Frenkel defect. Allsolids above absolute zero have a numberof point defects, the concentration ofwhich depends on temperature. Point de-fects can also be produced by strain or byirradiation.

Dislocations (or line defects) are alsoproduced by strain in solids. These defectsare irregularities extending over a numberof lattice points along a line of atoms.

definite proportions, law of See con-stant proportions.

degassing The removal of adsorbed,dissolved, or absorbed gases from liquidsor solids.

degenerate Describing different quan-tum states that have the same energy. Forinstance, the five d orbitals in a transition-metal atom all have the same energy butdifferent values of the magnetic quantumnumber m. Differences in energy occur if amagnetic field is applied or if the arrange-ment of ligands around the atom is notsymmetrical. The degeneracy is then saidto be ‘lifted’.

degrees absolute See absolute tempera-ture.

degrees Kelvin See absolute tempera-ture.

degrees of freedom 1. The independentways in which particles can take up energy.In a monatomic gas, such as helium orargon, the atoms have three translationaldegrees of freedom, corresponding to theirmotion in space in three mutually perpen-dicular directions (i.e. along x, y, and z co-ordinates). The mean energy per atom foreach degree of freedom is kT/2, where k isthe Boltzmann constant and T the thermo-dynamic temperature; the mean energy peratom is thus 3kT/2.

A diatomic gas, in addition to threetranslational degrees, also has two rota-tional degrees of freedom located about thetwo axes perpendicular to the bond, andone vibrational degree of freedom locatedalong the axis of the bond. The rotationaldegrees also each contribute kT/2 to the av-erage energy. The vibrational degree con-tributes kT (kT/2 for kinetic energy andkT/2 for potential energy). Thus, the aver-age energy per molecule for a diatomicmolecule is 3kT/2 (translation) + kT (rota-tion) + kT (vibration) = 7kT/2.

Linear triatomic molecules also havetwo significant rotational degrees of free-dom; nonlinear molecules have three. Fornonlinear polyatomic molecules, the num-ber of vibrational degrees of freedom is 3N– 6, where N is the number of atoms in themolecule.

The molar energy of a gas is the averageenergy per molecule multiplied by the Avo-gadro constant. For a monatomic gas, forexample, it is 3RT/2.2. The independent physical quantities(e.g. pressure, temperature, etc.) that de-fine the state of a given system. See phaserule.

dehydration 1. Removal of water froma substance.2. Removal of the elements of water (i.e.hydrogen and oxygen in a 2:1 ratio) toform a new compound.

deionization A method of removingions from a solution using ion exchange.The term is commonly applied to the pu-

decompostion

68

rification of tap water; deionized water hassuperseded distilled water in many chemi-cal applications. See ion exchange.

deliquescent Describing a solid HYGRO-SCOPIC compound that absorbs so muchwater from the atmosphere that a solutioneventually forms.

delocalization Diffusion of one or moreof a molecule’s bonding electrons over twoor more of the bonds between the mole-cule’s atoms.

delocalized bond (nonlocalized bond)A type of molecular bonding in which theelectrons forming the delocalized bond areno longer regarded as remaining betweentwo atoms but instead are spread over sev-eral atoms or even the whole molecule.

The electron density of the delocalizedbond is spread by means of a delocalizedmolecular ORBITAL and may be regarded asa series of pi bonds extending over severalatoms, for example the C–O pi bonds inthe carbonate ion. See also metallic bond;resonance.

delta brass See delta metal.

delta metal (delta brass) A strong alloyof copper and zinc, containing also a littleiron. Its main use is for making cartridgecases.

dendritic growth Growth of crystals ina branching (‘treelike’) habit.

denitrification See nitrogen cycle.

density (mass density) Symbol: ρ Themass per unit volume of a given substance.It is typically measured in grams per cubicdecimeter (g m–3), which is equivalent tograms per liter (g L–1), or in kilograms percubic meter (kg m–3). See also relative den-sity.

density functional theory A method ofcalculating the electronic structure of mol-ecules using the electron density. Thetheory was developed by Walter Kohn(1923– ) and his colleagues in the mid-

1960s and has been used extensively inchemistry and solid state physics since thattime.

depolarizer A substance used in avoltaic cell to prevent polarization. For ex-ample, hydrogen bubbles forming on theelectrode can be removed by an oxidizingagent such as manganese(IV) oxide(MnO2).

depression of freezing point A COL-LIGATIVE PROPERTY of solutions in whichthe freezing point of a given solvent is low-ered by the presence of a solute. Theamount of the reduction is proportional tothe molal concentration (m) of the solute.The depression depends only on this con-centration and is independent of solutecomposition. The formula is:

∆t = KfCM,where ∆t is the lowering of the tempera-ture, Kf is the proportionality constant(also known as the freezing point constantor the cryoscopic constant), and Cm is themolal concentration. Note that the unit ofKf is the kelvin kilogram mole–1 (K kgmol–1). The formula can be applied to themeasurement of relative molecular masswith considerable precision. A knownmass of pure solvent is slowly frozen, withstirring, in a suitable cold bath and thefreezing temperature measured using aBeckmann thermometer. A known mass ofsolute of known molecular mass is intro-duced, the solvent thawed out, and thecooling process and measurement re-peated. The addition is repeated severaltimes and an average value of Kf for thesolvent obtained by plotting ∆t against Cm.The whole process is then repeated usingthe unknown solute and its relative mo-lecular mass determined using the value ofKf previously obtained.

The effect is applied to more precisemeasurement of relative molecular mass byusing a pair of Dewar flasks (pure solventand solution) and measuring ∆t by meansof thermocouples. The theoretical explana-tion for the effect is similar to that for low-ering of vapor pressure. The freezing pointof the solvent is that point at which thecurve representing the vapor pressure

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depression of freezing point

above the liquid phase intersects the curverepresenting the vapor pressure above thefrozen solvent. The addition of solute de-presses the former curve but as the solidphase that separates is always pure solvent(i.e. above the eutectic point), there is noattendant depression of the latter curve.Consequently the point of intersection isdepressed, resulting in a lowering of thefreezing point. See also lowering of vaporpressure. Compare elevation of boilingpoint.

derivative A compound obtained by re-action from another compound. The termis most often used in organic chemistry ofcompounds that have the same generalstructure as the parent compound.

derived unit A measurement unit de-fined in terms of the BASE UNITS of a systemof measurement, and not directly from astandard value of the quantity it measures.For example, the newton is a unit of forcedefined in base SI UNITS as a kilogram metersecond–2 (kg m s–2).

desalination Any of various techniquesfor removing the salts (mainly sodiumchloride) from seawater to make it fit fordrinking, irrigation, use in water-cooledengines, and for making steam for steamturbines. There are various methods.Those based on distillation depend on acheap source of heat energy, of which solarenergy is the most promising, especially inhot climates. Flash evaporation (evapora-tion under reduced pressure) and freezingto make pure ice are other methods, as areelectrodialysis, ion exchange, reverse os-mosis, and the use of molecular sieves.

desiccation Removal of moisture froma substance.

desiccator A laboratory apparatus fordrying solids or for keeping solids free ofmoisture. It is a container in which is kepta hygroscopic material (e.g. calcium chlo-ride or silica gel).

destructive distillation The process ofheating an organic substance such that it

wholly or partially decomposes in the ab-sence of air to produce volatile products,which are subsequently condensed. The de-structive distillation of coal was theprocess for manufacturing coal gas andcoal tar. Formerly, methanol was made bythe destructive distillation of wood.

detergents A group of substances thatimprove the cleansing action of solvents,particularly water. The majority of deter-gents, including soap, have the same basicstructure. Their molecules have a nonpolarhydrocarbon chain (tail) that lacks anaffinity for water molecules and is there-fore said to be hydrophobic (water-fearingor repelling). Attached to this tail is a smallpolar group (head) that has an affinity forwater molecules and is said to be hy-drophilic (water-loving or attracting).

Detergents reduce the surface tension ofwater and thus improve its wetting powerby anchoring their hydrophilic heads in thewater with their hydrophobic tails pro-truding above it. Consequently the watersurface is broken up, enabling the water tospread over the material to be cleaned andpenetrate between the material and dirt towhich it has been exposed. With the assis-tance of agitation, the dirt can then befloated off. At the same time the hy-drophobic tails of the detergent moleculesdissolve in grease and oils. The protrudinghydrophilic heads of the molecules repeleach other, causing the grease or oil to rollup and form tiny drops, which float offinto the water to form an emulsion.

Unlike soaps synthetic detergents,which are often derived from petrochemi-cals, do not form insoluble scums withhard water. Synthetic detergents are ofthree types. Anionic detergents form ionsconsisting of a hydrocarbon chain to whichis attached either a sulfonate group,–SO2–O–, or a sulfate group, –O–SO2–O–.The corresponding metal salts are solublein water. Cationic detergents have organicpositive ions of the type RNH3

+, in whichR has a long hydrocarbon chain. Nonionicdetergents are complex chemical com-pounds called ethoxylates. They owe theirdetergent properties to the presence of anumber of oxygen atoms in one part of the

derivative

70

molecule, which are capable of forming hy-drogen bonds with the surface water mol-ecules, thus reducing the surface tension ofthe water.

deuterated compound A compound inwhich one or more 1H atoms have been re-placed by deuterium (2H) atoms.

deuteride A compound of deuteriumwith other elements; i.e. a hydride in whichthe deuterium isotope is present ratherthan 1H. See hydride.

deuterium (heavy hydrogen) Symbol: D,2H A naturally occurring, stable isotopeof hydrogen in which the nucleus containsone proton and one neutron. The atomicmass is thus approximately twice that of1H. Chemically it behaves almost identi-cally to hydrogen, forming analogous com-pounds, although reactions of deuteriumcompounds are often slower than those ofcorresponding 1H compounds. This fact ismade use of in KINETICS where the rate of areaction may depend on transfer of a hy-drogen atom (i.e. a kinetic isotope effect).

deuterium oxide (heavy water; (D2O) Anaturally occurring form of water in whichdeuterium isotopes (2H) have replaced thefar more common isotope of hydrogen(1H) in the water molecule. Deuteriumoxide represents only about 0.003% bymass of the water on Earth, from which itcan be separated by either electrolysis orfractional distillation. The chief use of deu-terium oxide is to slow down fast neutronsin nuclear fission reactors designed to useunenriched uranium (238U) as fuel.

deuteron The nucleus of the deuteriumatom, 2H+ or D+. It is used to bombardother nuclei in nuclear accelerators.

Dewar flask (vacuum flask) A double-walled container of thin glass with thespace between the walls evacuated andsealed to stop conduction and convectionof energy through it. The glass is often sil-vered to reduce radiation.

dextro-form See optical activity.

dextrorotatory See optical activity.

D-form See optical activity.

diagonal relationship There is a gen-eral trend in the periodic table for elec-tronegativity to increase from left to rightalong a period and to decrease down agroup. Thus a move of ‘one across and onedown’, i.e. a diagonal move in the table,gives rise to effects that tend to cancel eachother. There is a similar general trend andcombined effect for size, which decreasesalong a period but increases down a group.These diagonal relationships give rise tosimilarities in chemical properties, whichare particularly noticeable for the follow-ing diagonal pairs. Li–Mg; Be–Al; B–Si.Li–Mg: 1. both have carbonates that giveCO2 on heating; 2. both burn in air to givethe normal oxide only; 3. both form a ni-tride; 4. both form hydrated chlorides thathydrolyze slowly. Be–Al: 1. both give hy-drogen with alkalis; 2. both give water-in-soluble hydroxides that dissolve in alkali;3. both form complex ions of the typeMCl3–; 4. both have covalently bridgedchlorides. B–Si: 1. both form acidic oxidesof a giant covalent-molecule type with highmelting points; 2. both form low-stabilityhydrides that ignite in air; 3. both formreadily hydrolyzable chlorides that fume inair; 4. both have amorphous and crys-talline forms and both form glasses withbasic oxides, such as Na2O.

diamagnetism See magnetism.

diamond An allotrope of carbon andthe hardest naturally occurring substanceknown. It is used for jewelry and, industri-ally, for cutting and drilling equipment. Indiamond, each carbon atom is surroundedby four equally spaced carbon atomsarranged tetrahedrally. The carbon atomsform a three-dimensional network witheach carbon-carbon bond equal to 0.154nm and set at an angle of 109.5° with itsneighbors. Millions of atoms are cova-lently bonded in diamonds to form a giantmolecular structure, the great strength ofwhich results from the strong covalentbonds between carbon atoms. Diamonds

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diamond

can be formed synthetically from graphiteunder extreme temperature and pressure inthe presence of a catalyst. Although small,such diamonds are of adequate size formany industrial uses. Natural diamondsare thought to form deep in the Earth’scrust and are often associated with ancientvolcanic vents when discovered. Major de-posits occur in Africa, Australia, Brazil,Canada, and Russia. See also carbon.

diatomaceous earth See diatomite.

diatomic Describing a molecule thatconsists of two atoms. Hydrogen (H2),oxygen (O2), nitrogen (N2), and the halo-gens are examples of elements that formidatomic molecules. Sodium chloride(NaCl) is another example of a diatomicmolecule.

diatomite (diatomaceous earth; kiesel-guhr) A whitish powdered mineral con-sisting mainly of SILICON(IV) OXIDE derivedfrom the shells of diatoms. It is used tomake fireproof cements and as an ab-sorbent in the manufacture of dynamite.

dibasic acid An acid that has two activeprotons, such as sulfuric acid. Dibasicacids can give rise to two series of salts. Forexample, sulfuric acid (H2SO4) formssulfates (SO4

2–) and hydrogensulfates(HSO4

–).

diboron trioxide See boron oxide.

dicarbide See carbide.

dichlorine oxide (chlorine monoxide;chlorine(I) oxide; Cl2O) An orange gasmade by passing chlorine over mercury(II)oxide. It is a strong oxidizing agent anddissolves in water to give chloric(I) acid.

dichromate(VI) See chromium; potas-sium dichromate.

dielectric constant (relative permittiv-ity) A quantity that characterizes how amedium reduces the electric field strengthassociated with a distribution of electriccharges. If two point charges Q1 and Q2

are a distance d apart in a medium with apermittivity ε this means that the force Fbetween the charges is given by F =(1/4πε)(Q1Q2/d2).The dielectric constant (relative permittiv-ity) εr of the medium is given by εr = ε/ε0,where ε0 is the permittivity of free space.The dielectric constant of air is very slightlygreater than 1, while that of water is about80. The value of the dielectric constant of amedium has important physical and chem-ical consequences, particularly for ions insolutions.

diffusion Movement of a gas, liquid, orsolid as a result of the random thermal mo-tion of its particles (atoms or molecules). Adrop of ink in water, for example, willslowly spread throughout the liquid. Diffu-sion in solids takes place very slowly atnormal temperatures. See also Graham’slaw.

dihydrate A crystalline compound hav-ing two molecules of water of crystalliza-tion per molecule of compound.

diiodine hexachloride (iodine trichlo-ride; I2Cl6) A yellow crystalline solidmade by reacting excess chlorine with io-dine. It is a strong oxidizing agent. At 70°Cit dissociates into iodine monochloride andchlorine.

dilead(II) lead(IV) oxide (red lead;Pb3O4) A powder made by heatinglead(II) oxide or lead(II) carbonate hydrox-ide at 400°C. It is black when hot and redor orange when cold. On strong heating itdecomposes to lead(II) oxide and oxygen.Dilead(II) lead (IV) oxide is used as a pig-ment and in glass making. It is not stoi-chiometric and tends to have less oxygenthan denoted by its formula.

diluent A solvent that is added to reducethe strength of a solution.

dilute Denoting a solution in which theamount of solute is low relative the amountof solvent. The term is always relative andincludes dilution at trace level as well as thecommon term bench dilute acid, which

diatomaceous earth

72

usually means a 2M solution. Compareconcentrated.

dimensionless units Units defined interms of the ratio of two comparable quan-tities expressed in like units, which there-fore reduce to one. For example, the radianand steradian, both SI derived units, are di-mensionless because they are expressed asratios of meter per meter and square meterper square meter, respectively. See SI units.

dimer A compound (or molecule)formed by combination or association oftwo molecules of a monomer. For instance,aluminum chloride (AlCl3) is a dimer(Al2Cl6) in the vapor phase.

dimorphism See polymorphism.

dinitrogen oxide (nitrous oxide; N2O) Acolorless gas with a faintly sweet odor andtaste. It is appreciably soluble in water butmore soluble in ethanol. It is preparedcommercially by carefully heating ammo-nium nitrate:

NH4NO3(s) → N2O(g) + 2H2O(g)Dinitrogen oxide is fairly easily decom-

posed on heating to temperatures above520°C, giving nitrogen and oxygen. Thegas is used as a mild anesthetic in medicineand dentistry, being marketed in small steelcylinders. It is sometimes called laughinggas because it induces a feeling of elation.It is also used as an aerosol propellent.

dinitrogen tetroxide (N2O4) A color-less gas that becomes a pale yellow liquidbelow 21°C and solidifies below –11°C.On heating, the gas dissociates to nitrogendioxide molecules:

N2O4(g) → 2NO2(g)This dissociation is complete at 140°C.Liquid dinitrogen tetroxide has good sol-vent properties and is used as a nitratingagent.

diphosphane (diphosphine; P2H4) Ayellow liquid that can be condensed outfrom PHOSPHINE in a freezing mixture. It ig-nites spontaneously in air.

diphosphine See diphosphane.

dipolar bond See coordinate bond.

dipole–dipole interactions The inter-action of two molecules resulting fromtheir DIPOLE MOMENTS. The strength of thisinteraction depends on the size of the di-pole moments of the molecules and theirdistance apart. The dipole moments thatcan interact include permanent dipole mo-ments in molecules such as water, induceddipole moments caused by the presence ofpolar molecules in close proximity, thusgiving rise to induced dipole–dipole inter-actions, and the charge separation associ-ated with VAN DER WAALS FORCES.

dipole moment Symbols: µ, p A quanti-tative measure of polarity in either a bond(bond moment) or a molecule as a whole(molecular dipole moment). The unit is thedebye (equivalent to 3.335 64 × 10–30

coulomb meter). Molecules such as HF,H2O, and NH3, possess dipole moments;CCl4, N2, and PF5 do not.

The molecular dipole moment can beestimated by vector addition of individualbond moments if the bond angles areknown. The possession of a dipole momentpermits direct interaction with electricfields or interaction with the electric com-ponent of radiation.

Dirac, Paul Adrien Maurice (1902–84) English physicist. Dirac was one of thefounders of quantum mechanics. In a re-markable series of papers in the second halfof the 1920s he formulated quantum me-chanics in a general way that incorporatedthe matrix mechanics of Werner HEISEN-BERG and the wave mechanics of ErwinSCHRÖDINGER as special cases. He sharedthe 1933 Nobel Prize for physics withSchrödinger.

dislocation See defect.

disodium hydrogen orthophosphateSee disodium hydrogen phosphate(V).

disodium hydrogen phosphate(V) (di-sodium hydrogen orthophosphate; Na2-HPO4) A white solid prepared bytitrating phosphoric acid with sodium hy-

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disodium hydrogen phosphate(V)

droxide solution using phenolphthalein asthe indicator. On evaporation the solutionyields efflorescent monoclinic crystals ofthe dodecahydrate, Na2HPO4.12H2O. Theeffloresced salt contains 7H2O. Disodiumhydrogen phosphate is used in the textileindustry.

disodium oxide See sodium monoxide.

disodium tetraborate decahydrate(borax; Na2B4O7.10H2O) A white crys-talline solid, sparingly soluble in coldwater but readily soluble in hot water. Itoccurs naturally as salt deposits in dry lakebeds, especially in California. It is an im-portant industrial material, being used inthe manufacture of enamels and heat-resis-tant glass, as a paper glaze, and as a sourceof borium compounds. It is also used inlaundry products and as a mild antiseptic.

In solution hydrolysis occurs:B4O7

2– + 7H2O = 2OH– + 4H3BO3See also borax-bead test.

dispersed phase See colloid.

dispersing agent A compound used tohelp produce EMULSIONS or dispersions ofIMMISCIBLE liquids, such as water and oil.

dispersion force See van der Waalsforce.

displacement pump A commonly useddevice for transporting liquids and gasesaround chemical plants. It works on theprinciple of the bicycle pump: a pistonraises the pressure of the fluid and, when itis high enough, a valve opens and the fluidis discharged through an outlet pipe. As thepiston moves back the pressure falls andthe cycle continues. Displacement pumpscan be used to generate very high pressures(e.g. in the synthesis of ammonia) but be-cause of the system of valves, they are moreexpensive than other types of pump. Com-pare centrifugal pump.

displacement reaction A chemical re-action in which an atom or group displacesanother atom or group from a molecule. A

common example is the displacement ofhydrogen from acids by metals:

Zn + 2HCl → ZnCl2 + H2See also double decomposition.

disproportionation A chemical reac-tion in which there is simultaneous oxida-tion and reduction of the same compound.An example is the reaction of copper(I)chloride to copper and copper(II) chloride:

2CuCl → Cu + CuCl2in which there is simultaneous oxidation(to Cu(II)) and reduction (to Cu(0)).

Another example is the reaction ofchlorine with water:

Cl2 + H2O → 2H+ + Cl– + ClO–

in which there is reduction to Cl– and oxi-dation to ClO–.

dissociation Breakdown of a moleculeinto two molecules, atoms, radicals, orions. Often the reaction is reversible, as inthe ionic dissociation of weak acids inwater:

HCN + H2O ˆ H3O+ + CN–

See also dissociation constant.

dissociation constant Symbol: K TheEQUILIBRIUM CONSTANT of a reversible dis-sociation reaction. For example, the disso-ciation constant of a reaction:

AB ˆ A + Bis given by:

K = [A][B]/[AB]where the brackets denote concentration(activity).

Often the degree of dissociation is used– the fraction (α) of the original compoundthat has dissociated at equilibrium. For anoriginal amount of AB of n moles in a vol-ume V, the dissociation constant is givenby:

K = α2n/(1 – α)VNote that this expression is for dissociationinto two molecules.

Acid dissociation constants (or acidityconstants, symbol: Ka) are dissociationconstants for the dissociation into ions insolution:

HA + H2O ˆ H3O+ + A–

If the concentration of water is taken asunity, the acidity constant is given by:

Ka = [H3O+][A–]/[HA]

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74

The acidity constant is a measure of thestrength of the acid. Base dissociation con-stants (symbol Kb) are similarly defined.The expression:

K = α2n/(1 – α)Vapplied to an acid is known as Ostwald’sdilution law. In particular if α is small (aweak acid) then K = α2n/V, or α = C√V,where C is a constant. The degree of disso-ciation is then proportional to the squareroot of the dilution.

distillation The process of boiling a liq-uid and condensing the vapor. Distillationis used to purify liquids or to separate com-ponents of a liquid mixture. See also frac-tional distillation; steam distillation;vacuum distillation.

distilled water Water that has been pu-rified by distillation in order to separate itfrom dissolved solids or other substances.Laboratory-grade water is usually distilledseveral times.

disulfur dichloride (sulfur monochlo-ride; S2Cl2) A red fuming liquid with astrong smell. It is prepared by passing chlo-rine over molten sulfur and is used toharden (vulcanize) rubber.

dithionate A salt of dithionic acid.Dithionates are reducing agents.

dithionic acid (hyposulfuric acid;H2S2O6) A strong sulfur oxyacid that de-composes slowly on standing or heating.

dithionite (sulfinate) A salt of dithio-nous acid. Dithionites are powerful reduc-ing agents.

dithionous acid (sulfinic acid; hyposul-furous acid; H2S2O4) An unstable sulfuroxyacid that is known only in solution.

divalent (bivalent) Having a valence oftwo.

dl-form See optical activity.

Döbereiner’s triads Groups of threechemically similar elements in which the

central member, when placed in order ofincreasing relative atomic mass, has a rela-tive atomic mass approximately equal tothe average of the outer two. Other chemi-cal and physical properties of the centralmember also lie between those of the firstand last members of the triad. Germanchemist Johann Döbereiner (1780–1849)noted this relationship in 1817; each triadis now recognized as comprised of consec-utive members of a group of the periodictable; e.g. calcium, strontium, and barium;chlorine, bromine, and iodine.

dolomite (pearl spar) A typically white,pink, or colorless mineral, CaCO3.-MgCO3, used as an ore of magnesium andfor the lining of open-hearth steel furnacesand Bessemer converters.

donor 1. The atom, ion, or moleculethat provides the pair of electrons in form-ing a covalent bond.2. The impurity atoms used in dopingsemiconductors.

doping The incorporation of impuritieswithin the crystal lattice of a solid so as toalter its physical properties. For instance,silicon, when doped with boron, becomessemiconducting.

double bond A covalent bond betweentwo atoms that includes two pairs of elec-trons, one pair being the single bond equiv-alent (the sigma bond) and the otherforming an additional bond, the pi bond (πbond). It is conventionally represented bytwo lines, for example H2C=O. See multi-ple bond; orbital.

double decomposition (metathesis) Achemical technique for making an insolu-ble salt by mixing two soluble salts. Theions from the two reactants ‘change part-ners’ to form a new soluble compound andan insoluble compound, which is precipi-tated. For example, mixing solutions ofpotassium iodide (KI) and lead(II) nitrate(Pb(NO3)2) forms a solution of potassiumnitrate (KNO3) and a yellow precipitate oflead(II) iodide (PbI2).

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double decompostion

double salt When equivalent quantitiesof certain salts are mixed in aqueous solu-tion and the solution evaporated, a salthaving two different anions or cations mayform, e.g. FeSO4.(NH4)2SO4.6H2O. Inaqueous solution the salt behaves as a mix-ture of the two individuals. These salts arecalled double salts to distinguish themfrom complex salts, which yield complexions in solution. See also alum.

Downs process An electrolytic processfor making sodium and chlorine fromfused sodium chloride. The process takesplace in a Downs cell, which has a centrallylocated graphite anode surrounded by acylindrical steel cathode. A coaxial steelgrid keeps the electrodes and their prod-ucts separate. Chlorine gas is collected viaa cone-shaped dome above the anode,while molten sodium floats on top of thedenser electrolyte to be siphoned off via acollecting pipe.

Dow process An industrial methodwhereby magnesium is extracted from sea-water via the precipitation of magnesiumhydroxide (Mg(OH)2) when calcium hy-droxide (Ca(OH)2) is added, followed bySOLVATION of the precipitated hydroxideby hydrochloric acid (HCl).

dry cell A voltaic cell in which the elec-trolyte is typically in the form of a jelly orpaste and the metal container holding theelectrolyte forms the negative electrode.Dry cells are extensively used for flash-lights, toys, and other portable applica-tions.

dry ice See carbon dioxide.

dubnium A radioactive synthetic trans-actinide element first made at Dubna, a

town near Moscow, in 1967 by bombard-ing americium ions with neon ions. It wasalmost simultaneously made in the U.S.A.by bombarding californium nuclei with ni-trogen nuclei.

Symbol: Db; m.p., b.p., and r.d. un-known, p.n. 105; most stable isotope262Db (half-life 34 s).

Dulong and Petit’s law The law thatstates that the molar heat capacity of asolid element is approximately equal to3R, where R is the GAS CONSTANT (25 J K–1

mol–1). The law applies only to elementswith simple crystal structures at normaltemperatures. At lower temperatures themolar heat capacity falls with decreasingtemperature. Molar heat capacity was for-merly called atomic heat – the product ofthe relative atomic mass of a substance andits specific heat capacity.

The law was derived by Frenchchemists Pierre Dulong (1785–1838) andAlexis Petit (1791–1820) in 1819.

Duralumin (Trademark) A strong light-weight aluminum alloy containing 3–4%copper, with small amounts of magnesium,manganese, and sometimes silicon. It iswidely used in aircraft bodies and similarapplications requiring strength combinedwith lightness.

dysprosium A very soft malleable sil-very-white element of the lanthanoid seriesof metals. It occurs in association withother lanthanoids. It is used in lasers andcompact disks, and as a neutron absorberin nuclear reactors; it is also a constituentof certain magnetic alloys.Symbol: Dy; m.p. 1412°C; b.p. 2562°C;r.d. 8.55 (20°C); p.n. 66; most commonisotope 164 Dy; r.a.m. 162.50.

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76

ebullioscopic constant See elevation ofboiling point.

ebullition The boiling or bubbling of aliquid.

Edison cell See nickel–iron accumula-tor.

edta Abbreviation for ethylenedi-aminetetraacetic acid, a tetrabasic car-boxylic acid. It is important in inorganicchemistry because the carboxylate ion canact as a ligand in forming complexes. Edtais a hexadentate ligand, able to coordinateto a metal ion at the four COO– groups andthe two nitrogen atoms. It is able to formoctahedral complexes with certain metalions.

effervescence The evolution of gas inthe form of bubbles in a liquid.

efficiency Symbol: η A measure used forprocesses of energy transfer; the ratio ofthe useful energy produced by a system ordevice to the energy input. For a reversibleheat engine, the idealized maximum effi-ciency is given by

η = (T1 – T2)/T1where T1 is the temperature of the heat assourced and T2 is the temperature of the

heat given out at the end of the process. Seealso Carnot’s principle; Carnot cycle.

efflorescence The process in which acrystalline hydrated solid loses water ofcrystallization to the air. A powdery de-posit is gradually formed on the surface ofthe compound.

Eigen, Manfred (1927– ) Germanchemist. Eigen was a pioneer of the studyof very fast chemical reactions. Starting in1954, he studied such reactions by relax-ation techniques in which very quickchanges in the pressure, temperature, andelectric field were applied to the system andthe consequences studied using spec-troscopy. He shared the 1967 Nobel Prizefor chemistry with Ronald NORRISH andGeorge PORTER for this work. SubsequentlyEigen used relaxation techniques to studyvery fast biochemical reactions.

eigenfunction A function that is asolution of an EIGENVALUE equation. An ex-ample of an eigenfunction is the WAVE-FUNCTION for a quantum mechanicalsystem described by the SCHRÖDINGER

EQUATION.

eigenvalue One of the set of possiblevalues of numbers that make up the solu-

77

E

N

C

CH2

H2CH2

HOOC

HOOC

CH2

CH2

N

COOH

COOH

CH2

Edta

tions of an eigenvalue equation. An eigen-value equation has the general form: Ôψ =Eψ, where Ô is an operator which operateson the EIGENFUNCTION ψ and E is a numberwhich multiplies the same eigenfunction.The solution of an eigenvalue equationconsists in finding the sets of eigenfunc-tions ψ and eigenvalues E that satisfy theeigenvalue equation. The fundamentalequations of quantum mechanics such asthe SCHRÖDINGER EQUATION are eigenvalueequations. In the case of the Schrödingerequation the eigenfunctions are the WAVE-FUNCTIONS for the quantum states of thesystem and the eigenvalues are the valuesof the quantized energy levels associatedwith these quantum states.

einsteinium A radioactive transuranicelement of the actinoid series, not foundnaturally on Earth and originally discov-ered in the radioactive fallout of the firstlarge hydrogen bomb. It can be producedin milligram quantities by bombarding239Pu with neutrons to give 253Es (half-life20.47 days). Several other short-lived iso-topes have also been synthesized.

Symbol: Es; m.p. 860 ± 30°C; b.p. andr.d. unknown; p.n. 99; most stable isotope254Es (half-life 276 days).

electrochemical equivalent Symbol:z The mass of an element released from asolution of its ions when a current of oneampere flows for one second during elec-trolysis (i.e. by one coulomb of electricity).

electrochemical series (activity series;electromotive series) A series giving theactivities of metals for reactions that in-volve ions in solution. In decreasing orderof activity, the series for the most commer-cially important metals is

K, Ba, Ca, Na, Mg, Al, Mn, Zn, Cr, Fe, Cd, Co, Ni, Sn,Pb, H, Cu, Hg, Ag, Pt, Au

Any member of the series will displace ionsof a lower member from solution. For ex-ample, zinc metal will displace Cu2+ ions:

Zn(s) + Cu2+(aq) → Zn2+(aq) + Cu(s)Zinc has a greater tendency than copper

to form positive ions in solution. Similarly,

metals above hydrogen displace hydrogenfrom acids:

Zn + 2HCl → ZnCl2 + H2The series is based on ELECTRODE POTEN-TIALS, which measure the tendency of el-ements to form positive ions. The series isone of increasing electrode potential forhalf cells of the type Mn+|M. Thus, copper(EŠ for Cu2+|Cu = + 0.34 V) is lower thanzinc (EŠ for Zn2+|Zn = –0.76V). The hy-drogen half cell by definition has the valueEŠ = 0.

electrochemistry The study of the for-mation and behavior of ions in solutions. Itincludes electrolysis and the generation ofelectricity by chemical reactions in cells.

electrochromatography See electro-phoresis.

electrode Any part of an electrical de-vice or system that emits or collects elec-trons or other charge carriers. An electrodemay also be used to deflect charged parti-cles by the action of the electrostatic fieldthat it produces. See also half cell.

electrodeposition (electroplating) Theprocess of depositing a layer of solid(metal) on an electrode by means of elec-trolysis. Positive ions in solution gain elec-trons at the cathode and are deposited as atoms. Copper, for instance, can bedeposited on a metal cathode from an acid-ified copper sulfate solution. Electro-deposition allows metals to be plated with other, more decorative or corrosion-resistant metals, e.g. chromium on steel.

electrode potential (reduction potential)Symbol: E A measure of the tendency ofan element to form ions in solution. For ex-ample, a metal in a solution containing M+

ions may dissolve in the solution as M+

ions; the metal then has an excess of elec-trons and the solution an excess of positiveions. Thus, the metal becomes negativewith respect to the solution. Alternatively,the positive ions may gain electrons fromthe metal and be deposited as metal atoms.In this case, the metal becomes positivelycharged with respect to the solution. In ei-

einsteinium

78

ther case, a potential difference is devel-oped between solid and solution, and anequilibrium state will be reached at whichfurther reaction is prevented. The equilib-rium value of this potential differencewould give an indication of the tendency toform aqueous ions.

It is not, however, possible to measurethis value for an isolated half cell, becauseany measurement requires a circuit, whichsets up another half cell in the solution. Toovercome this limitation, electrode poten-tials are defined by comparison with a HY-DROGEN ELECTRODE (a type of half cell),which is connected to the half cell underinvestigation by a salt bridge. The e.m.f. ofthe full cell can then be measured.

In referring to a given half cell the morereduced form is written on the right for ahalf-cell reaction. For the half cell Cu2+|Cu,the half-cell reaction is a reduction:

Cu2+(aq) + 2e– → CuThe cell formed in comparison with a hy-drogen electrode is:

Pt(s)H2(g)|H+(aq)|Cu2+(aq)|CuThe e.m.f. of this cell is +0.34 volt (V)measured under standard conditions.Thus, the standard electrode potential(symbol: EŠ) is +0.34 V for the half cellCu2+|Cu. The standard conditions are 1.0molar solutions of all ionic species, stan-dard pressure, and a temperature of 298 K.

Half cells can also be formed by a solu-tion of two different ions (e.g. Fe2+ andFe3+). In such cases, a platinum electrode, ahalf cell employing a platinum plate, isused under standard conditions.

electrodialysis A method of removingions from water by selective flow throughmembranes under the influence of an elec-tric field. In the simplest arrangement a cellis divided into three compartments by twosemipermeable membranes, one permeableto positive ions and the other permeable tonegative ions. Electrodes are placed in theouter compartments of the cell – the posi-tive electrode next to the membrane thatallows negative ions to pass, and the nega-tive electrode next to the membrane thatallows positive ions to pass. In thisarrangement, positive and negative ionspass through the membranes, leaving

deionized water in the center compartmentof the cell. In practice, an array of alternat-ing membranes is used. Electrodialysis isused to make drinking water in areaswhere the available water supply is brack-ish. See also desalination.

electrolysis The production of chemicalchange by passing electric charge throughcertain conducting liquids known as elec-trolytes. Electrolytes may be solutions ormolten salts, etc. The current is conductedby the migration of ions – positive ones(cations) to the cathode (negative elec-trode), and negative ones (anions) to theanode (positive electrode). Reactions takeplace at the electrodes via the transfer ofelectrons to or from them.

In the electrolysis of water (containing asmall amount of acid to make it conductadequately) hydrogen gas is given off at thecathode and oxygen is evolved at theanode. At the cathode the reaction is:

H+ + e– → H2H → H2(g)

At the anode:OH– → e– + OH

2OH → H2O + O2O → O2(g)

In certain cases the electrode materialmay dissolve. For instance, in the electrol-ysis of copper(II) sulfate solution with cop-per electrodes, copper atoms of the anodedissolve as copper ions:

Cu → 2e– + Cu2+

See Faraday’s laws.

electrolyte A liquid containing positiveand negative ions that conducts electricityby the flow of those charges. Electrolytescan be solutions of acids or metal salts(‘ionic compounds’), usually in water. Al-ternatively they may be molten ionic com-pounds, which also allow ions to movefreely. Liquid metals in which conductionis by free electrons rather than ions are notclassified as electrolytes. See also electroly-sis.

electrolytic Relating to the behavior orreactions of ions in solution.

electrolytic cell See cell; electrolysis.

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electrolytic cell

electrolytic corrosion Corrosion byelectrochemical reaction, e.g. rusting.

electrolytic refining A method of puri-fying metals by electrolysis. Copper is puri-fied by making the impure metal the anodein an electrolytic cell containing an acidi-fied copper sulfate electrolyte. The cathodeis a thin strip of pure copper. The copper atthe anode dissolves as Cu2+ ions and purecopper is deposited on the cathode. In thisparticular process, gold and silver are ob-tained as by-products, deposited as ananode sludge on the bottom of the cell.

electromagnetic radiation Energy wavespropagated by electric and magnetic fieldsthat oscillate at right angles to one anotheras they travel through space. Electromag-netic radiation forms a whole electromag-netic spectrum, depending on frequency,ranging from high-frequency radio wavesto low-frequency gamma rays.

Electromagnetic radiation can bethought of as waves (electromagneticwaves) or as streams of photons. The fre-quency (v) and wavelength (λ) are relatedby

λv = cwhere c is the speed of light. The energycarried depends on the frequency.

electromagnetic spectrum See electro-magnetic radiation.

electromagnetic waves See electro-magnetic radiation.

electromotive force (e.m.f.) The great-

est potential difference that can be suppliedby a source of electrical energy. The unit isthe volt (V).

electromotive series See electrochemi-cal series.

electron An elementary particle of nega-tive charge (–1.602 192 × 10–19 coulomb)and rest mass 9.109 558 × 10–31 kilogram.Electrons are present in all atoms in shellsaround the nucleus.

electron affinity Symbol: A The energyreleased when an atom (or molecule orgroup) gains an electron in the gas phase toform a negative ion. It is thus the energy of:

A + e– → A–

A positive value of A (often in electron-volts) indicates that heat is given out. Oftenthe molar enthalpy is given for this processof electron attachment (∆H). Here theunits are joules per mole (J mol–1), and, bythe usual convention, a negative value indi-cates that energy is released.

electron configuration See configura-tion.

electron-deficient compounds Com-pounds in which the number of electronsavailable for bonding is insufficient for thebonds to consist of conventional two-elec-tron covalent bonds. In diborane, B2H6,for example, each boron atom has two ter-minal hydrogen atoms bound by conven-tional electron-pair bonds and in additiontwo hydrogen atoms bridging the boronatoms (B–H–B). In each bridge there are

electrolytic corrosion

80

ELECTROMAGNETIC SPECTRUM(note: the figures are only approximate)

Radiation Wavelength (m) Frequency (Hz)gamma radiation –10–10 1019–

x-rays 10–12 –10–9 1017 –1020

ultraviolet radiation 10–9 –10–7 1015 –1017

visible radiation 10–7 –10–6 1014 –1015

infrared radiation 10–6 –10–4 1012 –1014

microwaves 10–4 –1 109 –1013

radio waves 1 – –109

only two electrons for the bonding orbital.See also boron hydride; multicenter bond.

electron diffraction A technique usedto help determine the structure of sub-stances, principally the shapes of moleculesin the gaseous phase. A beam of electronsdirected through a gas at low pressure pro-duces a series of concentric rings on a pho-tographic plate. The dimensions of theserings are related to the interatomic dis-tances in the molecules. See also x-ray dif-fraction.

electron donor See reduction.

electronegative Describing an atom ormolecule that attracts electrons, formingnegative ions. Examples of electronegativeelements include the halogens (fluorine,chlorine etc.), which readily form negativeions, e.g. F–, Cl–, etc. See also electronega-tivity.

electronegativity A measure of the ten-dency of an atom in a molecule to attractelectrons to itself. Elements to the right-hand side of the periodic table are stronglyelectronegative (values from 2.5 to 4);those on the left-hand side have low elec-tronegativities (0.8–1.5) and are sometimescalled ELECTROPOSITIVE elements. Differentelectronegativities of atoms in the samemolecule give rise to polar bonds andsometimes to polar molecules.

As the concept of electronegativity isnot precisely defined it cannot be preciselymeasured and several electronegativityscales exist. Although the actual values be-tween these scales differ, they are in goodrelative agreement. See also electron affin-ity; ionization potential.

electronic energy level See energylevel.

electronic spectra of molecules Thespectra associated with transitions betweenthe electronic states of molecules. Thesetransitions correspond to the visible or ul-traviolet regions of the electromagneticspectrum. There are changes in vibrationaland rotational energy when electronic

transitions occur. This has the effect thatthere are spectral bands associated withchanges in vibrational motion, with thesebands having fine structure due to changesin rotational motion. Because electronictransitions are associated with changes invibrational motion the corresponding spec-tra are sometimes called vibrational spec-tra. In contrast to the cases of rotationaland vibrational spectroscopy, all moleculescan give rise to electronic spectra. The elec-tronic spectra of molecules are used to ob-tain information about energy levels inmolecules, interatomic distances, dissocia-tion energies of molecules, and force con-stants of chemical bonds.

electronic transition The demotion orpromotion of an electron between elec-tronic energy levels in an atom or molecule.

electron pair Two electrons in one or-bital with opposing spins (spin paired),such as the electrons in a covalent bond orlone pair.

electron spin See spin.

electron spin resonance (ESR) A tech-nique similar to that used in NUCLEAR MAG-NETIC RESONANCE, but applied to unpairedelectrons in a molecule rather than to thenuclei. It is a powerful means by which tostudy free radicals and transition-metalcomplexes.

electron-transfer reaction A chemicalreaction that involves the transfer, addi-tion, or removal of electrons. Many elec-tron-transfer reactions involve complexesof transition metals. The rates of such re-actions vary enormously and can be ex-plained in terms of the way in whichmolecules of the solvent that start off sol-vating the reactants rearrange so as to sol-vate the products.

electronvolt Symbol: eV A unit of en-ergy equal in SI derived units to 1.602192× 10–19 joule. It is defined as the energy re-quired to move an electron charge across apotential difference of one volt. It is oftenused to measure the kinetic energies of ele-

81

electronvolt

mentary particles or ions, or the ionizationpotentials of molecules.

electrophoresis The application of anelectric field between two electrodes lo-cated at either side of a solution in order tocause charged particles of a colloid to movethrough the solution. The technique is usedto separate and identify colloidal sub-stances such as carbohydrates, proteins,and nucleic acids. Various experimentalarrangements are used. One simple tech-nique uses a strip of adsorbent papersoaked in a buffer solution with electrodesplaced at two points on the paper. Thistechnique is sometimes called electrochro-matography. In gel electrophoresis, used toseparate DNA fragments, the solution isreplaced by a layer of gel.

electroplating The process of coating asolid surface with a layer of metal bymeans of electrolysis (i.e. by electrodeposi-tion).

electropositive Describing an atom ormolecule that tends to lose electrons, form-ing positive ions. Examples of electroposi-tive elements include the alkali metals(lithium, sodium, etc.), which readily formpositive ions, e.g. Li+, Na+, etc. See alsoelectronegative.

electrovalent bond (ionic bond) Abinding force between the ions in com-pounds in which the ions are formed bycomplete transfer of electrons from one el-ement to another element or radical. Forexample, Na + Cl becomes Na+ + Cl–. Theelectrovalent bond arises from the excessof the net attractive force between the ionsof opposite charge over the net repulsiveforce between ions of like charge. The mag-nitude of electrovalent interactions is of theorder 102–103 kJ mol–1 and electrovalentcompounds are generally solids with rigidlattices of closely packed ions. Thestrengths of electrovalent bonds vary withthe reciprocal of the inter-ionic distancesand are discussed in terms of lattice ener-gies.

electrum 1. A naturally occurring or ar-

tifically created alloy of gold and silver(containing up to 45% silver) that resem-bles pure gold in appearance.2. A NICKEL-SILVER containing 52% copper,26% nickel, and 22% zinc.

element A substance that cannot bechemically decomposed into more simplesubstances. The atoms of an element allhave the same number of protons (given bythe element’s proton number) and thus thesame number of electrons, which deter-mines the element’s chemical activity.

At present there are 114 reported chem-ical elements, although research is continu-ing to synthesize new ones. The elementsfrom hydrogen (proton number 1) to ura-nium (92) all occur naturally on Earth,with the exception of technetium (43),which is produced artificially by particlebombardment. Neptunium (93), pluto-nium (94), americium (95) and curium (96)also occur naturally in very small quanti-ties in uranium ores. Technetium and el-ements with proton numbers higher than84 (polonium) are radioactive. Radioactiveisotopes also exist for other elements, ei-ther naturally in small amounts or synthet-ically via particle bombardment. Theelements with proton number higher than92 are the transuranic elements. Thetransuranics are all synthesized. Thus, nep-tunium and plutonium can be made byneutron bombardment of uranium nucleiwhile the other transuranics are made byhigh-energy collision processes betweennuclei. The higher proton number elementshave been detected only in extremelyminute quantities – in some cases, only afew atoms have been produced.

There has been some controversy aboutthe naming of the higher synthetic elementsbecause of disputes about their discovery.A long-standing problem concerned el-ement-104 (rutherfordium) which was for-merly also known by its Russian name ofkurchatovium. More recent confusion hasbeen caused by differences between namessuggested by the International Union ofPure and Applied Chemistry (IUPAC) andthe names suggested by the AmericanChemical Union (ACU).The original IUPAC names (1994) were:

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82

mendelevium (Md, 101) nobelium (No, 102) lawrencium (Lr, 103) dubnium (Db, 104) joliotium (Jl, 105) rutherfordium (Rf, 106) bohrium (Bh, 107) hahnium (Hn, 108) meitnerium (Mt, 109)

The ACU names were: mendelevium (Md, 101) nobelium (No, 102) lawrencium (Lr, 103) rutherfordium (Rf, 104) hahnium (Ha, 105) seaborgium (Sg, 106) nielsbohrium (Ns, 107) hassium (Hs, 108) meitnerium (Mt, 109)

A compromise list of names was adoptedby IUPAC in 1997:

mendelevium (Md, 101) nobelium (No, 102) lawrencium (Lr, 103) rutherfordium (Rf, 104) dubnium (Db, 105) seaborgium (Sg, 106) bohrium (Bh, 107) hassium (Hs, 108) meitnerium (Mt, 109)

These are the names used in this dictionary.Dubnium is named after Dubna in Russia,where much work has been done on syn-thetic elements. Hassium is the Latin namefor the German state of Hesse, where theGerman synthetic element research groupis located.

In addition to the above elements, fiveother elements have been reported withproton numbers 110, 111, 112, 114, and116. Element 110 has been named darm-stadtium (Ds, 110). So far the balance ofthese elements have not been named andhave their temporary systematic IUPACnames (see unun-).These are: unununium (Uuu, 111), unun-bium (Uub, 112), ununquadium (Uuq,114) and ununhexium (Uuh, 116).

elevation of boiling point A colliga-tive property of solutions in which the boil-ing point of a solution is raised relative tothat of the pure solvent. The elevation is di-

rectly proportional to the number of solutemolecules introduced rather than to anyspecific aspect of the solute composition.The proportionality constant, kB, is calledthe boiling-point elevation constant orsometimes the ebullioscopic constant. Therelationship is

∆t = kBCmwhere ∆t is the rise in boiling point and Cmis the molal concentration; the units of kBare kelvins kilograms moles–1 (K kg mol–1).The property permits the measurement ofthe relative molecular mass of involatilesolutes. Compare depression of freezingpoint.

Elinvar (Trademark) A steel alloy ofchromium, iron, and nickel containingsome tungsten and manganese. It is usedfor making hairsprings for clocks andwatches because its elasticity does not varywith temperature.

elixir of life See alchemy.

elution The removal of an adsorbedsubstance in a chromatography column orion-exchange column using a solvent (elu-ent), giving a solution called the eluate. TheCHROMATOGRAPHY column can selectivelyadsorb one or more components from themixture. To ensure efficient recovery ofthese components graded elution is used.The eluent is changed in a regular mannerstarting with a nonpolar solvent and grad-ually replacing it by a more polar one. Thiswill wash the strongly polar componentsfrom the column.

emery A mineral of corundum (alu-minum oxide, A12O3) containing somemagnetitie (Fe3O4), hematite (Fe2O3), orspinel (MgAl2O4). It is extremely hard andis used as an abrasive and polishing ma-terial.

e.m.f. See electromotive force.

emission spectrum See spectrum.

empirical formula The formula of acompound showing the simplest ratio ofthe atoms present. The empirical formula

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empirical formula

is the formula obtained by experimentalanalysis of a compound. It can be related toa molecular formula only if the relativemolecular mass is known. For exampleP2O5 is the empirical formula of phospho-rus(V) oxide although its molecular for-mula is P4O10. Compare molecularformula; structural formula.

emulsion A colloid in which a liquidphase (small droplets with a diameterrange 10–5–10–7 centimeter) is dispersed orsuspended in a liquid medium. Emulsionsare classed as lyophobic (solvent-repellingand generally unstable) or lyophilic (sol-vent-attracting and generally stable).

en See ethylenediamine.

enantiomer (enantiomorph) A com-pound whose structure is not superimpos-able on its mirror image: one of any pair ofoptical isomers. See also isomerism; opticalactivity.

enantiomorph See enantiomer.

enantiotropy The existence of differentstable allotropes of an element at differenttemperatures; sulfur, for example, exhibitsenantiotropy. The phase diagram for anenantiotropic element has a point at whichall the allotropes can coexist in a stableequilibrium. At temperatures above orbelow this point, one of the allotropes willbe more stable than the other(s). See alsoallotropy; monotropy.

enclosure compound See clathrate.

endothermic Describing a process inwhich heat is absorbed (i.e. heat flowsfrom outside the system, or the tempera-ture falls). The dissolving of a salt in water,for instance, is often an endothermicprocess. Compare exothermic.

end point See equivalence point; volu-metric analysis.

energy Symbol: W A property of a sys-tem; a measure of its capacity to do work.Energy and work have the same SI derived

unit: the joule (J). It is convenient to divideenergy into kinetic energy (energy of mo-tion) and potential energy (‘stored’ en-ergy). Names are given to many differentforms of energy depending on their source,i.e. chemical, electrical, nuclear, etc.; theonly real difference lies in the system underdiscussion. For example, chemical energyis the kinetic and potential energies of elec-trons in a chemical compound.

energy level One of the discrete energiesthat an atom or molecule, for instance, canhave according to quantum theory. Thus inan ATOM there are certain definite shellsand orbitals that the electrons can be in,corresponding to definite electronic energylevels of the atom. Similarly, a vibrating orrotating molecule can have discrete vibra-tional and rotational energy levels.

energy profile A diagram that traces thechanges in the energy of a system duringthe course of a reaction. Energy profiles areobtained by plotting the potential energyof the reacting particles against the reac-tion coordinate, i.e. the pathway for whichthe energy is a minimum. To obtain the re-action coordinate the energy of the total in-teracting system is plotted against positionfor the molecules.

enthalpy Symbol: H The sum of the in-ternal energy (U) and the product of pres-sure (p) and volume (V) of a system:

H = U + pVIn a chemical reaction carried out at

constant pressure, the change in enthalpymeasured is the internal energy change plusthe work done by the volume change:

∆H = ∆U + p∆VIn SI units enthalpy is measured in joules(J) or kilojoules (kJ).

entropy Symbol: S In any system thatundergoes a reversible change, the changeof entropy is defined as the heat Q ab-sorbed divided by the thermodynamic tem-perature T:

∆S = ∆Q/TA given system is said to have a certain en-tropy, although absolute entropies are sel-dom used: it is rather the change in entropy

emulsion

84

that is important, particularly as the en-tropy of a system measures the availabilityof energy to do work.

In any real (i.e. irreversible) change in aclosed system entropy always increases. Al-though the total energy of the system hasnot changed, according to the first law ofTHERMODYNAMICS, the available energy isless as a consequence of the second law ofthermodynamics.

The concept of entropy has beenwidened to take in the general idea of dis-order – the higher the entropy, the moredisordered the system. For instance, achemical reaction involving polymeriza-tion may well have a decrease in entropybecause there is a change to a more orderedsystem. The ‘thermal’ definition of entropyis actually a special case of this idea of dis-order, because it explains how transferredenergy is distributed among particles ofmatter.

Epsom salt See magnesium sulfate.

equation See chemical equation.

equation of state An equation that in-terrelates the pressure, temperature, andvolume of a system, such as a gas. Theequation for an ideal gas and the VAN DER

WAALS EQUATION are examples. See gaslaws.

equilibrium In a reversible chemical re-action:

A + B ˆ C + DThe reactants are forming the products:

A + B → C + Dwhich also react to give the original reac-tants:

C + D → A + BThe concentrations of A, B, C, and Dchange with time until a state is reached atwhich both reactions are taking place atthe same rate. The concentrations (or pres-sures) of the components are then constant– the system is said to be in a state of chem-ical equilibrium. Note that the equilibriumis a dynamic one; the reactions still takeplace but at equal rates. The relative pro-portions of the components determine the‘position’ of the equilibrium, which may be

changed by changing the conditions (e.g.temperature or pressure).

equilibrium constant Symbol: Kc,Kp In a chemical equilibrium of the type

xA + yB ˆ zC + wDThe expression:

[A]x[B]y/[C]z[D]w

where the square brackets indicate concen-trations, is a constant (Kc) when the systemis at equilibrium. Kc is the equilibrium con-stant of the given reaction; its units dependon the stoichiometry of the reaction. Forgas reactions, pressures are often used in-stead of concentration. The equilibriumconstant is then Kp, where Kp = Kc

n. Here nis the number of moles of product minusthe number of moles of reactant; for in-stance, in

3H2 + N2 ˆ 2NH3n is 2 – (1 + 3) = –2.

equipartition of energy The principlethat the total energy of a molecule is, onaverage, equally distributed among theavailable DEGREES OF FREEDOM. It is onlyapproximately true in most cases.

equivalence point The point in a titra-tion at which the reactants have beenadded in equivalent proportions, so thatthere is no excess of either. It differsslightly from the end point, which is theobserved point of complete reaction, be-cause of the effect of the indicator, errors,etc.

equivalent proportions, law of (law ofreciprocal proportions) When two chem-ical elements both form compounds with athird element, a compound of the first twoelements will contain each in the same rel-ative proportions in which they exist incompounds with the third element. For ex-ample, the mass ratio of carbon to hydro-gen in methane (CH4) is 12:4; the massratio of oxygen to hydrogen in water(H2O) is 16:2. In carbon monoxide (CO),the ratio of carbon to oxygen is 12:16.

equivalent weight A measure of ‘com-bining power’ formerly used in calcula-tions for chemical reactions. The

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equivalent weight

equivalent weight of an element is the num-ber of grams that could combine with ordisplace one gram of hydrogen (or 8 gramsof oxygen or 35.5 grams of chlorine). It isthe relative atomic mass (atomic weight)divided by the valence. For a compoundthe equivalent weight depends on the reac-tion considered. An acid, for instance, inacid-base reactions has an equivalentweight equal to its molecular weight di-vided by the number of acidic hydrogenatoms.

erbium A soft, malleable, silvery el-ement of the lanthanoid series of metals. Itoccurs in association with other lan-thanoids in such minerals as apatite,gadolinite, monazite, and bastnaesite. Itdoes not oxidize as easily as other lan-thanoids. It is used to make vanadium al-loys more workable, to make pinkpigments for paints and ceramics, and as asignal amplifier in fiber-optic cables.

Symbol: Er; m.p. 1529°C; b.p. 2863°C;r.d. 9.066 (25°C); p.n. 68; most commonisotope 166Er; r.a.m. 167.26.

Erlenmeyer flask A conical glass labo-ratory flask with a narrow neck and widebottom.

ESR See electron spin resonance.

ethanoate (acetate; CH3COO–) A saltor ester of ETHANOIC ACID (acetic acid).

ethanoic acid (acetic acid; CH3COOH) A colorless viscous liquid or glassy solid or-ganic acid with a pungent odor. It is theacid in vinegar. Below 16.6°C it solidifiesto a glassy solid known as glacial ethanoicacid. It is made by the oxidation of ethanolor butane, or by the bacterial fermentationof beer or wine, and is used as a foodpreservative and for making polymers.

ethanol (ethyl alcohol; alcohol; C2H5OH)A colorless volatile liquid alcohol. Ethanoloccurs in intoxicating drinks, as the resultof the anaerobic fermentation of simplesugars:

C6H12O6 → 2C2H5OH + 2CO2

Apart from its use in drinks, alcohol isused as a solvent and to form ethanal (acetaldehyde). Formerly, the main sourcewas by fermentation of molasses, but nowcatalytic hydration of ethene is used tomanufacture industrial ethanol.

ethene (ethylene; C2H4) A gaseousalkene. Ethene is not normally present inthe gaseous fraction of crude oil but can beobtained from heavier fractions by cat-alytic cracking. This is the principal indus-trial source. The compound is important asa starting material in the organic-chemicalsindustry (e.g. in the manufacture ofethanol) and as the starting material for theproduction of polyethene. See also Zeise’ssalt.

ethoxylate See detergents.

ethyl alcohol See ethanol.

ethylene See ethene.

ethylenediamine An organic com-pound, H2NCH2CH2NH2. It is importantin inorganic chemistry because it functionsas a bidentantate ligand, coordinating to ametal ion by the lone pairs on the two ni-trogen atoms. In the names of complexes itis given the abbreviation en.

ethylenediaminetetraacetic acid Seeedta.

ethyne (acetylene; C2H2) An unstablecolorless, gaseous, organic compound.Traditionally ethyne was made by the ac-tion of water on calcium dicarbide, and thegas has found use in oxy-acetylene weldingtorches, since its combustion with oxygenproduces a flame of very high temperature.It is also important in the organic chemi-cals industry for the production of vinylcompounds.

ethynide See carbide.

eudiometer An apparatus for the volu-metric analysis of gases. It often consists ofa sealable graduated glass tube housing

erbium

86

wires that are used to spark a reaction be-tween gas mixtures.

europium A silvery-white element ofthe lanthanoid series of metals. It occurs inassociation with other lanthanoids in suchminerals as monazite, bastnaesite, gadolin-ite, and samarskite. It is highly reactive andwill ignite in air when heated above 150°C.Its main use is in a mixture of europiumand yttrium oxides widely employed as thered phosphor in television screens. Themetal is used in some superconducting al-loys.

Symbol: Eu; m.p. 822°C; b.p. 1597°C;r.d. 5.23 (25°C); p.n. 63; most commonisotope 153Eu; r.a.m. 151.965.

eutectic A mixture of two substances insuch proportions that no other mixture ofthe same substances has a lower freezingpoint. If a solution containing the propor-tions of the eutectic is cooled no solid is de-posited until the freezing point of theeutectic is reached, when two solid phasesare simultaneously deposited in the sameproportions as the mixture; these two solidphases form the eutectic. If one of the sub-stances is water the eutectic can also be re-ferred to as a cryohydrate.

evaporation 1. A change of state fromliquid to gas (or vapor). Evaporation cantake place at any temperature, althoughthe rate increases with temperature. Evap-oration occurs because some molecules inthe liquid, if they are near the surface andmoving in the right direction, have enoughenergy to escape into the gas phase. Be-cause these are the molecules with higherkinetic energies, the liquid is cooled as a re-sult.2. A change from solid to vapor, especiallyoccurring at high temperatures close to themelting point of the solid. Thin films ofmetal can be evaporated onto a surface inthis way.

exa- Symbol: E A prefix used with SIunits, denoting 1018.

excitation The process by which an

atom, molecule, etc. is elevated from itsground to an excited state.

excitation energy The energy requiredto change an atom, molecule, etc. from onequantum state to a state with a higher en-ergy. The excitation energy (sometimescalled excitation potential) is the differencebetween two energy levels of the system.

excitation potential See excitation en-ergy.

excited state The state of an atom, mol-ecule, or other system when it has an en-ergy greater than that of its GROUND STATE.

exclusion principle See atom.

exothermic Denoting a chemical reac-tion in which heat is evolved (i.e. heatflows from the system or the temperaturerises). Combustion is an example of anexothermic process. Compare endother-mic.

explosive A substance or mixture thatcan rapidly decompose upon detonation,producing large amounts of heat and gases.The three most important classes of explo-sives are:1. Propellants, which burn steadily, and

are used as rocket fuels,2. Initiators, which are very sensitive and

are used in small amounts to detonateless sensitive explosives, and

3. High explosives, which need an initiatorbut are very powerful.

Eyring, Henry (1901–81) Americanchemist. Eyring was a prolific chemistwhose main contribution was to the theoryof chemical reactions. In the late 1920s andearly 1930s he calculated a number of po-tential energy surfaces in terms of quantummechanics. He then expressed the temper-ature dependence of the rate of chemicalreactions in terms of the parameters of po-tential energy surfaces. Eyring gave an ac-count of this work in the book The Theoryof Rate Processes (1941) which he wrotewith Samuel Glasstone and Keith Laidler.

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Eyring, Henry

face-centered cubic crystal (f.c.c.)(cubic close packing) A close-packed crys-tal structure made up of layers in whicheach atom, ion, or molecule is surroundedby six others arranged hexagonally. Thelayers are packed one on top of the other,with the second layer fitting into the re-cesses left in the first layer, and the thirdlayer fitting into yet a different set of holesformed by the second layer. If the layers aredesignated A, B, C, the packing is AB-CABC. This is in contrast to HEXAGONAL

CLOSE PACKED CRYSTAL, in which the layersare arranged ABABAB. Copper and alu-minum have face-centered cubic crystalstructures. See close packing.

fac-isomer See isomerism.

Fahrenheit scale A temperature scale inwhich the temperature of pure melting iceat standard pressure is fixed at 32° and thetemperature of pure boiling water at stan-dard pressure is fixed at 212°. The scale isnot used for scientific purposes. To convertfrom degrees Fahrenheit (°F) to degreesCelsius (°C) the formula

°F = 1.8(°C) + 32is used. See also Celsius scale; temperaturescale.

Fajan’s rules Rules that deal with varia-tions in the degree of covalent character inionic compounds in terms of polarizationeffects. Decrease in size and increase incharge for cations is regarded as makingthem more polarizing, while for anions anincrease in both charge and size makesthem more polarizable. Thus covalentcharacter is said to increase with increasingpolarizing power of the cation and/or in-creasing polarizability of the anion. Thesewere originally stated as rules one and two.

Fajan’s third rule states that covalent char-acter is greater for cations with a nonraregas configuration than for those with acomplete octet, charge and size being ap-proximately equal. For example, Cu+ ismore polarizing than Na+ (ions of approx-imately the same size) because the d elec-trons around Cu+ do not shield the nucleusas effectively as the complete octet in Na+.

farad Symbol: F The SI derived unit ofcapacitance. When the plates of a capacitorare charged by one coulomb and there is apotential difference of one volt betweenthem, the capacitor is said to have a capac-itance of one farad. One farad = 1 coulombvolt–1 (CV–1).

Faraday, Michael (1791–1867) Eng-lish physicist and chemist. Faraday was re-sponsible for a remarkably large number ofimportant discoveries in chemistry andphysics. His early work was mostly de-voted to chemistry. In 1820 he preparedchlorides of carbon. In 1835 he discoveredbenzene. In the 1830s Faraday discoveredwhat are now known as FARADAY’S LAWS ofelectrolysis. He also introduced the termsanode, cathode, cation, anion, electrode,and electrolyte. Faraday made fundamen-tal contributions to the study of electricityand magnetism. He constructed the firstelectric motor and discovered electromag-netic induction. He also developed the con-cept of electric and magnetic fields. Hisinvestigations of matter in magnetic fieldsled to the discovery of diamagnetism andparamagnetism. In 1845 he discoveredFaraday rotation, i.e. the rotation of theplane of polarization when polarized lightpasses through matter in a magnetic field.

faraday Symbol: F A non-SI unit of elec-

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F

tric charge equal to the charge required todischarge one mole of a singly-charged ion.In SI units one faraday is equal to96.48670 kilocoulombs (kC). See Fara-day’s laws.

Faraday constant See Faraday’s laws.

Faraday’s laws (of electrolysis) Twolaws resulting from the work of MichaelFaraday on electrolysis:1. The amount of chemical change pro-

duced is proportional to the electriccharge passed.

2. The amount of chemical change pro-duced by a given charge depends on theion concerned.More strictly it is proportional to the

relative ionic mass of the ion, and thecharge on it. Thus the charge Q needed todeposit m grams of an ion of relative ionicmass M carrying a charge Z is given by:

Q = FmZ/MF, the Faraday constant, has a value of onefaraday, i.e. 9.648670 × 104 coulombs.

f-block elements The block of elementsin the periodic table that have two outer selectrons and an incomplete penultimatesubshell of f electrons. The f block consistsof the lanthanoids (cerium to lutetium) andthe actinoids (thorium to lawrencium).

f.c.c. See face-centered cubic crystal.

feldspar (felspar) A member of a largegroup of very abundant crystalline igneousrocks, mainly silicates of aluminum withcalcium, potassium, and sodium. The al-kali feldspars, e.g. orthoclase, microcline,have mainly potassium with a little sodiumand no calcium; the plagioclase feldspars,e.g. albite, anorthite, labradorite, varyfrom sodium-only to calcium-only, withlittle or no potassium. Feldspars are used inmaking ceramics, enamels, and glazes.

femto- Symbol: f A prefix used with SIunits denoting 10–15. For example, 1 fem-tometer (fm) = 10–15 meter (m).

femtochemistry The study of atomicand molecular processes, particularly

chemical reactions, that take place ontimescales of about a femtosecond (10–15s).Femtochemistry became possible with theconstruction of lasers that could be pulsedin femtoseconds. Femtochemistry can beregarded as very high speed ‘photography’of chemical reactions. It has enabled enti-ties that exist only for a short time, such asactivated complexes, to be studied. The de-tailed mechanisms of many chemical reac-tions have been investigated in this way,notably by the Egyptian-born Americanchemist Ahmed Zewail.

fermi A non-SI unit of length equal toone femtometer (10–15 meter) in SI units. Itwas formerly used in atomic and nuclearphysics.

fermium A highly radioactive trans-uranic element of the actinoid series, notfound naturally on Earth. It was discov-ered in the fallout from the first hydrogenbomb. It can be produced in very smallquantities by bombarding 239Pu with neu-trons to give 253Fm (half-life 3 days). Sev-eral other short-lived isotopes have alsobeen synthesized.

Symbol: Fm; m.p., b.p. and r.d. un-known; p.n. 100; most stable isotope257Fm (half-life 100.5 days).

ferric chloride See iron(III) chloride.

ferric oxide See iron(III) oxide.

ferric sulfate See iron(III) sulfate.

ferricyanide See cyanoferrate.

ferrite A nonconducting, magnetic,metal oxide ceramic having the general for-mula MO.Fe2O3, where M is a divalenttransition metal such as cobalt, nickel, orzinc. Ferrites are used to make powerfulmagnets for use in high-frequency elec-tronic devices such as radar sets.

ferrocene (Fe(C5H5)2) An orange crys-talline solid. It is an example of a sandwichcompound, in which an iron(II) ion is co-ordinated to two cyclopentadienyl ions.The bonding involves overlap of d orbitals

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ferrocene

on the iron with the pi electrons in the cy-clopentadienyl rings. The compound meltsat 173°C, is soluble in organic solvents,and can undergo substitution reactions onthe rings. It can be oxidized to the bluecation (C5H5)2Fe+. The systematic name isdi-π-cyclopentadienyl iron(II).

ferrocyanide See cyanoferrate.

ferromagnetism See magnetism.

ferrosoferric oxide See triiron tetrox-ide.

ferrous chloride See iron(II) chloride.

ferrous oxide See iron(II) oxide.

ferrous sulfate See iron(II) sulfate.

fertilizer A substance added to soil to in-crease its fertility. Artificial fertilizers gen-erally consist of a mixture of chemicalsdesigned to chiefly supply three main nu-trients: nitrogen (N), phosphorus (P), andpotassium (K), and are consequentlyknown as PKN fertilizers. Typical chemi-cals used include ammonium nitrate, am-monium phosphate, ammonium sulfate,and potassium carbonate.

Fick’s law A law describing the diffu-sion that occurs when solutions of differentconcentrations come into contact, withmolecules moving from regions of higherconcentration to regions of lower concen-tration. Fick’s law states that the rate ofdiffusion dn/dt, called the diffusive fluxand denoted J, across an area A is given by:dn/dt = J = –DA∂c/∂x, where D is a con-stant called the diffusion constant, ∂c/∂x isthe concentration gradient of the soluteand dn/dt is the amount of solute crossingthe area A per unit time. D is constant fora specific solute and solvent at a specifictemperature. Fick’s law was formulated bythe German physiologist Adolf Eugen Fick(1829–1901) in 1855.

filler A solid material used to modify thephysical properties or reduce the cost ofsynthetic compounds, such as rubbers,

plastics, paints, and resins. Slate powder,glass fiber, mica, and cotton are all used asfillers.

filter See filtration.

filter pump A type of vacuum pump inwhich a jet of water forced through a noz-zle carries air molecules out of the system.Filter pumps cannot produce pressuresbelow the vapor pressure of water. Theyare used in the laboratory for vacuum fil-tration, distillation, and similar techniquesrequiring a low-grade vacuum.

filtrate See filtration.

filtration The process of removing sus-pended particles from a fluid by passing orforcing the fluid through a porous material(the filter). The fluid that passes throughthe filter is the filtrate. In laboratory filtra-tion, filter paper or sintered glass is com-monly used.

fine structure Closely spaced lines seenat high resolution in a spectral line or band.Fine structure may be caused by vibrationof the molecules or by electron spin. Hy-perfine structure, seen at very high resolu-tion, is caused by the atomic nucleusaffecting the possible energy levels of theatom.

fireclay A refractory clay containinghigh proportions of alumina (aluminumoxide) and silica (silicon(IV) oxide), usedfor making firebricks and furnace linings.

firedamp Methane and other com-bustible gases occurring in coal mines. Ex-plosions or fires can occur if quantities ofthese gases accumulate. The deoxygenated,suffocating air left after an explosion orfire, consisting chiefly of carbon dioxideand nitrogen, is called blackdamp. After-damp is the poisonous carbon monoxideformed.

first-order reaction A reaction inwhich the rate of reaction is proportionalto the concentration of one of the reactingsubstances. The concentration of this re-

ferrocyanide

90

acting substance is raised to the power one;i.e. rate = k[A]. For example, the decompo-sition of hydrogen peroxide is a first-orderreaction,

rate = k[H2O2]Similarly the rate of decay of radioactivematerial is a first-order reaction,

rate = k[radioactive material]For a first-order reaction, the time for adefinite fraction of the reactant to be con-sumed is independent of the original con-centration. The units of k, the RATE

CONSTANT, are s–1.

Fischer, Ernst Otto (1918–94) Germaninorganic chemist. Fischer is noted for hiswork on inorganic complexes. In 1951 twochemists, T. Kealy and P. Pauson, were at-tempting to join two five-carbon (cyclo-pentadiene) rings together and discovereda compound C5H5FeC5H5, which theyproposed had an iron atom joined to acarbon atom on each ring. Fischer, on re-flection, considered such a structure inade-quate for he failed to see how it couldprovide sufficient stability with its carbon–iron–carbon bonds. The British chemistGeoffrey WILKINSON suggested a morenovel structure in which the iron atom wassandwiched between two parallel rings andthus formed bonds with the electrons in therings, rather than with individual carbonatoms. Compounds of this type are calledSANDWICH COMPOUNDS. By careful x-rayanalysis Fischer confirmed the proposedstructure of FERROCENE, as the compoundwas called, and for this work shared theNobel Prize for chemistry with Wilkinsonin 1973.

fixation of nitrogen See nitrogen fixa-tion.

fixing, photographic A stage in the de-velopment of exposed photographic filmor paper in which unexposed silver halidesin the emulsion are dissolved away. Thefixing bath typically contains a solution ofammonium or sodium thiosulfate(IV), andis employed after the initial development(chemical reduction) of the latent silverimage.

flame test A preliminary test in qualita-tive analysis in which a small sample of achemical is introduced into a nonluminousBunsen-burner flame on a clean platinumwire. The flame vaporizes part of the sam-ple and excites some of the atoms, whichemit light at wavelengths characteristic ofthe metallic elements in the sample. Thusthe observation of certain colors in theflame indicates the presence of these el-ements. The same principles are applied inthe modern instrumental method of spec-trographic analysis.

flare stack A special chimney in an oilrefinery, rig, or other chemical plant at thetop of which unwanted gases are burnt.

flash photolysis A technique for inves-tigating free radicals in gases. The gas isheld at low pressure in a long glass orquartz tube, and an absorption spectrumtaken using a beam of light passing downthe tube. The gas can be subjected to a verybrief intense flash of light from a lamp out-side the tube, producing free radicals,which are identified by their spectra. Meas-urements of the intensity of spectral lineswith time can be made using an oscillo-scope, and the kinetics of very fast reac-tions can thus be investigated.

flash point The lowest temperature atwhich vapor given off by a flammable liq-uid will be sufficient to momentarily sus-tain a flame in the presence of a spark.

flocculation The combining of the par-ticles of a finely divided precipitate, such asa colloid, into flocculent clumps that sinkand are easier to filter off. See coagulation.

flocculent See flocculation.

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flocculent

FLAME TEST COLORSElement Flame colorbarium greencalcium brick redlithium crimsonpotassium pale lilacsodium yellowstrontium red

flotation See froth flotation.

fluid A state of matter that is not a solid,i.e. a liquid or a gas. All fluids can flow,and the resistance they pose to such flow iscalled VISCOSITY.

fluidization The suspension of a finely-divided solid, e.g. a catalyst, in an upward-flowing liquid or gas. This suspensionmimics many properties of liquids, includ-ing allowing the solids to ‘float’ in it andachieve a uniform temperature with thefluid medium. Fluidized beds so preparedare used industrially to carry out catalyticreactions and to minimize the productionof pollutants from coal-fired furnaces.

fluorescein A fluorescent dye used as anabsorption indicator and water flowmarker.

fluorescence The absorption of energyby atoms, molecules, etc., followed by im-mediate emission of electromagnetic radia-tion as the particles make transitions tolower energy states. Compare phosphores-cence.

fluoridation The introduction of smallquantities of fluoride compounds, e.g.sodium fluoride (NaF), into the water sup-ply as a public-health measure to reducethe incidence of tooth decay.

fluoride See halide.

fluorination See halogenation.

fluorine A slightly greenish-yellow,highly reactive, poisonous, gaseous el-ement belonging to the halogens; i.e. group17 (formerly VIIA) of the periodic table, ofwhich it is the first element. It occurs no-tably as fluorite (CaF2) and cryolite(Na3AlF3) but traces of fluorine are alsowidely distributed with other minerals. It isslightly more abundant than chlorine, ac-counting for about 0.065% of the Earth’scrust. Fluorine’s high reactivity delayed itsisolation until 1886, and means it is neverfound as a free element. Fluorine is nowprepared by electrolysis of a molten mix-

ture of potassium fluoride and hydrogenfluoride electrolytes, using copper or steelapparatus. Its preparation by chemicalmethods is impossible.

Fluorine compounds are used in thesteel industry, glass and enamels, uraniumprocessing, aluminum production, and in arange of fine organic chemicals. Fluorine isthe most reactive and most electronegativeelement known; in fact it reacts directlywith almost all the elements, including therare gas xenon. As the most powerful oxi-dizing agent known, it has the pronouncedcharacteristic of bringing out the higheroxidation states when combined withother elements. Ionic fluorides contain thesmall F– ion, which is very similar in reac-tivity to the O2– ion. With the more elec-tronegative elements, fluorine forms anextensive range of compounds, e.g. SF6,NF3, and PF5, in which the bonding is es-sentially covalent.

Fluorine reacts explosively with hydro-gen, even in the dark, to form hydrogenfluoride, HF, which polymerizes as a resultof hydrogen bonding and has a boilingpoint very much higher than that of HCl(HF b.p. 19°C; HCl b.p. –85°C). Unlikethe other halogens, fluorine does not formhigher oxides or oxyacids; oxygen difluo-ride in fact reacts with water to give hy-drogen fluoride.

Fluorine exhibits a strong electronwithdrawing effect on adjacent bonds, thusCF3COOH is a strong acid whereasCH3COOH is not. Although the coordina-tion number seldom exceeds one there arecases in which fluorine bridging occurs,e.g. (SbF5)n. The element is sufficiently re-active to combine directly with the rare gasxenon at 400°C to form XeF2 and XeF4. At600°C and high pressure it will even formXeF6.

Fluorine and hydrogen fluoride are ex-tremely dangerous and should be used onlyin purpose-built apparatus; gloves and faceshields should be used when working withhydrofluoric acid, and accidental exposureshould be treated as a hospital emergency.

Symbol: F; m.p. –219.62°C; b.p.–188.14°C; d. 1.696 kg m–3 (0°C); p.n. 9;r.a.m. 18.99840.32.

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fluorite (fluorspar) See calcium fluoride.

fluorite structure See calcium-fluoridestructure.

fluorspar See calcium fluoride.

flux 1. A substance used to keep metalsurfaces free of oxide in soldering. See sol-der.2. A substance used in smelting metals toreact with silicates and other impuritiesand form a low-melting slag.

fluxional molecule A molecule inwhich the constituent atoms change theirrelative positions so quickly at room tem-perature that the normal concept of struc-ture is inadequate; i.e. no specific structureexists for longer than about 10–2 secondand the relative positions become indistin-guishable. For example ClF3 at –60°C hasa distinct ‘T’ shape but at room tempera-ture the fluorine atoms are visualized asmoving rapidly over the surface of thechlorine atom in a state of exchange andare effectively identical.

formula (chemical formula) A represen-tation of a chemical compound using sym-bols for the atoms and subscript numbersto show the numbers of atoms present. Seeempirical formula; general formula; mo-lecular formula; structural formula.

fraction See fractional distillation.

fractional crystallization Crystalliza-tion of one component from a mixture insolution. When two or more substances arepresent in a liquid (or in solution), on grad-ual cooling to a lower temperature onesubstance will preferentially form crystals,leaving the other substance in the liquid (ordissolved) state. Fractional crystallizationcan thus be used to purify or separate sub-stances.

fractional distillation (fractionation)Distillation in which a mixture of liquids isseparated into components (fractions) de-pending on their boiling points. A commontechnique is to use a long vertical column

(fractionating column) containing an inertmaterial (e.g. glass beads) attached to thevessel containing the mixture. There is adecreasing temperature gradient from thebottom to the top of the column. Vaporrises in the column until it reaches a part atwhich it condenses back to liquid, and runsback down the column. A steady state isreached at which the more volatile compo-nents are concentrated near the top of thecolumn and the less volatile componentsare near the bottom. It is possible to drawoff fractions at points on the column. Onan industrial scale, large towers containingperforated trays are used for extractingfractions in petroleum refining.

fractionating column See fractionaldistillation.

fractionation See fractional distillation.

francium A radioactive element of thealkali-metal group, the seventh element ofgroup 1 (formerly IA) of the periodic table.It is found on Earth only as a short-livedproduct of radioactive decay, occurring inuranium ores in minute quantities. At least22 isotopes are known.

Symbol: Fr; m.p. 27°C; b.p. 677°C; r.d.2.4°; p.n. 87; most stable isotope 223Fr(half-life 21.8 minutes).

Frasch process See sulfur.

free electron An electron that can movefrom one atom or molecule to anotherunder the influence of an applied electricfield. Movement of free electrons in a con-ductor constitutes an electric current. Freeelectrons are also involved in conductingheat and in METALLIC BONDS.

free energy A measure of the ability of asystem to do useful work. See Gibbs func-tion; Helmholtz function.

free radical An atom or group of atomswith a single unpaired electron. Free radi-cals are produced by breaking a covalentbond; for example:

CH3Cl → CH3• + Cl•

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free radical

They are often formed in light-inducedreactions and as a result of chemical de-composition at high temperatures. Freeradicals are extremely reactive and can bestabilized and isolated only under specialconditions.

freezing The process by which a liquid isconverted into a solid by cooling; the re-verse of melting.

freezing mixtures Two or more sub-stances mixed together to produce a tem-perature below the freezing point of water(0°C). A mixture of sodium chloride andice in water, for example, can yield a tem-perature as low as –20°C.

freezing point The temperature atwhich a liquid is in equilibrium with itssolid phase at standard pressure and belowwhich the liquid freezes or solidifies. Thistemperature is always the same for a par-ticular pure liquid and is numerically equalto the melting point of the solid.

freezing point constant See depressionof freezing point.

French chalk See talc.

Frenkel defect see defect.

frontier orbital theory A theory of thechemical and spectroscopic properties ofmolecules that emphasizes the frontier or-bitals, i.e. the highest occupied molecularorbital (HOMO) and the lowest unoccu-pied molecular orbital (LUMO). Thetheory was developed by the Japanesechemist Kenichi Fukui (1919–98) in the1950s. Frontier orbital theory has some-times been used in conjunction with theWOODWARD–HOFFMANN RULES.

froth flotation (flotation) An industrialtechnique for separating the required partsof ores from unwanted gangue. The min-eral is pulverized and mixed with water, towhich a frothing agent is added. The mix-ture is aerated, after which particles of oneconstituent are carried to the surface by

bubbles of air, which adhere to it preferen-tially. The froth is then skimmed off. Vari-ous additives are also used to modify thesurface properties of the mineral particles(e.g. to increase the adherence of air bub-bles).

fuel cell A type of cell in which fuel isconverted directly into electricity. In oneform, hydrogen gas and oxygen gas are fedto the surfaces of two separately compart-mentalized porous nickel electrodes im-mersed in a potassium hydroxide solution.The oxygen reacts to form hydroxyl (OH–)ions, which it releases into the solution,leaving a positive charge on one electrode.The hydrogen reacts with the OH– ions inthe solution to form water, giving up elec-trons to leave a negative charge on theother electrode. Large fuel cells can gener-ate tens of amperes. Usually the e.m.f. isabout 0.9 volt and the efficiency around60%.

fullerene See buckminsterfullerene.

fullerite See buckminsterfullerene.

fuller’s earth A natural clay, the pre-dominate mineral in bentonite, used as anabsorbent and decolorizing agent, and as adrilling mud and industrial catalyst.

fuming sulfuric acid See oleum.

fundamental units The BASE UNITS of ameasurement system, especially those thatdeal with force and motion. In most sys-tems these are the units for length, mass,and time, plus at least one electrical unit.The fundamental units in the SI are consid-ered to be the meter, the kilogram, the sec-ond, and the ampere.

fused Describing a solid that has beenmelted and solidified into a single mass.Fused silica, for example, is produced bymelting sand.

fused ring See ring.

fusion Melting.

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94

gadolinium A ductile malleable silveryelement of the lanthanoid series of metals.It occurs in association with other lan-thanoids in such minerals as gaolinite,bastnaesite, monazite, and xenotime. It isalso a product of nuclear fission and can befound in association with uranium ores.Gadolinium is used in alloys and magnets,and in electronic components designed towithstand very high temperatures. Twoisotopes, gadolinium-155 and gadolinium-157, are the best neutron absorbers knownand thus find use in the nuclear industry.

Symbol: Gd; m.p. 1313°C; b.p.3266°C; r.d. 7.9 (25°C); p.n. 64; mostcommon isotope 158Gd; r.a.m. 157.25.

galena (lead glance) A mineral form oflead(II) sulfide (PbS), usually occurring asgrayish-blue cubes or octahedrons. It is theprincipal ore of lead.

gallium A soft silvery low-meltingmetallic element belonging to group 13(formerly IIIA) of the periodic table. It isfound in minute quantities in several ores,including sphalerite (ZnS), bauxite(Al2O3.H2O), and kaolinite (Al2(OH)4-S12O5. Gallium is used in low-melting al-loys, high-temperature thermometers, andas a doping impurity in semiconductors.Gallium arsenide is a semiconductor usedin light-emitting diodes and in microwaveapparatus.

Symbol: Ga; m.p. 29.78°C; b.p.2403°C; r.d. 5.907 (solid at 20°C), 6.114(liquid); p.n. 31; most common isotope696a; r.a.m. 69.723.

galvanic cell See cell.

galvanizing The process by which steel

is coated with zinc by dipping it in a bathof molten zinc, or by electrodeposition.The zinc is preferentially attacked by car-bonic acid (H2CO3), thus protecting thesteel by forming zinc carbonates.

gamma radiation A form of ELECTRO-MAGNETIC RADIATION emitted by changes inthe nuclei of atoms. Gamma rays have veryhigh frequencies and thus short wave-length. Gamma radiation shows particleproperties much more often than waveproperties. The radiant energy (W) of agamma photon, given by

W = hvwhere h is the Planck constant and v the ve-locity, can be very high. A gamma photonof 1024 hertz (Hz) has an energy of 6.6 ×10–10 joule (J).

gamma rays Streams of gamma radia-tion.

gangue The waste rock or other unde-sirable material occurring in an ore.

gas The state of matter in which forcesof attraction between the particles of a sub-stance are small. The particles have free-dom of movement and gases therefore haveno fixed shape or volume. The atoms andmolecules of a gas are in a continual stateof motion and are continually collidingwith each other and with the walls of anycontaining vessel. These collisions createthe pressure of a gas.

gas chromatography A techniquewidely used for the separation and analysisof mixtures. Gas chromatography employsa column packed with either a solid sta-tionary phase (gas-solid chromatography

95

G

or GSC) or a solid coated with a non-volatile liquid (gas-liquid chromatographyor GLC). The whole column is placed in athermostatically controlled heating jacket.A volatile sample is introduced into the col-umn using a syringe, and an unreactive car-rier gas, such as nitrogen, passed throughit. The components of the sample will becarried along in this mobile phase. How-ever, some of the components will clingmore readily to the stationary phase thanothers, either because they become at-tached to the solid surface or because theydissolve in the liquid. The time taken fordifferent components to pass through thecolumn is characteristic and can be used toidentify them. The emergent sample ispassed through a detector, which registersthe presence of the different components inthe carrier gas.

Two types of detector are in commonuse: the katharometer, which measureschanges in thermal conductivity, and theflame-ionization detector, which turns thevolatile components into ions and registersthe change in electrical conductivity.

gas constant (universal gas constant)Symbol: R The universal constant 8.31451J mol–1 K–1 appearing in the equation ofstate for an ideal gas. See gas laws.

gaseous diffusion separation A tech-nique for the separation of gases that relieson their slightly different atomic masses.For example, the isotopes of uranium (235Uand 238U) can be separated by first prepar-ing gaseous uranium hexafluoride (UF6)from uranium ore. If this is then made topass through a series of fine pores, the mol-ecules containing the lighter isotope of ura-nium will pass through more quickly. Asthe gases pass through successive ‘di-aphragms’, the proportion of lighter mol-ecules increases. A separation of over 99%is thus attainable. Gaseous diffusion sepa-ration has also been applied to the separa-tion of the isotopes of hydrogen.

gas equation See gas laws.

gas laws Laws relating the temperature,pressure, and volume of a fixed mass of

gas. The main gas laws are BOYLE’S LAW

and CHARLES’ LAW. The laws are notobeyed exactly by any real gas, but manycommon gases obey them under certainconditions, particularly at high tempera-tures and low pressures. A gas that wouldobey the laws over all pressures (p) andtemperatures (T) is a perfect or ideal gas.

Boyle’s and Charles’ laws can be com-bined into a gas equation of state for idealgases:

pVm = RTwhere Vm is the molar volume and R is themolar gas constant. For n moles of gas,then

pV = nRTAll real gases deviate to some extent

from the gas laws, which are applicableonly to idealized systems of particles ofnegligible volume with no intermolecularforces. There are several modified equa-tions of state that give a better descriptionof the behavior of real gases, the bestknown being the VAN DER WAALS EQUATION.

gas-liquid chromatography See gaschromatography.

gas-solid chromatography See gaschromatography.

gauss Symbol: G The unit of magneticflux density in the c.g.s. system. It is equalto 10–4 tesla (T) in SI units.

Gay-Lussac, Joseph-Louis (1778–1850)French chemist and physicist. Gay-Lussacis best remembered for his work on gases.In 1802 he formulated the law sometimescalled Charles’ law (after the work ofJacques Charles in 1787) or Gay-Lussac’slaw that, if the pressure remains constant,gases expand equally for a given increase intemperature. In 1805 he found that twovolumes of hydrogen combine with onevolume of oxygen to produce water. In1808 he formulated what is known as Gay-Lussac’s law of combining volumes, whichstates that gases combine in simple propor-tions by volume, with the volumes of theproducts being related to the volume of thereactants. He was an early investigator ofboron, iodine, and the cyanides. He also in-

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96

vented the Gay-Lussac tower for recover-ing nitrogen oxides produced in the manu-facture of sulphuric acid.

Gay-Lussac’s law 1. Gases react in vol-umes that are in simple ratios to each otherand to their products, if they are gases, as-suming that all volumes are measured atthe same temperature and pressure.2. See Charles’ law.

gel A lyophilic colloid that is normallystable but may be induced to coagulatepartially under certain conditions (e.g.lower temperatures). It is these conditionsthat produce a pseudo-solid or easily de-formable jellylike mass called a gel, inwhich intertwining particles enclose thewhole dispersing medium. Gels may be fur-ther subdivided into elastic gels (e.g.gelatin) and rigid gels (e.g. silica gel).

gel electrophoresis see electrophoresis.

gel filtration See molecular sieve.

gem positions Positions in a moleculeon the same atom. For example, 1,1-dichloroethane (CH3CHCl2) is a gem di-halide.

general formula A representation ofthe chemical formula common to a groupof compounds. For example, ROH (whereR is a hydrocarbon group) is the generalformula for an alcohol. See also empiricalformula; molecular formula; structural for-mula.

geochemistry The study of the chem-istry of the Earth. This includes the chemi-cal composition of the Earth, theabundance of the elements and their iso-topes, the atomic structure of minerals,and chemical processes that occur at thehigh temperatures and pressures inside theEarth.

geometrical isomerism See isomerism.

germanium A hard, brittle, grayish-white metalloid element belonging togroup 14 (formerly IVA) of the periodic

table. It is found in sulfide ores such as ar-gyrodite (4Ag2S.GeS2) and in zinc ores andcoal. Most germanium is recovered duringzinc or copper refining as a by-product.Germanium was extensively used in earlysemiconductor devices but has now beenlargely superseded by silicon. It is used asan alloying agent and catalyst, and in phos-phors and infrared equipment.

Symbol: Ge; m.p. 937.45°C; b.p.2830°C; r.d. 5.323 (20°C); p.n. 32; mostcommon isotope 74Ge; r.a.m. 72.61.

germanium(IV) oxide (GeO2) A com-pound made by strongly heating germa-nium in air or by hydrolyzinggermanium(IV) chloride. It occurs in twoforms. One is slightly soluble in waterforming an acidic solution; the other is in-soluble in water. Germanium oxide dis-solves in alkalis to form the anion GeO3

2–.

German silver See nickel-silver.

Gibbs, Josiah Willard (1839–1903)American mathematician and physicist.Between 1873 and 1878 Gibbs foundedthe subject of chemical thermodynamics ina series of lengthy papers. In one of thesepapers entitled On the Equilibrium of Het-erogeneous Substances, he put forward thephase rule concerning physical systemswith different phases. His work on chemi-cal thermodynamics also included the con-cepts of the Gibbs free energy and chemicalpotential. In addition, he contributed towork on the thermodynamics of surfaces.Gibbs was one of the pioneers of statisticalmechanics. He gave a classic exposition ofthe subject in the book Elementary Princi-ples of Statistical Mechanics (1902).

Gibbs free energy See Gibbs function.

Gibbs function (Gibbs free energy)Symbol: G A thermodynamic function de-fined by

G = H – TSwhere H is the enthalpy, T the thermody-namic temperature, S the entropy, and Gthe energy absorbed or liberated. It is use-ful for specifying the conditions of chemi-cal equilibrium for reactions for constant

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Gibbs function

temperature and pressure (G is a mini-mum). It is named for U.S. physicist andmathematician Josiah Willard Gibbs(1839–1903), who proposed it in 1877.See also free energy.

giga- Symbol: G A prefix used with SIunits, denoting 109. For example, 1 giga-hertz (GHz) = 109 hertz (Hz).

glass A hard transparent material madeby heating calcium oxide (lime, CaO),sodium carbonate (Na2CO3), and sand (sil-icon(IV) oxide, SiO2). This produces a cal-cium silicate (Ca2SiO4) – the normal typeof glass, called soda glass. Special types ofglass can be obtained by incorporatingboron oxide (B2O3) in the glass (borosili-cate glass, used for laboratory apparatus)or by including metals other than calcium,e.g. lead or barium.

Glass is an amorphous substance, in thesense that there is no long-range orderingof the atoms on a lattice. It can be regardedas a supercooled liquid, which has not crys-tallized. Solids with similar noncrystallinestructures, e.g. obsidian, are also calledglasses.

Glauber’s salt See sodium sulfate.

GLC Gas-liquid chromatography. Seegas chromatography.

gold A ductile malleable lustrous yellowtransition metal, the third element of group11 (formerly subgroup IB) of the periodictable. Gold is largely unreactive and istherefore found native, often in veins in ig-neous rock or in placer deposits. Nativegold almost always contains 2–20% silver.Gold is used in jewelry, often alloyed withcopper, and in electronics and coloredglass. The purity of gold is traditionallystated in karats (or carats), with 24 karatsindicating pure gold. Thus, a ring stated tobe 18 karat gold would consist of an alloyof 18 parts gold and six parts other metals.

Symbol: Au; m.p. 1064.43°C; b.p.2807°C; r.d. 19.320 (20°C); p.n. 79; mostcommon isotope 197Au; r.a.m. 196.96654.

gold(III) chloride (gold trichloride; auric

chloride; AuCl3) A compound preparedby dissolving gold in aqua regia. The brightyellow crystals (chloroauric acid, HAuCl4)produced on evaporation are heated toform dark red crystals of gold(III) chloride.The chloride decomposes easily (at 175°C)to give gold(I) chloride (AuCl) and chlo-rine; at higher temperatures it decomposesfurther to give gold and chlorine. Gold(III)chloride is used in photography. It exists asa dimer, Au2Cl6.

Goldschmidt, Victor Moritz (1888–1947) Swiss-born Norwegian chemist.Goldschmidt is frequently described as thefounder of geochemistry. He pioneered theapplication of chemical thermodynamicsto processes in geology. After the develop-ment of x-ray crystallography by theBraggs, he and his colleagues worked outthe structure of over 200 crystals. This ledhim to produce tables of atomic and ionicradii. Goldschmidt also devoted a lot of at-tention to the problem of determining thecosmic abundance of the chemical el-ements. He summarized his work in thebook Geochemistry (1954), which waspublished posthumously.

Goldschmidt process A process for ex-tracting certain metals from their oxides byreduction with aluminum, named for Ger-man chemist Hans Goldschmidt (1861–1923), who discovered it. See thermite.

gold trichloride See gold(III) chloride.

Graham’s law (of diffusion) The prin-ciple that gases diffuse at a rate that is in-versely proportional to the square root oftheir density. It was discovered in 1829 bythe Scottish chemist Thomas Graham(1805–69). Light molecules, in otherwords, diffuse faster than heavy molecules.The principle is used in the separation ofisotopes.

grain A crystal in a metal that has beenprevented from attaining its regular geo-metrical form.

gram (gramme) Symbol: g A metric unitof mass defined as 10–3 kilogram. It was

giga-

98

the fundamental unit of mass in the c.g.s.system.

granulation A process for enlargingparticles to improve the flow properties ofsolid reactants and products in industrialchemical processes. The larger a particle,and the freer from fine materials in a solid,the more easily it will flow. Dry granula-tion produces pellets from dry materials,which are crushed into the desired size.Wet granulation involves the addition of aliquid to the material, and the resultingpaste is extruded and dried before cuttinginto the required size.

graphite An allotrope of CARBON. Theatoms are arranged in layers as a series offlat, hexagonal rings. Graphite is a goodconductor of heat and electricity. The lay-ers cleave easily, making graphite useful asa solid lubricant.

gravimetric analysis A method ofquantitative analysis in which the final an-alytical measurement is made by determi-nation of mass. There are many variationsin the method but in essence they all con-sist of:1. taking a sample whose mass has been ac-

curately determined into solution;2. precipitating a known compound by a

quantitative reaction;3. performing digestion and coagulation

procedures;4. filtering and washing;5. drying and determining mass as a pure

compound.Filtration is a key element in the

method and a variety of special filter pa-pers and sinter-glass filters are available.

gray Symbol: Gy The SI derived unit ofabsorbed energy dose per unit mass result-ing from the passage of ionizing radiationthrough living tissue. One gray is an energyabsorption of one joule per kilogram ofmass.

gray cast iron See cast iron.

ground state The state of lowest energyof an atom (or ion, molecule, etc.). For ex-

ample, the hydrogen atom in the groundstate has its single electron in the 1s orbital.If energy is provided (e.g. by electromag-netic radiation, electron impact, high tem-perature, etc.) it is possible to formhydrogen atoms in an EXCITED STATE (forexample, with the electron in the 2s or-bital).

group In the periodic table, a series ofchemically similar elements that have simi-lar electronic configurations. A group isthus a column of the periodic table. For ex-ample, the alkali metals, all of which haveouter s1 configurations, belong to group 1.See also periodic table.

group 0 elements See rare gases.

group 1 elements See alkali metals.

group 2 elements See alkaline-earthmetals.

group 3-12 elements See transition el-ements.

group 13 elements A group of elementsin the periodic table consisting of the el-ements boron (B), aluminum (Al), gallium(Ga), indium (In), and thallium (Tl). Theseelements were formerly classified as groupIII elements and belonged to the IIIA sub-group. The IIIB subgroup consisted of theelements scandium (Sc), yttrium (Y), lan-thanum (La), and actinium (Ac).

The group 13 elements all have threeouter electrons (s2p1) with no partly filledinner shells. The group is the first group ofelements in the p-block. As with all thegroups there is an increase in metallic char-acter as the group is descended.

The boron atom is both small and has ahigh ionization potential, so bonds toboron are largely covalent with only smalldegrees of polarization. In fact boron isclassed as a metalloid. It has volatile hy-drides and a weakly acidic oxide. As thegroup is descended the ionization poten-tials tend to decrease and the atomic radiiincrease, leading to increasingly polar in-teractions and the formation of distinctM3+ ions. The increase in metallic charac-

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group 13 elements

ter is clearly illustrated by the hydroxides:boric acid B(OH)3 is acidic; aluminum andgallium hydroxides are amphoteric, dis-solving in acids to give Al3+ and Ga3+ andin bases to give aluminates and gallates; thehydroxides of indium and thallium are dis-tinctly basic. As the elements get heavierthe bond energies with other elements be-come generally smaller. The monovalentstate (removal of the p electron only) be-comes progressively stable. For examplegallium(I) is known in the gas phase and incoordination complexes (sometimes as‘GaCl2’). Monovalent indium halides areknown, for example InX, and thallium(I) isdistinctly more stable than thallium(III).

group 14 elements A group of elementsin the periodic table consisting of the el-ements carbon (C), silicon (Si), germanium(Ge), tin (Sn), and lead (Pb). These el-ements were formerly classified as groupIV elements and belonged to the IVA sub-group. The IVB subgroup consisted of theelements titanium (Ti), zirconium (Zr), andhafnium (Hf), which are transition metals.

The group 14 elements all have elec-tronic structures with four outer electrons(s2p2) and no partly filled inner shells. Thegroup shows the common trend towardmetallic character with the heavier el-ements; thus carbon is a typical nonmetal,silicon and germanium are metalloids, andtin and lead are characteristically metallic.As observed with other groups, the firstmember of the group is quite different fromthe rest. The carbon atom is smaller andhas a higher ionization potential, bothfavouring predominance of covalence, butadditional factors are:1. The widespread nature of extensive

catenation in carbon compounds.2. The possibility of strong overlap of p or-

bitals or double bonds3. The unavailability of d orbitals in car-

bon compounds.Significant differences are:1. Both oxides of carbon are gaseous, CO

and CO2; the other elements form solidoxides, e.g. SiO2, Pb3O4.

2. The heavier elements readily expandtheir coordination number, e.g. SiF6

2–

and SnCl62–.

group 15 elements A group of elementsin the periodic table consisting of the el-ements nitrogen (N), phosphorus (P), ar-senic (As), antimony (Sb), and bismuth(Bi). These elements were formerly classi-fied as group V elements and belonged tothe VA subgroup. The VB subgroup con-sisted of the elements vanadium (V), nio-bium (Nb), and tantalum (Ta), which aretransition elements.

The group 15 elements all have elec-tronic configurations corresponding toouter s2p3 electrons with no vacancies ininner levels. The ionization potentials arehigh and the lighter members of the group,N and P, are distinctly electronegative andnonmetallic in character. Arsenic and anti-mony are metalloids and bismuth is weaklymetallic (Bi2O3 dissolves in acids to giveBi(OH)3).

Nitrogen is dissimilar from the othermembers of the group in that it forms sta-ble multiple bonds, it is limited to coordi-nation number 4, e.g. NH4

+, it issufficiently electronegative to form hydro-gen bonds and the nitride ion N3–, and inthat its oxides are irregular with the rest ofthe group.

Group members display a remarkablenumber of allotropes, showing the trendfrom nonmetallic forms through to metal-lic forms. Thus nitrogen has only the di-atomic form; phosphorus has a highlyreactive form P1 (brown), and a tetrahedralform P4 (white), forms based on brokentetrahedra Pn (red and violet), and a hexag-onal layer-type lattice (black). Arsenic andantimony have the As4 and Sb4 forms,which are less stable than the layer-typelattice form; bismuth has the layer-latticeform only.

group 16 elements (chalcogens) Agroup of elements of the periodic table con-sisting of the elements oxygen (O), sulfur(S), selenium (Se), tellurium (Te), and polo-nium (Po). These elements were formerlyclassified as group VI elements and be-longed to the VIA subgroup. The VIB sub-group consisted of the elements chromium(Cr), molybdenum (Mo), and tungsten(W), which are transition elements.

group 14 elements

100

The electronic configurations of thegroup 6 elements are all s2p4 with no va-cant inner orbitals. These configurationsare all just two electrons short of a rare-gasstructure. Consequently they have highelectron affinities and are almost entirelynonmetallic in character.

The group shows the normal propertyof a trend toward metallic character as it isdescended. Selenium, tellurium, and polo-nium have ‘metallic’ allotropes, and polo-nium has generally metalloid-typeproperties where it has been possible tostudy these (Po is very rare). All the el-ements combine with a large number ofother elements, both metallic and non-metallic, but in contrast to compounds ofthe halogens they are more generally insol-uble in water, and even when soluble donot ionize readily.

Oxygen behaves like the first membersof other groups in displaying great differ-ences from the rest of its group. For exam-ple, oxygen forms hydrogen bondswhereas sulfur and the others do not; oxy-gen cannot expand its outer shell and pos-itive oxidation states are uncommon (OF2,O2F2, O2

+PtF6–). Oxygen is paramagnetic.

Excluding oxygen, group 16 shows similartrends with increasing size to those ingroup 15. For example, with increasingsize there is decreasing stability of H2X,greater tendency to form complexes suchas SeBr6

2–, decreasing stability of high for-mal positive oxidation states, and marginalmetallic properties (e.g. Po forms a hy-droxide).

group 17 elements A group of elementsin the periodic table consisting of the el-ements fluorine (F), chlorine (Cl), bromine(Br), iodine (I), and astatine (At). Formerlythey belonged to group VII and were clas-sified in the VIIA subgroup. The VIIB sub-group consisted of the elements manganese(Mn), technetium (Te), and rhenium (Re),

which are transition elements. See halo-gens.

group 18 elements See rare gases.

group theory A branch of mathematicsused to analyze symmetry in a systematicway. A group consists of a set of elementsA, B, C, etc. and a rule for forming theproduct of these elements, such that certainconditions are satisfied:1. Every product AB of two elements A and

B is also an element of the set.2. Any three elements A, B, C, must satify

A(BC) = (AB)C.3. There is an element of the set, denoted I,

which is the identity element, i.e. for allA in the set AI = IA =A.

4. For each element A of the set there is aninverse, denoted A–1, which also belongsto the set which satisfies the relationAA–1 = A–1 = A = I.In the case of symmetry operations the

product is the successive performance ofthese operations. If a group has a finitenumber of elements it is said to be a dis-crete group. The point groups, which arisefrom the rotations and reflections of bodiessuch as isolated molecules are discretegroups.

GSC Gas-solid chromatography. See gaschromatography.

gunmetal See bronze.

gunpowder A finely powdered mixtureof sulfur, charcoal, and potassium nitrate,used as an explosive.

gypsum See calcium sulfate.

gyromagnetic ratio The ratio of themagnetic moment of a system to the angu-lar momentum of that system. The gyro-magnetic ratio is a useful quantity in thetheory of electrons and atomic nuclei.

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gyromagnetic ratio

Haber, Fritz (1868–1934) Germanphysical chemist. Haber is noted for hisdiscovery of the industrial process for syn-thesizing ammonia from nitrogen and hy-drogen. The need at the time was fornitrogen compounds for use as fertilizers –most plants cannot utilize free nitrogenfrom the air, and need ‘fixed’ nitrogen. Themain source was deposits of nitrate salts inChile, but these would have a limited life.Haber, in an attempt to solve this problem,began investigating the reaction:

N2 + 3H2 ˆ 2NH3Under normal conditions the yield is verylow. Haber (1907–09) showed that practi-cal yields could be achieved at high tem-peratures (250°C) and pressures (250atmospheres) using a catalyst (iron is thecatalyst now used). The process was devel-oped industrially by Carl BOSCH around1913 and is still the main method for thefixation of nitrogen. Haber received theNobel Prize for chemistry for this work in1918.

Haber process An important industrialprocess for the manufacture of ammonia,which is used for fertilizers and for makingnitric acid. The reaction is the equilibrium:

N2 + 3H2 ˆ 3NH3The nitrogen used is obtained by fractionaldistillation of liquid air and the hydrogenby the oxidation of hydrocarbons (fromnatural gas). The nitrogen and hydrogenare purified and mixed in the correct pro-portions. The equilibrium amount of am-monia is favoured by low temperatures,but in practice the reaction rate would betoo slow to be economic. An optimum tem-perature of about 450°C is therefore used,along with a catalyst. High pressure alsofavors the reaction and a pressure of about250 atmospheres is used. The catalyst is

iron with small amounts of potassium andaluminum oxides present. The yield isabout 15%. Liquid ammonia is condensedout of the gas equilibrium mixture at–50°C.

habit See crystal habit.

hafnium A bright silvery transitionmetal, the third element of group 4 (for-merly subgroup IVB) of the periodic table.It is found in zirconium ores. Hafnium isdifficult to work and can burn in air. It isused in control rods for nuclear reactorsand in certain specialized alloys and ce-ramics.

Symbol: Hf; m.p. 2230°C; b.p. 5197°C;r.d. 13.31 (20°C); p.n. 72; most commonisotope 180Hf; r.a.m. 178.49.

hahnium Symbol: Ha or Hn A nameformerly suggested for element-105, nowknown as dubnium (Db), and also for el-ement-108, now known as hassium (Hs).See element.

half cell An electrode in contact with asolution of ions. In general there will be ane.m.f. set up between electrode and solu-tion by transfer of electrons to or from theelectrode. The e.m.f. of a half cell cannotbe measured directly, since setting up a cir-cuit results in the formation of another halfcell. See electrode potential.

half-life (half-life period; Symbol: t½) Thetime taken for half the nuclei of a sample ofa radioactive nuclide to decay. The half-lifeof a nuclide is a measure of its stability.(Stable nuclei can be thought of as havinginfinitely long half-lives.) If N0 is the origi-nal number of nuclei, the number remain-

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H

ing at the end of one half-life is N0/2, at theend of two half-lives is N0/4, etc.

half sandwich compound See sand-wich compound.

halide A compound containing a halo-gen. The inorganic halides of electroposi-tive elements contain ions of the typeNa+Cl– (sodium chloride) or K+Br– (potas-sium bromide). Transition metal halidesoften have some covalent bonding. Non-metal halides are covalent compounds,which are usually volatile. Examples aretetrachloromethane (CCl4) and silicontetrachloride (SiCl4). Halides are named asbromides, chlorides, fluorides, or iodides.

halite (rock salt) A naturally occurringmineral form of sodium chloride, NaCl. Itforms colorless or white crystals whenpure, but is often colored by impurities.

halogenation A reaction in which aHALOGEN atom is introduced into a gener-ally organic molecule. Halogenations arespecified as chlorinations, brominations,fluorinations, etc., according to the el-ement involved. There are several methods.1. Direct reaction with the element using

high temperature or ultraviolet radia-tion:

CH4 + Cl2 → CH3Cl + HCl2. Addition to a double bond:

H2C:CH2 + HCl → C2H5Cl3. Reaction of a hydroxyl group with a

halogenating agent, such as PCl3:C2H5OH → C2H5Cl + OH–

4. In aromatic compounds direct substitu-tion can occur using aluminum chlorideas a catalyst:

2C6H6 + Cl2 → 2C6H5Cl5. Alternatively in aromatic compounds,

the chlorine can be introduced by react-ing the diazonium ion with copper(I)chloride:

C6H5N2+ + Cl– → C6H5Cl + N2

halogens A group of elements (group17, formerly VIIA, of the periodic table)consisting of fluorine, chlorine, bromine,iodine, and the short-lived element asta-tine. The halogens all have outer valence

shells that are one electron short of a rare-gas configuration. Because of this, thehalogens are characterized by high electronaffinities and high electronegativities, fluo-rine being the most electronegative elementknown. The high electronegativities of thehalogens favor the formation of bothuninegative ions, X–, particularly com-bined with electropositive elements (e.g.NaCl, KBr, CsI, CaF2), and the single co-valent bond –X with elements of moderateto high electronegativity (e.g. HCl, SiF4,SF4, BrCH3, Cl2O). The halogens all formdiatomic molecules, X2, which are charac-terized by their high reactivities with awide range of other elements and com-pounds. As a group, the elements increasein both size and polarizability as the protonnumber increases and there is an attendantdecrease in electronegativity and reactivity.This means that there is no chemistry ofpositive species apart from a few cationiciodine compounds. The heavier halogensCl, Br, and I all form oxo-species with for-mal positive oxidation numbers +1 and +5,chlorine and iodine also forming +3 and +7species (e.g. HOCl, HBrO3, I2O5, HIO4).

The elements decrease in oxidizingpower in the order F2>Cl2>Br2>I2 and theions X– may be arranged in order of in-creasing reducing power F–<Cl–<Br–<Iŋd

ash;. Thus any halogen will displace the el-ements below it from their salts in solution,for example

Cl2 + 2Br– → Br2 + 2Cl–

A wide range of organic halides isformed in which the C–F bond is charac-teristically resistant to chemical attack; theC–Cl bond is also fairly stable, particularlyin aryl compounds but the alkyl halogencompounds become increasingly suscepti-ble to nucleophilic attack and are generallymore reactive.

hammer mill A device used in the chem-ical industry for crushing and grindingsolid materials at high speeds to a specifiedsize. The impact between the particles,grinding plates, and grinding hammers pul-verizes the particles. Hammer mills can beused for a greater variety of soft materialthan other types of grinding equipment.Compare ball mill.

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hardness (of water) Hard water is waterthat will not readily form a lather withsoap owing to the presence of dissolvedcalcium, iron, and magnesium compounds.Such compounds can react with soap toproduce insoluble salts, which collect assolid scum. The effectiveness of the cleans-ing solution is thus reduced. Hardness ofwater is of two types, temporary hardnessand permanent hardness. Only temporaryhardness can be removed by boiling thewater.

Hardness of water is usually expressedby assuming that all the hardness is due todissolved calcium carbonate, which is pre-sent as ions. It can be estimated by titrationwith a standard soap solution or with edta.See also permanent hardness; detergents;temporary hardness; water softening.

hard vacuum See vacuum.

hard water See hardness.

hassium A transactinide element that isformed artificially.

Symbol: Hs; p.n. 108; most stable iso-tope 265Hs (half-life 2 × 10–3s).

hausmannite See manganese.

heat Energy transferred as a result of atemperature difference. The term is oftenloosely used to mean internal energy (i.e.the total kinetic and potential energy of theparticles). It is common in chemistry to de-fine such quantities as heat of combustion,heat of neutralization, etc. These are in factmolar enthalpies for the change, given thesymbol ∆HMŠ. The superscript symbol de-notes standard conditions, while the sub-script M indicates that the enthalpy changeis for one mole. The unit is usually the kilo-joule per mole (kJ mol–1). By convention,∆H is negative for an exothermic reaction.Molar enthalpy changes stated for chemi-cal reactions are changes for standard con-ditions, which are defined as 298 K (25°C)and 101325 Pa (1 atmosphere). Thus, thestandard molar enthalpy of reaction is theenthalpy change for reaction of substancesunder these conditions producing reactantsunder the same conditions. The substances

involved must be in their normal equilib-rium physical states under these conditions(e.g. carbon as graphite, water as the liq-uid, etc.). Note that the measured enthalpychange will not usually be the standardchange. In addition, it is common to spec-ify the entity involved. For instance∆HfŠ(H2O) is the standard molar en-thalpy of formation for one mole of H2Ospecies.

heat engine (thermodynamic engine) Adevice for converting heat energy intowork. Heat engines operate by transferringenergy from a high-temperature source to alow-temperature sink. The theoretical op-eration of heat engines is useful in thetheory of thermodynamics. See Carnotcycle.

heat exchangers Devices that enablethe heat from a hot fluid to be transferredto a cool fluid without allowing the twofluids to come into contact. The normalarrangement is for one of the fluids to flowin a coiled tube through a jacket containingthe second fluid. Both the cooling and heat-ing effect may be of benefit in conservingthe energy used in a chemical plant and incontrolling a process.

heat of atomization The energy re-quired in dissociating one mole of a sub-stance into atoms. See heat.

heat of combustion The energy liber-ated when one mole of a substance burns inexcess oxygen. See heat.

heat of crystallization The energy lib-erated when one mole of a substance crys-tallizes from a saturated solution of thatsubstance.

heat of dissociation The energy re-quired to dissociate one mole of a sub-stance into its constituent elements.

heat of formation The energy changewhen one mole of a substance is formedfrom its elements. See heat.

heat of neutralization The energy lib-

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erated when one mole of an acid or base isneutralized.

heat of reaction The energy changewhen molar amounts of given substancesreact completely. See heat.

heat of solution The energy changewhen one mole of a substance is dissolvedin a given solvent to infinite dilution (inpractice, to form a dilute solution).

heavy hydrogen See deuterium.

heavy-metal pollution See pollution.

heavy water See deuterium oxide.

hecto- Symbol: h A prefix used with SIunits, denoting 102. For example, 1 hec-tometer (hm) = 102 meters (m).

Heisenberg, Werner Karl (1901–76)German physicist. Heisenberg was one ofthe founders of quantum mechanics. In1925, along with Max BORN and PascualJordan, he formulated matrix mechanics.In 1926 he studied the spectrum of mo-lecular hydrogen and predicted the exis-tence of ortho- and para-hydrogen fromthe splitting of the spectral lines. His mostfamous contribution is the HEISENBERG UN-CERTAINTY PRINCIPLE, put forward in 1927.

Heisenberg uncertainty principle Theimpossibility of making simultaneousmeasurements of both the position and themomentum of a subatomic particle (e.g. anelectron) with unlimited accuracy. The un-certainty arises because, in order to detectthe particle, radiation has to be ‘bounced’off it, and this process itself disrupts theparticle’s position. Heisenberg’s uncer-tainty principle is not a consequence of ‘experimental error’. It represents a funda-mental limit to objective scientific observa-tion, and arises from the wave–particleduality of particles and radiation. In onedirection, the uncertainty in position ∆x and momentum ∆p are related by ∆x∆p ∼ h/4π, where h is the Planck con-stant. It is named for the German physicist

Werner Heisenberg, who discovered it in1927.

Heitler, Walter (1904–81) German-born physicist. Heitler is most famous for aclassic paper he wrote with Friz London in1927 in which they showed that the chem-ical bond in the hydrogen molecule couldbe described by quantum mechanics.Heitler extended this work to more com-plicated molecules and was a pioneer of theuse of group theory in quantum mechanics.He wrote a classic book entitled TheQuantum Theory of Radiation, the thirdedition of which was published in 1954.

helicate See supramolecular chemistry.

helium An inert colorless odorlessmonatomic gas, the first member of therare gases; i.e. group 18 (formerly VIIIA or0) of the periodic table. Helium has theelectronic configuration 1s2 and consists ofa nucleus of two protons and two neutrons(equivalent to an alpha particle) with twoextra-nuclear electrons. It has an extremelyhigh ionization potential and is completelyresistant to chemical attack of any sort.Helium is the second most abundant el-ement in the universe, the primary processin the Sun and other stars being the nuclearfusion of hydrogen to give helium. Never-theless the gas accounts for only 5.2 ×10–4% of the Earth’s atmosphere althoughsome natural gas deposits contain up to7%.

Helium is recovered commercially bythe fractional distillation of natural gasand also forms part of ammonia plant tailgas if natural gas is used as a feedstock. Itsapplications are in fields in which inertnessis required and where cheaper alternatives,such as nitrogen, would prove too reactive.Examples include high-temperature metal-lurgy, powder technology, and use as acoolant in nuclear reactors. Helium is alsofavoured over nitrogen for diluting oxygenin breathing mixtures designed for deep-sea diving due to its lower solubility inblood and as a pressurizer for liquefied gasfuels in rockets (due to its total inertness).It is also used as an ideal gas for balloons

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helium

(no fire risk) and for low-temperaturephysics research.

Helium is unusual in that it is the onlyknown substance for which there is notriple point (i.e., no combination of pres-sure and temperature at which all threephases can co-exist in equilibrium). This isbecause the interatomic forces, which nor-mally participate in the formation ofsolids, are so weak in helium that they areof the same order as the zero-point energy.At 2.186 K helium undergoes a transitionfrom liquid helium I to liquid helium II, thelatter being a true liquid but exhibiting su-perconductivity and an immeasurably lowviscosity (SUPERFLUIDITY). This low viscos-ity allows the liquid to spread in layers afew atoms thick, described by some as anaction like ‘flowing uphill’.

Helium has two naturally occurring iso-topes, of which helium-4 (4He) is the mostcommon. Helium-3 (3He) is formed in nu-clear reactions and by the decay of tritium.This isotope also undergoes a phase changeat temperatures close to absolute zero.

Symbol: He; m.p. 0.95 K (under pres-sure); b.p. 4.216 K; r.d. 0.178; p.n. 2; mostcommon isotope 4He; r.a.m. 4.002602.

Helmholtz free energy See Helmholtzfunction.

Helmholtz function (Helmholtz free en-ergy) Symbol: F A thermodynamic func-tion defined by

F = U – TSwhere U is the internal energy, T the ther-modynamic temperature, and S the en-tropy. It is a measure of the ability of asystem to do useful work in an isothermalprocess. See also free energy.

hematite A reddish-brown, gray, orblack mineral form of iron(III) oxide(Fe2O3), found in igneous, metamorphic,and sedimentary rocks. It is the principalore of iron and is also used as a pigment.

hemihydrate A crystalline compoundhaving one molecule of water of crystal-lization per two molecules of compound(e.g. 2CaSO4.H2O).

hemimorphite See calamine.

henry Symbol: H The SI derived unit ofinductance, equal to the inductance of aclosed circuit that has a magnetic flux ofone weber per ampere of current in the cir-cuit. 1 H = 1 Wb A–1. It is named for U.S.physicist Joseph Henry (1797–1878).

Henry’s law The concentration (C) of agas in solution is proportional to the par-tial pressure (p) of that gas in equilibriumwith the solution, i.e. p = kC, where k is aproportionality constant. The relationshipis similar in form to that for Raoult’s law,which deals with ideal solutions.

A consequence of Henry’s law is thatthe ‘volume solubility’of a gas is indepen-dent of pressure. It was discovered in 1801by British chemist William Henry (1774–1836).

heptahydrate A crystalline hydratedcompound containing one molecule ofcompound per seven molecules of water ofcrystallization.

heptavalent (septivalent) Having a va-lence of seven.

hertz Symbol: Hz The SI derived unit offrequency, defined as one cycle per second(s–1). Note that the hertz is used for regu-larly repeated processes, such as vibrationsor wave motions. It is named for the Ger-man physicist Heinrich Hertz (1857–94).

Herzberg, Gerhard (1904–99) German-born Canadian spectroscopist. Herzberginvestigated and interpreted the spectra ofmany molecules. He wrote a series of clas-sic, definitive books on atomic and mo-lecular spectra between the 1930s and the1970s. Herzberg was awarded the NobelPrize for chemistry in 1971.

Hess’s law (law of constant heat summa-tion) A derivative of the first law of ther-modynamics. It states that the total heatchange for a given chemical reaction in-volving alternative series of steps is inde-pendent of the route taken. It is named forthe Swiss-born Russian chemist Germain

Helmholtz free energy

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Hess (1802–50), who stated it in 1840.

heterogeneous Relating to more thanone phase. A heterogeneous mixture, forinstance, contains two or more distinctphases.

heterogeneous catalyst See catalyst.

heterolysis See heterolytic fission.

heterolytic fission (heterolysis) Thebreaking of a covalent bond so that bothelectrons of the bond remain with one frag-ment. A positive ion and a negative ion areproduced:

RX → R+ + X–

Compare homolytic fission.

hexacyanoferrate See cyanoferrate.

hexagonal close-packed crystal Aclose-packed crystal structure in which lay-ers of close-packed atoms, ions, or mol-ecules are stacked in an ABABABarrangement. In this arrangement, the Blayer fits into the recesses left in the Alayer, with the third layer then fitting in therecesses in the second layer that are directlyabove the atoms, etc. in the first layer. Zincand magnesium have hexagonal close-packed structures. See close packing. Com-pare face-centered cubic crystal.

hexagonal crystal See crystal system.

Hinshelwood, Sir Cyril Norman(1897–1967) British chemist. Hinshel-wood was a leading figure in the study ofthe kinetics and mechanisms of chemicalreactions. He wrote a classic book on thistopic entitled The Kinetics of ChemicalChange in Gaseous Systems (1926). In1956 he shared the Nobel Prize for chem-istry with Nikolay SEMENOV for his workon the mechanisms of chain reactions. Hin-shelwood worked extensively on chemicalreactions in bacteria, summing up his re-search in the book The Chemical Kineticsof the Bacterial Cell (1954).

holmium A soft malleable silvery el-ement of the lanthanoid series of metals. It

occurs in association with other lan-thanoids in such minerals as euxenite,gadolinite, monazite, samarskite, andxenotime. It has a few applications in elec-tronics due to the highly magnetic natureof its compounds.

Symbol: Ho; m.p. 1474°C; b.p.2695°C; r.d. 8.795 (25°C); p.n. 67; mostcommon isotope 165Ho; r.a.m. 164.93032.

HOMO See frontier orbital theory.

homogeneous Relating to a singlephase. A homogeneous mixture, for in-stance, consists of only one phase.

homogeneous catalyst See catalyst.

homolysis See homolytic fission.

homolytic fission (homolysis) Thebreaking of a covalent bond so that oneelectron from the bond is left on each frag-ment. Two free radicals result:

RR′ → R• + R′•Compare heterolytic fission.

hornblende Any of a group of dark-col-ored rock-forming minerals consisting ofcomplex silicates of calcium, sodium, alu-minum, iron, and magnesium. It is a majorcomponent of granite and other igneousand metamorphic rocks.

host–guest chemistry A branch ofsupramolecular chemistry in which a mo-lecular structure acts as a ‘host’ to hold a‘guest’ ion or molecule. The guest may becoordinated to the host or may be trappedby its structure. See also supramolecularchemistry.

HPLC High-performance liquid chro-matography; a sensitive analytical tech-nique, similar to gas-liquid chroma-tography but using a liquid carrier. Thecarrier is specifically choosen for the par-ticular substance to be detected.

Humphreys series See hydrogen atomspectrum.

Hund’s rule A rule that states that the

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Hund's rule

electronic configuration in degenerate or-bitals will have the minimum number ofpaired electrons. It is named for the Ger-man physicist Friedrich Hund (1896–1993), who stated it in 1925.

hybrid orbital See orbital.

hydrate A compound coordinated withwater molecules. When water is bound upin a compound it is known as the WATER OF

CRYSTALLIZATION.

hydration The solvation of species suchas ions in water.

hydraulic cement See cement.

hydrazine (N2H4) A colorless liquidthat can be prepared by the oxidation ofammonia with sodium chlorate(I) or by thegas phase reaction of ammonia with chlo-rine. Hydrazine is a weak base, formingsalts (e.g. N2H4.HCl) with strong acids. Itis a powerful reducing agent, reducing saltsof the noble metals to their respective met-als. Anhydrous hydrazine ignites sponta-neously in oxygen and reacts violently withoxidizing agents. The aqueous solution,hydrazine hydrate, has been used as a fuelfor jet engines and for rockets.

hydrazine hydrate See hydrazine.

hydrazoic acid (hydrogen azide; HN3) Acolorless liquid with a nauseating smell. Itis highly poisonous and explodes in thepresence of oxygen and oxidizing agents. Itcan be made by distilling a mixture ofsodium azide (NaN3) and a dilute acid. It isusually used as an aqueous solution. Thesalts of hydrazoic acid (azides), especiallylead azide (Pb(N3)2), are used in detonatorsbecause they explode when given a me-chanical shock.

hydride A compound of hydrogen. Ionichydrides are formed with highly elec-tropositive elements and contain the H– ion(hydride ion). Nonmetals form covalenthydrides, as in methane (CH4) or silane(SiH4). The boron hydrides are electron-deficient covalent compounds. Many tran-

sition metals absorb hydrogen to form in-terstitial hydrides.

hydrobromic acid (HBr) A colorlessliquid produced by dissolving HYDROGEN

BROMIDE in water. It shows the typicalproperties of a strong acid and is a strongreducing agent. A convenient way of pro-ducing hydrobromic acid is to bubble hy-drogen sulfide through bromine water.Although it is not as strong as hydrochloricacid it dissociates extensively in water andis a good proton donor.

hydrocarbon Any compound contain-ing only the elements carbon and hydro-gen. Methane and ethane are examples.

hydrochloric acid (HCl) A colorlessfuming liquid made by dissolving HYDRO-GEN CHLORIDE to water:

HCl(g) + H2O1(l). → H3O+(aq) +Cl–(aq)

Dissociation into ions is extensive andhydrochloric acid shows the typical prop-erties of a strong acid. It reacts with car-bonates to give carbon dioxide and yieldshydrogen when reacted with all but themost unreactive metals. Hydrochloric acidis used in the manufacture of dyes, drugs,and photographic materials. It is also usedto pickle metals, i.e. clean their surfacesprior to electroplating. It donates protonswith ease and is the strongest of the hydro-halic acids. The concentrated acid is oxi-dized to chlorine by such agents aspotassium manganate(VII) and man-ganese(IV) oxide.

hydrocyanic acid (prussic acid; HCN)A highly poisonous weak acid formedwhen hydrogen cyanide gas dissolves inwater. Its salts are cyanides. Hydrogencyanide is used in making acrylic plastics.

hydrofluoric acid (HF) A colorless liq-uid produced by dissolving HYDROGEN FLU-ORIDE in water. It is a weak acid, but willdissolve most silicates and hence can beused to etch glass. As the interatomic dis-tance in HF is relatively small, the H–FBOND ENERGY is very high and hydrogen

hybrid orbital

108

fluoride is not a good proton donor. Itdoes, however, form hydrogen bonds.

hydrogen A colorless odorless gaseouselement traditionally placed in group 1(formerly IA) of the periodic table. Al-though it has some similarities to both thealkali metals (group 1) and the halogens(group 17), hydrogen is now not normallyclassified in any particular periodic group.It is the most abundant element in the uni-verse and the ninth most abundant elementin the Earth’s crust and atmosphere (bymass). It occurs in nature principally as aconstituent of water, compounds formedby organisms, and hydrocarbon deposits(coal, oil, and natural gas); traces of mo-lecular hydrogen are found in some naturalgases and in the upper atmosphere.

The gas may be prepared in the labora-tory by the reaction of dilute hydrochloricacid with a metal that lies above hydrogenin the electromotive series, magnesium andzinc being commonly used:

Zn + 2HCl → H2 + ZnCl2Reactions of the amphoteric metals zinc

and aluminum with dilute aqueous alkali,to form zincates or aluminates, are alsoconvenient means of obtaining laboratoryhydrogen. Although electrolysis of dilutemineral acids may be used as well, caremust be taken to avoid mixing the hydro-gen released at the cathode with the oxygenreleased at the anode, because such mix-tures are potentially explosive. Industri-ally, hydrogen is obtained as a by-productof the electrolytic cells used in the produc-tion of sodium hydroxide, i.e. by reactingthe Na/Hg amalgam (produced whensodium liberated from the brine solutionalloys with the mercury cathode) withwater, or by the water-gas route in whichsteam is decomposed by hot coke.

The main use of hydrogen is as a chem-ical feedstock for the manufacture of am-monia and a range of organic compounds.Small-scale uses include reducing atmos-pheres for metallurgy, hydrogenation ofedible oils, and pharmaceutical manufac-turing. Hydrogen also has significant po-tential as a fuel, especially using fuel celltechnology.

The hydrogen atom consists of a proton(positive charge) with one extranuclearelectron (s1). Hydrogen is thus the simplestof all atoms, giving it a unique positionamong the elements. The chemistry of hy-drogen depends on one of three processes:1. Loss of the electron to form H+.2. Gain of an electron to form H–.3. Sharing of electrons by covalent bond

formation, as in H2 or HCl.The hydrogen atom with the 1s electron

removed would have an extremely smallionic radius and the positive hydrogen ionin fact occurs only in association withother species, as in H+NH3 or H+FH. Theion commonly written H+ in solution is ac-tually the solvated proton (hydroxoniumor hydronium) H3O+, which is formed byionization of acids, e.g.:

H2O + HCl → H3O+ + Cl–

It is believed that the lifetime of any oneH3O+ ion is extremely short, as the protonsappear to undergo very rapid exchange be-tween water molecules.

Hydrogen also forms a number of com-pounds in which it is regarded as gainingan electron and becoming H– (the hydrideion). It can form ionic HYDRIDES only withthe most electropositive elements (i.e.,those in groups 1 and 2). The ionic natureof these compounds is indicated by the factthat their melts are good conductors andthat hydrogen is liberated at the anode dur-ing their electrolysis. These hydrides areprepared by heating the element in astream of hydrogen:

H2 + 2M → 2MHExamples of hydrides with covalent

bonds are CH4, NH3, and HCl. These com-pounds are generally low-boiling. Generalmethods of preparation are:1. The hydrolysis of the appropriate ‘-ide’

compound, e.g. silicides give silane, ni-trides give ammonia, sulfides give H2S.

2. Reduction of a chloride; for example:SiCl4 + LiAlH4 → SiH4 + LiCl + AlCl3A third kind of hydride is that of metal-

lic hydrides formed between hydrogen andmany transition metals. Palladium in par-ticular is renowned for its ability to absorbhydrogen as PdHx, where x takes values upto 1.8. Titanium and zirconium behavesimilarly, but the exact nature of the com-

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hydrogen

pounds formed and the type of bond in-volved remains uncertain. There arechanges in the magnetic properties of themetal, indicating some electron interac-tion, and in the lattice dimensions of thecompound, but discrete phases of the typeMH or MH2 have not been isolated. Thesecompounds are sometimes referred to asthe interstitial hydrides.

Atomic nuclei possess the property of‘spin’ and for diatomic molecules there ex-ists the possibility of having the spins of ad-jacent nuclei aligned (ortho) or opposed(para). Because of the small mass of hydro-gen, these forms are more important in hy-drogen molecules than in other diatomicmolecules. The two forms are in equilib-rium, with parahydrogen dominant at lowtemperatures. However the equilibriumratio rises to 3 parts orthohydrogen to 1part parahydrogen at room temperatures.Although chemically identical, the meltingpoint and boiling point of the para formare both about 0.1°C lower than the 3:1equilibrium mixture.

Natural hydrogen in molecular or com-bined forms contains about one part in2000 of DEUTERIUM, symbol D, an isotopeof hydrogen that contains one proton andone neutron in its nucleus. The artificiallycreated radioactive isotope TRITIUM, sym-bol T, has one proton and two neutrons.Although the effect of isotopes on chemicalproperties is normally small, in the case ofhydrogen the difference in mass numberleads to a lowering of some reaction rates,a phenomenon known as the ‘isotope ef-fect’.

Hydrogen also exhibits two less com-mon forms of bonding. Boron hydridesform a wide variety of compounds inwhich the hydrogen acts as a bridgingspecies involving ‘three-center two-elec-tron’ bonds. Such species are said to be‘electron deficient’ because they do nothave sufficient electrons for conventionaltwo-electron covalent bonds. The second,less common, form is that of the coordi-nated hydrides in which the H– ion acts asa ligand bound to a transition metal atom.

Symbol: H; m.p. 14.01 K; b.p. 20.28 K;r.d. 0.089 88; p.n. 1; most common isotope1H; r.a.m. 1.0079.

hydrogenation The reaction of a com-pound with hydrogen. An example is thehydrogenation of nitrogen to form ammo-nia in the Haber process. In organic chem-istry, hydrogenation refers to the additionof hydrogen to multiple bonds, usuallywith the aid of a catalyst. Unsaturated nat-ural liquid vegetable oils can be hydro-genated to form saturated semisolid fats –a reaction used in making types of mar-garine and cooking oils. Consumption ofsuch hydrogenated oils is now thought tobe associated with increased risk of cardio-vascular disease. See Bergius process.

hydrogen atom spectrum The spec-trum of the hydrogen atom is characterizedby several series of sharp spectral lines de-scribed by simple laws. The general law forthese series of lines is:

1/λ = R (1/n12 – 1/n2

2),where λ is the wavelength associated witha spectral line, R is the RYDBERG CONSTANT

and n1 and n2 are integers, with n2≥n1.The first of these series to be discovered

was the Balmer series in which n1 = 2, n2 =3,4,5,… This series is in the visible regionand was discovered by the Swiss mathe-matician and physicist Johann JakobBalmer (1825–98) in 1885. The series inwhich n1 = 1 is the Lyman series which liesin the ultraviolet region. This series wasdiscovered by Theodore Lyman (1874–1954). The Lyman series is a conspicuousfeature of the spectrum of the Sun.

In the Paschen series (n1 = 3), the Brack-ett series (n1 = 4), the Pfund series (n1 = 5)and the Humphreys series (n1 = 6) the spec-tral lines occur in the infrared region.

The explanation for these regular serieslies in the existence of discrete, quantizedenergy levels. In 1913 Niels Bohr was ableto derive the formula for these series interms of the ad hoc quantum assumptionsof the BOHR THEORY. In the mid-1920s theformula was derived in a deductive wayfrom quantum mechanics. In the wave me-chanics formulation of quantum mechan-ics it is possible to derive the formulabecause the Schrödinger equation can besolved exactly for the hydrogen atom.

hydrogen azide See hydrazoic acid.

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110

hydrogen bond (H-bonding) An inter-molecular bond between molecules inwhich hydrogen is bound to a stronglyelectronegative element. Bond polarizationby the electronegative element X leads to apositive charge on hydrogen Xδ––Hδ+; thishydrogen can then interact directly withelectronegative elements of adjacent mol-ecules. The hydrogen bond is representedas a dotted line:

Xδ– – Hδ+ ....... Xδ– – Hδ+ …The length of a hydrogen bond is charac-teristically 0.15–0.2 nm. Hydrogen bond-ing may lead to the formation of dimers(for example, in carboxylic acids) and isused to explain the anomalously high boil-ing points of H2O and HF.

hydrogen bromide (HBr) A colorlesssharp-smelling gas that is very soluble inwater, giving HYDROBROMIC ACID. It is pro-duced by direct combination of hydrogenand bromine in the presence of a platinumcatalyst or by the reaction of phosphorustribromide with water. Hydrogen bromideis rather inactive chemically. It will notconduct electricity in the liquid state, indi-cating that it is a molecular compound.

hydrogencarbonate (bicarbonate) Asalt containing the ion –HCO3.

hydrogen chloride (HCl) A colorlessgas that has a strong irritating odor andfumes strongly in moist air. It is preparedby the action of concentrated sulfuric acidon sodium chloride. The gas is made in-dustrially by burning a stream of hydrogenin chlorine. It is not particularly reactivebut will form dense white clouds of ammo-nium chloride when mixed with ammonia.It is very soluble in water and ionizes al-most completely to give HYDROCHLORIC

ACID. It will also dissolve in many non-aqueous solvents, including toluene, butwill not ionize, indicating that the resultingsolution will show no acidic properties.Hydrogen chloride is used in the manufac-ture of organic chlorine compounds, suchas polyvinyl chloride (PVC).

Unlike the other hydrogen halides, hy-drogen chloride will not dissociate on heat-ing, indicating a strong H–Cl bond.

hydrogen cyanide See hydrocyanicacid.

hydrogen electrode (standard hydrogenhalf cell) A type of half cell based on hy-drogen, and assigned zero electrode poten-tial, so that other elements may becompared with it. The hydrogen is bubbledover a platinum electrode, coated in ‘plat-inum black’, in a 1 M acid solution. Hy-drogen is adsorbed on the platinum black,which has a high surface area, enabling theequilibrium

H(g) ˆ H+(aq) + e–

to be set up. The platinum is inert and hasno tendency to form platinum ions in solu-tion. See also electrode potential.

hydrogen fluoride (HF) A colorless liq-uid produced by the reaction of concen-trated sulfuric acid with calcium fluoride:

CaF2(s) + H2SO4(aq) → CaSO4(aq) +2HF(l)

It produces toxic corrosive fumes and dis-solves readily in water to give HYDROFLUO-RIC ACID.

Hydrogen fluoride is atypical of the hy-drogen halides inasmuch as the individualH–F units are associated into much largerunits, forming zigzag chains and rings.This effect is caused by hydrogen bondsthat form between the hydrogen and thehighly electronegative fluoride ions. Hy-drogen fluoride is used extensively as a cat-alyst in the petroleum industry.

hydrogen ion A positively charged hy-drogen atom, H+, i.e. a proton. Hydrogenions are produced by all acids in water, inwhich they are hydrated to hydroxonium(hydronium) ions, H3O+. See acid; pH.

hydrogen molecule ion (H2+) The sim-

plest type of molecule. It consists of twohydrogen nuclei and one electron. In theBORN–OPPENHEIMER APPROXIMATION inwhich the nuclei are regarded as beingfixed the SCHRÖDINGER EQUATION for thehydrogen molecule ion can be solved ex-actly. This enables ideas and approxima-tion techniques concerned with chemicalbonding to be tested quantitatively.

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hydrogen molecule ion

hydrogen peroxide (H2O2) A colorlesssyrupy liquid, usually used in solution inwater. Although it is stable when pure, oncontact with bases such as manganese(IV)oxide it gives off oxygen, themanganese(IV) oxide acting as a catalyst:

2H2O2 → 2H2O + O2Hydrogen peroxide can act as an oxi-

dizing agent, converting iron(II) ions toiron(III) ions, or as a reducing agent withpotassium manganate(VII). It is used as ableach and mild antiseptic, and as an oxi-dant in rocket fuel. The strength of H2O2solutions is usually given as volumestrength, i.e. the volume (in cubic decime-ters) of oxygen released at STP by the de-composition of one cubic decimeter of thesolution.

hydrogensulfate (bisulfate; HSO4–) An

acidic salt of sulfuric acid (H2SO4), inwhich only one of the acid’s hydrogenatoms has been replaced by a metal. An ex-ample is sodium hydrogensulfate,NaHSO4.

hydrogen sulfide (sulfuretted hydrogen;H2S) A colorless very poisonous gas withan odor of bad eggs. Hydrogen sulfide isprepared by reacting hydrochloric acidwith iron(II) sulfide. Its presence can be de-termined by mixing with lead nitrate solu-tion, with which H2S gives a blackprecipitate. Its aqueous solution is weaklyacidic. Hydrogen sulfide reduces iron(III)chloride to iron(II) chloride, forming hy-drochloric acid and a yellow precipitate ofsulfur. Hydrogen sulfide precipitates met-als from insoluble sulfides in acid solution,and is used in qualitative analysis. It burnswith a blue flame in oxygen to form sul-fur(IV) oxide and water. Natural gas con-tains some hydrogen sulfide, which isremoved before supply to the consumer.

hydrogensulfite (bisulfite; HSO3–) An

acidic salt of sulfurous acid (H2SO3), inwhich only one of the acid’s hydrogenatoms has been replaced by a metal. Anexample is sodium hydrogensulfite,NaHSO3.

hydrohalic Describing acids formed byhydrogen halides, e.g. HF, HCI, HBr,when dissolved in water.

hydroiodic acid See iodine.

hydrolysis A reaction between a com-pound and water, e.g.:Salts of weak acids

Na2CO3 + 2H2O → 2NaOH + H2CO3and certain inorganic halides

SiCl4 + 4H2O → Si(OH)4 + 4HCl

hydronium See acid; hydrogen ion.

hydrophilic See lyophilic.

hydrophobic See lyophobic.

hydrosol A colloid in aqueous solution.

hydroxide A compound containing theion OH– or the group –OH.

hydroxonium ion See acid; hydrogenion.

hydroxyl group A group (–OH) con-taining hydrogen and oxygen, characteris-tic of alcohols and some hydroxides. Itshould not be confused with the hydroxideion (OH–).

hygroscopic Describing a substancethat absorbs moisture from the atmos-phere. See also deliquescent.

hyperfine structure See fine structure.

hypo See sodium thiosulfate(IV).

hypobromous acid See bromic(I) acid.

hypochlorous acid See chloric(I) acid.

hypophosphorous acid See phosphinicacid.

hyposulfuric acid See dithionic acid.

hyposulfurous acid See dithionousacid.

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Iceland spar A pure transparent nearlyflawless mineral form of calcite (calciumcarbonate, CaCO3), noted for its propertyof birefringence (double refraction). Itcomes from Eskifjördhur in Iceland andhas been used in optics.

icosahedron A polyhedron bounded by20 triangular faces. The icosahedron is oneof the main structural units of boron andits compounds. Icosohedral symmetry alsooccurs in certain CLUSTER COMPOUNDS.

ideal gas (perfect gas) See gas laws; ki-netic theory.

ideal-gas scale See absolute tempera-ture.

ideal solution See Raoult’s law.

immiscible Describing two or more liq-uids that will not mix, such as oil andwater. After being shaken together and leftto stand they form separate layers.

indicator A compound that reversiblychanges color depending on the pH of thesolution in which it is dissolved. The visualobservation of this change is therefore aguide to the pH of the solution and it fol-lows that careful choice of indicators per-mits a wide range of end points to bedetected in acid–base titrations.

Redox titrations require either specificindicators, which detect one of the compo-nents of the reaction (e.g. starch for iodine,potassium thiocyanate for Fe3+), or trueredox indicators in which the transitionpotential of the indicator between oxidizedand reduced forms is important. The tran-sition potential of a redox indicator is anal-

ogous to the transition pH in acid–basesystems.

Complexometric titrations require indi-cators that complex with metal ions andchange color between the free state and thecomplex state. See also absorption indica-tor.

indium A soft malleable ductile silverymetallic element belonging to group 13(formerly IIIA) of the periodic table. A rareelement, it is found in minute quantities,primarily in zinc ores. It is used in alloys, inseveral electronic devices, and in electro-plating.

Symbol: In; m.p. 155.17°C; b.p.2080°C; r.d. 7.31 (25°C); p.n. 49; mostcommon isotope 115In; r.a.m. 114.818.

inert gases See rare gases.

infrared (IR) Electromagnetic radiationwith longer wavelengths than visible radia-tion. The wavelength range is approxi-mately 0.7 µm to 1 mm. Many materialstransparent to visible light are opaque toinfrared, including glass. Rock salt, quartz,germanium, or polyethene prisms andlenses are suitable for use with infrared. In-frared radiation is produced by movementof charges on the molecular scale; i.e. byvibrational or rotational motion of mol-ecules. Infrared spectroscopy is of particu-lar importance in organic chemistry andabsorption spectra are used extensively inidentifying compounds. Certain bonds be-tween pairs of atoms (C–C, C=C, C=O,etc.) have characteristic vibrational fre-quencies, which correspond to bands in theinfrared spectrum. Infrared spectra arethus used in finding the structures of neworganic compounds by indicating the pres-ence of certain groups. They are also used

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I

to ‘fingerprint’ and thus identify knowncompounds. At shorter wavelengths, in-frared absorption corresponds to transi-tions between rotational energy levels, andcan be used to find the dimensions of mol-ecules (by their moment of inertia).

inhibitor A substance that slows downthe rate of reaction. Hydrogen sulfide, hy-drogen cyanide, mercury salts, and arseniccompounds readily inhibit heterogeneouscatalysts by adsorption. For example, ar-senic compounds inhibit platinum catalystsin the oxidation of sulfur(IV) oxide to sul-fur(VI) oxide. Inhibitors must not be con-fused with negative catalysts; inhibitors donot change the pathway of a reaction.

inorganic chemistry The branch ofchemistry concerned with elements otherthan carbon and with the preparation,properties, and reactions of their com-pounds. Certain simple carbon compoundsare treated in inorganic chemistry, includ-ing the oxides, carbon disulfide, thehalides, hydrogen cyanide, and salts, suchas the cyanides, cyanates, carbonates, andhydrogencarbonates.

insoluble Describing a compound thathas a very low solubility (in a specified sol-vent).

instrumentation The measurement ofthe conditions and the control of processeswithin a chemical plant. The instrumentscan be classified into three groups: thosefor current information using mercurythermometers, weighing scales, and pres-sure gauges; those for recording viscosity,fluid flow, pressure, and temperature; andthose instruments that control and main-tain the desired conditions including pHand the flow of materials.

intercallation compound A com-pound that has a structure based on layersand in which there are layers of a differentcharacter interleaved in the basic structuralunits. For example, the micas phlogopite(KMg3(OH)2Si3AlO10) and muscovite(KAl2(OH)2Si3AlO10) are formed by inter-leaving K+ ions replacing a quarter of the

silicon layers in talc and pyrophyllite re-spectively. See also lamellar compound.

interhalogen See bromine.

intermediate 1. A compound that re-quires further chemical treatment to pro-duce a finished industrial product such as adye or pharmaceutical chemical.2. A transient chemical entity in a complexreaction. See also precursor.

intermediate bonding Describing aform of covalent bond that also has anionic or electrovalent character. See polarbond.

intermolecular forces Forces of attrac-tion between molecules rather than forceswithin the molecule (chemical bonding). Ifthese intermolecular forces are weak thematerial will be gaseous and as theirstrength progressively increases materialsbecome progressively liquids and solids.The intermolecular forces are divided intoH-bonding forces and van der Waalsforces, and the major component is theelectrostatic interaction of dipoles. See hy-drogen bond; van der Waals force.

internal energy Symbol: U The energyof a system that is the total of the kineticand potential energies of its constituentparticles (e.g. atoms and molecules). If thetemperature of a substance is raised, bytransferring energy to it, the internal en-ergy increases (the particles move faster).Similarly, work done on or by a system re-sults in an increase or decrease in the inter-nal energy. The relationship between heat,work, and internal energy is given by thefirst law of thermodynamics. Sometimesthe internal energy of a system is looselyspoken of as ‘heat’ or ‘heat energy’.Strictly, this is incorrect; heat is the trans-fer of energy as a result of a temperaturedifference.

internal resistance Resistance of asource of electricity. In the case of a cell,when a current is supplied, the potentialdifference (p.d.) between the terminals islower than the e.m.f. The difference (i.e.

inhibitor

114

e.m.f. – p.d.) is proportional to the currentsupplied. The internal resistance (r) is givenby:

r = (E – V)/Iwhere E is the e.m.f., V the potential dif-ference between the terminals, and I thecurrent.

interstitial See defect.

interstitial carbide See carbide.

interstitial compound A crystallinecompound in which atoms of a nonmetal(e.g. carbon, hydrogen, or boron) occupyinterstitial positions in the crystal lattice ofa metal (usually a transition metal). Inter-stitial compounds are often nonstoichio-metric. Their physical properties are oftensimilar to those of metals; e.g. they have ametallic luster and are electrical conduc-tors.

interstitial hydrides See hydrogen.

iodate See iodine.

iodic acid See iodic(V) acid.

iodic(V) acid (iodic acid; HIO3) A col-orless deliquescent crystalline solid pro-duced by the reaction of concentratednitric acid with iodine. Iodic(V) acid is astrong oxidizing agent. It will liberate io-dine from solutions containing iodide ionsand it reacts vigorously with organic mate-rials, often producing flames. It dissociatesextensively in water and hence is a strongacid.

iodic(VII) acid (periodic acid; H5IO6) Awhite crystalline solid made by low-tem-perature electrolysis of concentratediodic(V) acid. It exists in a number offorms, the most common of which isparaiodic(VII) acid. Iodic(VII) acid is apowerful oxidizing agent. It is a weak acid,which – by cautious heating under vacuum– can be converted to dimesoiodic(VII)acid (H4I2O9), and metaiodic(VII) acid(HIO4). All three will oxidize manganeseto manganate(VII), reactions that can be

used to determine the presence of smallamounts of manganese in steel.

iodide See halide.

iodine A dark-violet volatile solid el-ement belonging to the halogens i.e. group17 (formerly VIIA) of the periodic table, ofwhich it is the fourth element. It occurs inseawater and is concentrated by variousmarine organisms in the form of iodides.Significant deposits also occur in the formof iodates, i.e. salts containing iodineoxyanions. The element is convenientlyprepared by the oxidation of iodides inacid solution (using MnO2). Industrialmethods similarly use oxidation of iodidesor reduction of iodates to iodides by sul-fur(IV) oxide (sulfur dioxide) followed byoxidation, depending on the source of theraw materials. Iodine and its compoundsare used in chemical synthesis, photogra-phy, pharmaceuticals, and dyestuffs manu-facture. Trace amounts are essential in thediet to ensure proper production of thyroidhormones.

Iodine has the lowest electronegativityof the stable halogens and consequently isthe least reactive. It combines only slowlywith hydrogen to form hydroiodic acid,HI. Iodine also combines directly withmany electropositive elements, but does somuch more slowly than does bromine orchlorine. Because of the larger size of theiodine ion and the consequent low latticeenergies, the iodides are generally moresoluble than related bromides or chlorides.As with the other halides, iodides of Ag(I),Cu(I), Hg(I), and Pb(II) are insoluble unlesscomplexing ions are present.

Iodine also forms a range of covalentiodides with the metalloids and nonmetal-lic elements, but these are generally lessthermodynamically stable and are morereadily hydrolyzed than chlorine orbromine analogs.

Four oxides are known, of which io-dine(V) oxide, I2O5, is the most important.The other oxides, I2O4, I4O9, and I2O7 aremuch less stable and of uncertain structure.Like chlorine (but not bromine) iodineforms oxo-species based on IO–, IO2

–,IO3

–, and IO4–. The chemistry of the other

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iodine

oxo-species in solution is complex. Ele-mental iodine reacts with alkalis in a simi-lar way to bromine and chlorine.

The ionization potential of iodine is suf-ficiently low for it to form a number ofcompounds in which it is electropositive. Itforms I+ cations, for example, by reactionof solid silver nitrate with iodine solution.Iodine also exhibits the interesting prop-erty of forming solutions that are violetcolored in nondonor type solvents, butdark brown solutions in donor solvents,such as ethanol. Even though iodine solu-tions were once common as antisepticagents, the element is classified as toxic andcare should be taken to avoid eye intru-sions or excessive skin contact.

Symbol: I; m.p. 113.5°C; b.p. 184°C;r.d. 4.93 (20°C); p.n. 53; most commonisotope 127I; r.a.m. 126.90447.

iodine monochloride (ICl) A dark redliquid made by passing chlorine over io-dine. It has properties similar to those of itsconstituent halogens. Iodine monochlorideis used as a nonaqueous solvent and as aniodating agent in organic reactions.

iodine(V) oxide (iodine pentoxide;I2O5) A white crystalline solid made byheating iodic(V) acid to a temperature of200°C. It is a very strong oxidizing agent.

Iodine oxide is the acid anhydride ofiodic(V) acid, which is reformed whenwater is added to the oxide. Its main use isin titration work, measuring traces of car-bon monoxide in the air.

iodine pentoxide See iodine(V) oxide.

iodine trichloride See diiodine hexa-chloride.

ion An atom or molecule that has a neg-ative or positive charge as a result of losingor gaining one or more electrons. See elec-trolysis; ionization.

ion association The association of pairsof ions in electrolyte solutions occurringbecause of electrostatic attraction betweenthe ions. Ion association means that com-plete dissociation of electrolytes does not

occur. This has the effect that electricalconductivities that are found experimen-tally are lower than the expectations of theDEBYE–HÜCKEL THEORY. Ion association is adynamic phenomenon in which there israpid exchange between the ions in theelectrolyte solution. A theory to take ionassociation into account, and hence im-prove on the Debye–Hückel theory, wasproposed by Neils Bjerrum (1879–1958) in1926. Bjerrum found that ion associationbecomes more significant as the dielectricconstant of the solvent becomes lower.

ion exchange A process that takes placein certain insoluble materials, which con-tain ions capable of exchanging with ionshaving the same charge in the surroundingmedium. ZEOLITES, the first ion exchangematerials, were used for water softening.These have largely been replaced by syn-thetic resins made of an inert backbonematerial, such as polyphenylethene, towhich ionic groups are weakly attached. Ifthe ions exchanged are positive, the resin isa CATIONIC RESIN. An ANIONIC RESIN ex-changes negative ions. When all availableions on the resin have been exchanged (e.g.sodium ions replaced by calcium ions fromthe medium) the material can usually be re-generated by passing a concentrated solu-tion containing the original resin ion (e.g.sodium chloride) through it. In the exam-ple cited, the calcium ions will be replacedby sodium ions. Ion-exchange techniquesare used for a vast range of purificationand analytical purposes.

For example, solutions with ions thatcan be exchanged for OH– and H+ can beestimated by titrating the resulting solutionwith an acid or a base.

ionic bond See electrovalent bond.

ionic carbide See carbide.

ionic crystal A crystal composed of ionsof two or more elements. The positive andnegative ions are arranged in definite pat-terns and are held together by electrostaticattraction. Common examples are sodiumchloride and cesium chloride.

iodine monochloride

116

ionic product The product of ionic con-centrations in a solution. In the case ofpure water the equilibrium constant is:

KW = [H+][OH–]as a result of a small amount of self-ioniza-tion.

ionic radius A measure of the effectiveradius of an ion in a compound. For an iso-lated ion, the concept is not very meaning-ful, since the ion is a nucleus surrounded byan ‘electron cloud’. Values of ionic radiican be assigned, however, based on the dis-tances between ions in crystals.

Different methods exist for determiningionic radii and often different values arequoted for the same ion. The two mainmethods are those of Goldschmidt and ofPauling. The Goldschmidt radii are deter-mined by substituting data from one com-pound to another, to produce a set of ionicradii for different ions. The Pauling radiiare assigned by a more theoretical treat-ment for apportioning the distance be-tween an anion and a cation.

ionic strength For an ionic solution aquantity can be introduced that empha-sizes the charges of the ions present:

I = ½Σimiz2i

where m is the molality and z the ioniccharge. The summation is continued overall the different ions in the solution, i.

ionization The process of producingions. There are several ways in which ionsmay be formed from atoms or molecules.In certain chemical reactions ionization oc-curs by transfer of electrons; for example,sodium atoms and chlorine atoms react toform sodium chloride, which consists ofsodium ions (Na+) and chloride ions (Cl–).Certain molecules can ionize in solution;acids, for example, form hydrogen ions asin the reaction

H2SO4 → 2H+ + SO42–

The ‘driving force’ for ionization in asolution is solvation of the ions by mol-ecules of the solvent. H+, for example, issolvated as a hydroxonium (hydronium)ion, H3O+.

Ions can also be produced by ionizingradiation; i.e. by the impact of particles or

photons with sufficient energy to break upmolecules or detach electrons from atoms:A → A+ + e–. Negative ions can be formedby capture of electrons by atoms or mol-ecules: A + e– → A–.

ionization energy See ionization poten-tial.

ionization potential (IP; Symbol: I) Theenergy required to remove an electronfrom an atom (or molecule or group) in thegas phase, i.e. the energy required for theprocess:

M → M+ + e–

It gives a measure of the ability of metals toform positive ions. The second ionizationpotential is the energy required to removetwo electrons and form a doubly chargedion:

M → M2+ + e–

Ionization potentials stated in this wayare positive; often they are given in elec-tronvolts. Ionization energy is the energyrequired to ionize one mole of the sub-stance, and is usually stated in kilojoulesper mole (kJ mol–1).

In chemistry, the terms ‘second’, ‘third’,etc., ionization potentials are usually usedfor the formation of doubly, triply, etc.,charged ions. However, in spectroscopyand physics, they are often used with a dif-ferent meaning. The second ionization po-tential is the energy to remove the secondleast strongly bound electron in forming asingly charge ion. For lithium (ls22s1) itwould refer to removal of a 1s electron toproduce an excited ion with the configura-tion 1s12s1. Note also that ionization po-tentials are now stated as energies.

ion pair A positive ion and a negativeion in close proximity in solution, held bythe attractive force between their charges.See electrolysis.

IP See ionization potential.

IR See infrared.

iridium A hard brittle white transitionmetal, the third element of group 9 (for-merly part of subgroup VIIIB) of the peri-

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odic table. Iridium is found native and inassociation with platinum, to which it ischemically similar. Unusually high concen-trations of iridium are also found world-wide in clay bands associated with theboundary marking the end of the Creta-ceous Period 65 million years ago. Thisiridium is thought to have been depositedby the explosive impact of an ancient as-teroid, which in turn may account for thesubsequent mass extinctions of many or-ganisms, including the dinosaurs.

Iridium is highly resistant to corrosionmaking it desirable for alloys for high-pre-cision instruments and bearings. It is alsoused in electrical contacts, in pen points, inspark plugs, and in jewelry.

Symbol: Ir; m.p. 2410°C; b.p. 4130°C;r.d. 22.56 (17°C); p.n. 77; most commonisotope 193Ir; r.a.m. 192.217.

iron A malleable ductile medium-hardsilvery-gray ferromagnetic transition el-ement, the first element of group 8 (for-merly part of subgroup VIIIB) of theperiodic table. Iron occurs in many ores,especially hematite (Fe2O3), magnetite(Fe3O4), siderite (FeCO3), and pyrite(FeS2). It is extracted in blast furnacesusing coke, limestone, and hot air. Thecoke and oxygen in the air form carbonmonoxide, which then reduces the iron inthe ore to the metal. The limestone re-moves acidic impurities and forms a layerof slag above the molten iron at the base ofthe furnace. Carbon STEEL is formed by re-ducing the carbon content to between 0.1and 1.5%.

Iron is used to form structural supportsfor a wide variety of applications, includ-ing bridges, and as a catalyst in the Haberprocess for ammonia production. Iron isalso an essential element in the diet, be-cause it is required to make hemoglobin, acomponent of red blood cells which ab-sorbs oxygen in the lungs and transports itto the tissues.

Iron corrodes in air and moisture to hy-drated iron(III) oxide (rust).

The most stable oxidation state is +3,compounds of which are yellow or brown,but +2 (green) and +6 (easily reduced)states also exist. Solutions of iron(II) ions

give a green precipitate with sodium hy-droxide solution, whereas iron(III) ionsgive a brown precipitate. The concentra-tion of iron(II) ions can be estimated inacid solution by titration with standardpotassium manganate(VII) solution.Iron(III) ions must first be reduced toiron(II) ions with sulfur(IV) oxide (sulfurdioxide).

Symbol: Fe; m.p. 1535°C; b.p. 2750°C;r.d. 7.874 (20°C); p.n. 26; most commonisotope 56Fe; r.a.m. 55.845.

iron(II) chloride (ferrous chloride;FeCl2) A compound prepared by passingdry hydrogen chloride gas over heatediron. White feathery anhydrous crystals areproduced. Hydrated iron(II) chloride(FeCl2.6H2O) is prepared by reacting ex-cess iron with dilute or concentrated hy-drochloric acid. Green crystals of thehexahydrate are obtained on crystalliza-tion from solution. Iron(II) chloride isreadily soluble in water, producing anacidic solution due to salt hydrolysis.

iron(III) chloride (ferric chloride;FeCl3) A compound prepared in the an-hydrous state as dark red crystals by pass-ing dry chlorine over heated iron. Theproduct sublimes and is collected in acooled receiver as brownish-black irides-cent scales. The hydrated salt(FeCl3.6H2O) is prepared by adding excessiron(III) oxide to concentrated hydrochlo-ric acid. On crystallization yellow-browncrystals of the hexahydrate are formed.Iron(III) chloride is very soluble in waterand undergoes salt hydrolysis. At tempera-tures below 400°C the anhydrous salt ex-ists as a dimer, Fe2Cl6.

iron chromium oxide See chromite.

iron(II) oxide (ferrous oxide; FeO) Ablack powder formed by the careful reduc-tion of iron(III) oxide using either carbonmonoxide or hydrogen. It can also be pre-pared by heating iron(II) oxalate in the ab-sence of air. It is only really stable at hightemperatures and disproportionates slowlyon cooling to give iron(III) oxide and iron.Iron(II) oxide can be reduced by heating in

iron

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a stream of hydrogen. When exposed toair, it is oxidized to iron(III) oxide. It is abasic oxide, dissolving readily in diluteacids to form iron(II) salt solutions. Ifheated to a high temperature in an inert at-mosphere, iron(II) oxide disproportionatesto give iron and triiron tetroxide (Fe3O4).

iron(III) oxide (ferric oxide; Fe2O3) Arusty-brown solid prepared by the actionof heat on iron(III) hydroxide or iron(II)sulfate. It occurs in nature as the mineralHEMATITE. Industrially it is obtained byroasting iron pyrites. Iron(III) oxide dis-solves in dilute acids to produce solutionsof iron(III) salts. It is stable at red heat, de-composes around 1300°C to give triirontetroxide (Fe3O4), and can be reduced toiron by hydrogen at 1000°C. Iron(III)oxide is not ionic in character but has astructure similar to that of aluminumoxide.

iron(II) sulfate (ferrous sulfate; green vit-riol; FeSO4.7H2O) A compound that oc-curs in nature as the mineral melanterite(or copperas). It is made industrially fromiron pyrites. In the laboratory iron(II) sul-fate is prepared by dissolving excess iron indilute sulfuric acid. On crystallization,green crystals of the heptahydrate are ob-tained. Careful heating of the hydrated saltyields anhydrous iron(II) sulfate; on fur-ther heating the sulfate decomposes to giveiron(III) oxide, sulfur(IV) oxide, and sul-fur(VI) oxide. The hydrated crystals oxi-dize easily on exposure to air owing to theformation of basic iron(III) sulfate. Afreshly prepared solution of iron(II) sulfateabsorbs nitrogen monoxide (BROWN-RING

TEST).

Iron(II) sulfate crystals are isomor-phous with the sulfates of zinc, magne-sium, nickel, and cobalt.

iron(III) sulfate (ferric sulfate; Fe2(SO4)3)A compound prepared in the hydratedstate by the oxidation of iron(II) sulfatedissolved in dilute sulfuric acid using anoxidizing agent, such as hydrogen peroxideor concentrated nitric acid. On crystalliza-tion, the solution deposits a white mass ofthe nonahydrate (Fe2(SO4)3.9H2O). Theanhydrous salt can be prepared by gentlyheating the hydrated salt. Iron(III) sulfatedecomposes on heating to give iron(III)oxide and sulfur(VI) oxide. It forms alumswith the sulfates of the alkali metals.

irreversible change See reversible change.

irreversible reaction A reaction inwhich conversion to products is complete;i.e. there is little or no back reaction.

isobars 1. Two or more nuclides thathave the same NUCLEON NUMBERS but dif-ferent proton numbers. Actinium-89 andthorium-90 are isobars.2. Lines on a chart or graph joining pointsof equal pressure.

isoelectronic Describing compoundsthat have the same number of electrons.For example, carbon monoxide (CO) andnitrogen (N2) are isoelectronic.

isomer See isomerism.

isomerism The existence of two or morechemical compounds with the same mo-lecular formulae but different structuralformulae or different spatial arrangements

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isomerism

cis-isomer trans-isomer

M

Y

Y

X

X M

X

Y

X

Y

Isomerism: isomers of a square-planar complex

of atoms. The different forms are known asisomers.

In structural isomerism the structuralformulae of the compounds differ as tomolecular structure or to the position offunctional groups. For example, the com-pound C4H10 may be butane (with astraight chain of carbon atoms) or 2-methyl propane (CH3CH(CH3)CH3, witha branched chain). A particular case of thisoccurs in metal complexes when a ligandmay coordinate to a metal ion in two ways.For example, NO2 can coordinate throughN (the nitro ligand) or through O (the ni-trido ligand). Such ligands are said to beambidentate. Complexes that differ only inthe way in which the ligand coordinates aresaid to show linkage isomerism.

Stereoisomerism occurs when two com-pounds with the same molecular formulae

and the same groups differ only in thearrangement of the groups in space. Thereare various types of stereoisomerism.

Cis-trans (or syn-anti) isomerism occurswhen there is restricted rotation about abond between two atoms (e.g. a doublebond or a bond in a ring). Groups attachedto each atom may be on the same side ofthe bond (the cis- or syn-isomer) or oppo-site sides (the trans- or anti-isomer). Cis-trans isomerism occurs in square-planarcomplexes of the type MX2Y2, where M isa metal ion and X and Y are different lig-ands. If the X ligands are adjacent the iso-mer is a cis-isomer; if they are opposite, itis a trans-isomer. This type of isomerismcan also occur in octahedral complexes ofthe type MX2Y4. Octahedral complexes ofthe type MX3Y3 show a different type ofstereoisomerism. If the three X ligands are

isomorphism

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trans-isomer cis-isomer

X

Y

YM

Y

Y

X

mer-isomerfac-isomer

Y

X

Y

XM

Y

Y

X

X

Y

YM

Y

X

X

X

Y

YM

X

Y

Isomerism: isomers of octahedral complexes

in a plane that includes the M ion (with thethree Y ligands in a plane at right angles),then the structure is called the mer-isomer(‘mer’ stands for meridional). If the three X(and Y) ligands are all on a face of the oc-tahedron, the structure is the fac-isomer(‘fac’ stands for facial). Cis-trans isomerismwas formerly called geometrical isomerism.

Optical isomerism occurs when thecompound has no plane of symmetry andcan exist in left- and right-handed formsthat are mirror images of each other. Seealso optical activity.

isomorphism The existence of com-pounds with the same crystal structure.

isomorphism, law of See Mitscher-lich’s law.

isotherm A line on a chart or graph join-ing points of equal temperature. See alsoisothermal change.

isothermal change A process that takesplace at a constant temperature. Through-out an isothermal process, the system is inthermal equilibrium with its surroundings.For example, a cylinder of gas in contactwith a constant-temperature bath may becompressed slowly by a piston. The workdone appears as energy, which flows intothe bath to keep the gas at the same tem-perature. Isothermal changes are con-trasted with adiabatic changes, in which noenergy enters or leaves the system, and thetemperature changes. In practice noprocess is perfectly isothermal and none isperfectly adiabatic, although some can ap-proximate in behavior to one of these ideals.

isotones Two or more nuclides thathave the same neutron numbers but differ-ent proton numbers.

isotonic Describing solutions having thesame osmotic pressure.

isotopes Two or more species of thesame element differing in their mass num-bers because of differing numbers of neu-trons in their nuclei. The nuclei must havethe same number of protons (an element is

characterized by its proton number). Iso-topes of the same element have very similarproperties because they have the same elec-tron configuration, but differ slightly intheir physical properties. An unstable iso-tope is termed a radioisotope or radio-active isotope.

Isotopes of elements are useful in chem-istry for studies of the mechanisms ofchemical reactions. A standard technique isto label one of the atoms in a molecule byusing an isotope of a component element,which is known as a tracer. It is then possi-ble to follow the way in which this atombehaves throughout the course of the reac-tion. In labeling, radioisotopes are detectedby counters; stable isotopes can also beused, and detected by a mass spectrum.

Isotopes are also used in kinetic studies.For example, if the bond between twoatoms X–Y is broken in the rate-determin-ing step, and Y is replaced by a heavier iso-tope of the element, Y*, the reaction ratewill be slightly lower with the Y* present.This kinetic isotope effect is particularlynoticeable with hydrogen and deuteriumcompounds, because of the large relativedifference in mass.

isotope separation The separation ofdifferent isotopes of a chemical element,making use of the some differences in thephysical properties of these isotopes. Forsmall-scale isotope separation a MASS SPEC-TROMETER is frequently used. For large-scale isotope separation the methods usedinclude diffusion in the gas phase (used foruranium from the gas uranium hexafluo-ride), distillation (which was used to pro-duce heavy water), electrolysis, the use ofcentrifuges, and the use of lasers, with ex-citation of one isotope followed by its sep-aration using an electromagnetic field.

isotopic mass (isotopic weight) Themass number of a given isotope of an el-ement.

isotopic number The difference be-tween the number of neutrons in an atomand the number of protons.

isotopic weight See isotopic mass.

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isotopic weight

Jahn–Teller effect A distortion of mol-ecules that occurs so as to prevent the mol-ecule having degenerate valence orbitals.The Jahn–Teller effect is observed in metalcomplexes. Some complexes that would beexpected to be regular octahedra are dis-torted octahedra because of the effect. Itwas predicted theoretically by H. A. Jahnand Edward Teller in 1937.

jeweler’s rouge Iron(III) oxide (Fe2O3),used as a mild abrasive.

joliotium Symbol: Jl A name formerlysuggested for element-105, now known asdubnium (Db). See element.

joule Symbol: J The SI derived unit of allforms of energy and work, equal to the en-ergy required or work done when the pointof application of a force of one newtonmoves another object one meter in the di-rection of the force. 1 J = 1 N m. It isnamed for the British physicist James Joule(1818–89).

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J

kainite A mineral form of hydrated crys-talline magnesium sulfate (MgSO4) con-taining also some potassium chloride. It isused as a fertilizer and source of potassiumcompounds.

kaolin See China clay.

kaolinite A hydrated aluminosilicatemineral, Al2(OH)4Si2O5, the major con-stituent of CHINA CLAY. It is formed by theweathering of FELDSPARS from granites.

katal Symbol: kat The SI derived unit ofcatalytic activity, equal to the amount ofsubstance in moles that can be catalyzedper second. 1 kat = l mol s–1.

kelvin Symbol: K The SI base unit ofthermodynamic temperature. It is definedas the fraction 1/273.16 of the thermody-namic temperature of the triple point ofwater. Zero kelvin (0 K) is absolute zero.An interval of one kelvin is the same as aninterval of one degree Celsius on the Cel-sius scale. It is named for British physicistBaron William Thompson Kelvin (1824–1907), who devised the ABSOLUTE TEMPERA-TURE scale that now bears his name in1848.

kieselguhr See diatomite.

killed spirits Zinc(II) chloride solutionwhen used as a flux for solder.

kilo- Symbol: k A prefix used with SIunits denoting 103. For example, 1 kilome-ter (km) = 103 meters (m).

kilocalorie See calorie.

kilogram (kilogramme) Symbol: kg The

SI base unit of mass, equal to the mass ofthe international prototype of the kilo-gram, a cylinder consisting of a plat-inum–iridium alloy kept in a vault at Sèvresin France. The kilogram is the only SI baseunit that to date has yet to be defined interms of physical constants.

kilogramme See kilogram.

kilowatt-hour Symbol: kWh An m.k.s.system unit of electrical energy often usedto to measure electrical power consump-tion, equal to the energy transferred orwork done by one kilowatt of power in onehour. It has a value of 3.6 × 106 joules in SIunits.

kinetic energy See energy.

kinetic isotope effect See isotopes.

kinetics A branch of physical chemistryconcerned with the study of rates of chem-ical reactions and the effect of physicalconditions that influence the rate of reac-tion, e.g. temperature, light, concentration,etc. The measurement of these rates underdifferent conditions gives information onthe mechanism of the reaction, i.e. on thesequence of processes by which reactantsare converted into products.

kinetic theory A theory explainingphysical properties in terms of the motionof particles. The kinetic theory of gases as-sumes that the molecules or atoms of a gasare in continuous random motion and thepressure (p) exerted on the walls of a con-taining vessel arises from the bombard-ment by these fast moving particles. Whenthe temperature is raised the speeds in-crease; so consequently does the pressure.

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K

If more particles are introduced or the vol-ume is reduced there are more particles tobombard unit area of the walls and thepressure also increases. When a particlecollides with the wall it experiences a rateof change of momentum, which is propor-tional to the force exerted. For a largenumber of particles this provides a steadyforce per unit area (or pressure) on the wall.

Following certain additional assump-tions, the kinetic theory leads to an expres-sion for the pressure exerted by an idealgas. The assumptions are:1. The particles behave as if they are hard

smooth perfectly elastic points.2. They do not exert any appreciable force

on each other except during collisions.3. The volume occupied by the particles

themselves is a negligible fraction of thevolume of the gas.

4. The duration of each collision is negligi-ble compared with the time between col-lisions.By considering the change in momen-

tum on impact with the walls it can beshown that

p = ρc2/3

where ρ is the density of the gas and c is theroot-mean-square speed of the molecules.The mean-square speed of the molecules isproportional to the absolute temperature:

Nmc2 = RTSee also degrees of freedom.

Kipp’s apparatus An apparatus for theproduction of a gas from the reaction of aliquid on a solid. It consists of three globes,the upper globe being connected via a widetube to the lower globe. The upper globe isthe liquid reservoir. The middle globe con-tains the solid and also has a tap at whichthe gas may be drawn off. When the gas isdrawn off the liquid rises from the lowerglobe to enter the middle globe and reactswith the solid, thereby releasing more gas.When turned off the gas released forces theliquid back down into the lower globe andup into the reservoir, thus stopping the re-action. It is named for Dutch chemistPetrus Kipp (1808–84).

Kroll process A method of obtainingcertain metals by reducing the metal chlo-

ride with magnesium. Titanium can be ob-tained in this way:

TiCl4 + 2Mg → 2MgCl2 + TiIt is named for its inventor, Luxembourgmetallurgist William J. Kroll (1889–1973),who developed it in 1932.

Kroto, Sir Harold Walter (1939– )British chemist. Along with his Sussex Uni-versity colleague David Walton, Kroto hada long-standing interest in molecules con-taining carbon chains linked by alternatetriple and single bonds. Such chains withfive, seven, and nine carbon atoms hadbeen identified by radioastronomers inspace. In 1984 Kroto heard that the Amer-ican chemist Richard SMALLEY had devel-oped new techniques involving laserbombardment for the production of clus-ters of atoms. In 1985 he visited Smalley inHouston and persuaded him to direct hislaser beam at a graphite target. Clusters ofcarbon atoms were indeed produced but,more interesting than the small chains he was looking for, Kroto found a mass-spectrum signal for a molecule of exactly60 carbon atoms. The first suggestion wasthat it had a sandwichlike graphite struc-ture. However, such a planar fragmentwould have reactive carbon atoms at theedges, whereas C60 appeared to be stable.Kroto called C60 BUCKMINSTERFULLERENE.He shared the 1996 Nobel Prize for chem-istry with Smalley and with Robert CURL.

krypton A colorless odorless mon-atomic gas, the fourth member of the rare-gases; i.e. group 18 (formerly VIIIA or 0) ofthe periodic table. It occurs in minutequantities (0.001% by volume) in air, fromwhich it is recovered by fractional distilla-tion. Krypton is used in fluorescent lights,lasers, and pneumatic heart valves. It isknown to form unstable compounds withfluorine.

Symbol: Kr; m.p. –156.55°C; b.p.–152.3°C; mass density 3.749 (0°C) kgm–3; p.n. 36; most common isotope 84Kr;r.a.m. 83.80.

kurchatovium Symbol: Ku A name for-merly used for element-104 now known asrutherfordium (Rf). See element.

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label See isotopes.

labile Describing a complex having lig-ands that can easily be replaced by morestrongly bonded ligands.

lake A pigment made by combining anorganic dyestuff with an inorganic com-pound (e.g. aluminum oxide).

lamellar compound A compound witha crystal structure composed of thin platesor layers. Silicates form many compoundswith distinct layers. Typical examples aretalc (Mg3(OH)2Si4O10) and pyrophyllite(Al2(OH)2Si4O10). See also intercallationcompound.

lamp black A black pigment; a finely di-vided form of carbon formed by incom-plete combustion of an organic compound.

Langmuir, Irving (1881–1957) Ameri-can chemist. In 1919 Langmuir extendedthe model of electrons in atoms proposedby Gilbert LEWIS to try to account for thechemical valence of all atoms. In the 1920she investigated the chemistry of surfacesvery extensively. He won the 1932 NobelPrize for chemistry for his work on chemi-cal reactions at surfaces.

Langmuir isotherm An equation usedto describe the adsorption of a gas onto aplane surface at a fixed temperature. It canbe written in the form:

θ = bp/(1 + bp),where θ is the fraction of the surface cov-ered by the gas, p is the gas pressure and bis a constant known as the adsorption co-efficient, which is the equilibrium constantfor the adsorption process. This equationwas derived by the American chemist Irv-

ing Langmuir in 1916 using the kinetictheory of gases.

lanthanide contraction The decreasein atomic and ionic radii in the series oflanthanoid elements from lanthanum tolutetium as the atomic number increases.This contraction occurs because the 4felectrons in this series of elements arerather ineffective in shielding the outerelectrons from the attraction of a positivelycharged nucleus. In the case of the triposi-tive ions the radius of the lanthanum ion is1.17Å and the radius of the lutetium ion is1.00Å. A similar effect, the ACTINOID CON-TRACTION, occurs for 5f electrons.

lanthanides See lanthanoids.

lanthanoids (lanthanides; lanthanons;rare earths) A group of 15 silvery, reac-tive metals whose electronic configurationsdisplay back-filling of the 4f level. There isa maximum of 14 electrons in an f-orbital.The element lanthanum itself has no f-elec-trons (La [Xe]5d16s2) and is thus strictlynot a lanthanoid, but it is included by con-vention, thus giving a closely related seriesof 15 elements. Excluding lanthanum, theelements all have 4fx6s2 configurations, al-though gadolinium and lutecium have anadditional 5d1 electron.

The characteristic oxidation state isM3+ and the great similarity in the size ofthe ions leads to a very close similarity ofchemical properties and hence to great dif-ficulties of separation using conventionalmethods. In addition, cerium can assume aCe4+ state and ytterbium a Yb2+ state.Chromatographic and solvent-extractionmethods have been specially developed forthe lanthanoids.

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lanthanons See lanthanoids.

lanthanum A soft ductile malleable sil-very highly reactive metallic element ofgroup 3 (formerly IIIB) of the periodictable. It is usually considered to be the firstmember of the LANTHANOID series. It isfound associated with other lanthanoids inmany minerals, including monazite andbastnaesite. Lanthanum is used in severalalloys, especially those for lighter flints, be-cause it sparks or ignites readily whenscratched. It is also used as a catalyst, andin making optical glass to which it impartsenhanced refractive properties.

Symbol: La; m.p. 921°C; b.p. 3457°C;r.d. 6.145 (25°C); p.n. 57; most stable iso-tope 139La; r.a.m. 138.9055.

lapis lazuli See lazurite.

laser An acronym for Light Amplifica-tion by Stimulated Emission of Radiation.A laser device produces high-intensitymonochromatic coherent beams of light. Inthe laser process the molecules of a sample(such as ruby doped with Cr3+ ions) arepromoted to an excited state. Because thesample is in a cavity between two reflectivesurfaces, when a molecule emits sponta-neously, the photon so generated ricochetsbackward and forward. In this way othermolecules are stimulated to emit photonsof the same energy. If one of the reflectivesurfaces is partially transmitting this radia-tion can be tapped.

laser spectroscopy Spectroscopy thatuses lasers. The special features of lasers,notably their ability to produce beams ofcoherent monochromatic radiation, haveseveral important advantages over otherspectroscopic techniques. For example,spectroscopy that makes use of the RAMAN

EFFECT has greatly benefitted from the ap-plication of lasers because there are majorexperimental problems with other sourcesof light.

latent heat The heat evolved or ab-sorbed when a substance changes its phys-ical state, e.g. the latent heat of fusion is the

heat absorbed when a substance changesfrom a solid to a liquid.

laterite A red fine-grained type of clayformed in tropical climates by the weather-ing of igneous rocks. Its color comes fromthe presence of iron(III) hydroxide.

lattice A regular three-dimensionalarrangement of points. A lattice is used todescribe the positions of the particles(atoms, ions, or molecules) in a crystallinesolid. The lattice structure can be exam-ined by x-ray diffraction techniques.

lattice energy The energy released whenions of opposite charge are brought to-gether from infinity to form one mole of agiven crystal. The lattice energy is a meas-ure of the stability of a solid ionic sub-stance, with respect to ions in the gas. Seealso Born–Haber cycle.

lattice vibrations The vibrations of theatoms or ions in a crystal lattice about theirequilibrium positions. Such vibrationsoccur even at the absolute zero tempera-ture because of zero point energy. At lowtemperatures the vibrations are well de-scribed by the simple harmonic motion ofthe atoms. As the temperature is increasedthe anharmonicity of the vibrations be-comes more pronounced.

laughing gas See dinitrogen oxide.

Lavoisier, Antoine Laurent (1743–94) French chemist. Lavoisier is frequentlyreferred to as the founder of modern chem-istry. Perhaps his most significant contri-bution was to peform careful quantitativeexperiments that disproved the PHLOGIS-TON THEORY of combustion. This led himto establish that oxygen is one of the gasespresent in air. He also noticed the presenceof an inert gas in air, which was subse-quently named nitrogen. He summarizedhis work in the influential book Elemen-tary Treatise on Chemistry, which statedthe law of mass conservation in chemicalreactions. Lavoisier, who had been a taxfarmer, was executed in 1794, in the after-math of the French Revolution.

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law of conservation of energy Seeconservation of energy.

law of conservation of mass See con-servation of mass.

law of constant composition See con-stant proportions.

law of constant heat summation SeeHess’s law.

law of constant proportions See con-stant proportions.

law of definite proportions See con-stant proportions.

law of equivalent proportions Seeequivalent proportions.

law of isomorphism See Mitscherlich’slaw.

law of mass action See mass action,law of.

law of octaves See Newlands’ law.

law of reciprocal proportions Seeequivalent proportions.

lawrencium A radioactive synthetictransuranic element of the actinoid seriesof elements, first made in 1961 by bom-barding californium-252 targets withboron nuclei. It can also be made by bom-barding berkelium-249 targets with oxy-gen-18 nuclei. Several very short-livedisotopes have been synthesized.

Symbol: Lr; p.n. 103; m.p., b.p., andr.d. unknown; most stable isotope 262Lr(half-life 261 minutes).

laws of chemical combination Seechemical combination; laws of.

lazurite An uncommon typically azureblue mineral consisting of a silicate ofsodium, calcium, and aluminum with somesulfur. It and its parent rock, lapis lazuli,are widely used for ornaments and as semi-

precious gemstones. Lazurite was the orig-inal source of the pigment ultramarine.

LCAO (linear combination of atomicorbitals) See orbital.

leaching The washing out of a solublematerial from an insoluble solid by use of asolvent. This process is often carried out inbatch tanks or by dispersing the crushedsolid in a liquid.

lead A dense dull gray soft metallic toxicelement; the fifth member of group 14 (for-merly IVA) of the periodic table. It is theend product of most radioactive decay se-ries. It occurs in small quantities in a widevariety of minerals but only a few are eco-nomically important. The most importantof these is galena (PbS), found in Australia,Mexico, the USA, and Canada. Other eco-nomically important minerals are anglesite(PbSO4), litharge (PbO), and cerussite(PbCO3).

Galena is often associated with zincores and the smelting operations of the leadand zinc industries are thus closely inte-grated. Lead ore from galena is first con-centrated by froth flotation and theconcentrate then roasted and reduced asfollows:

2PbS + 3O2 → 2PbO + 2SO2PbS + 2O2 → PbSO4

2PbO + 2C → 2Pb + 2CO2PbO + PbS → 3Pb + SO2

PbSO4 + 2C → Pb + 2CO + SO2Additionally, various silver sulfides areoften found in small quantities withgalena, meaning that silver is often recov-ered in economic quantities with crudelead.

The outer s2p2 configurations of bothtin and lead give these metals similar prop-erties. There is, however, a much greaterpredominance of the divalent state withlead. Both lead oxides are amphoteric,lead(II) oxide (PbO) leading to plumbitesand lead(IV) oxide (PbO2) to plumbates ondissolution in alkalis. Lead also formsmixed oxides Pb2O3 (a yellow solid, betterwritten as Pb(II)Pb(IV)O3) and Pb3O4 (ared powder, better written as Pb2(II)-Pb(IV)O4). Both lead and tin have low

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lead

melting points and neither is attacked bydilute acids; they differ however in their re-action with concentrated nitric acid, withlead reacting as follows:

3Pb + 8HNO3 → 3Pb(NO3)2 + 2NO +4H2O

Tin, on the other hand, gives hydratedSn(IV) oxide. Concentrated hydrochloricacid will not attack lead.

Lead, like tin, forms halides, PbX2, withall the halogens. Lead(IV) chloride (PbCl4)is the only known lead(IV) halide, whereasall halides of SnX4 are known. The greatstability of the Pb(II) state leads to Pb(IV)compounds being powerful oxidizingagents.

Lead is used in lead–acid storage bat-teries, alloys, radiation shielding, andwater and sound proofing. It is also used inthe petrochemical, munitions, paint, andglass industries, although to a far lesser ex-tent than previously, due to the toxic na-ture of the metal and its compounds.

Symbol: Pb; m.p. 327.5°C; b.p.1830°C; r.d. 11.35 (20°C); p.n. 82; mostcommon isotope 208Pb; r.a.m. 207.2.

lead(II) acetate See lead(II) ethanoate.

lead–acid battery A type of batteryused in vehicles. It has two sets of plates:spongy lead plates connected in series tothe negative terminal, and lead(IV) oxideplates connected to the positive terminal.The material of the electrodes is held in ahard lead-alloy grid. The plates are inter-leaved. The electrolyte is dilute sulfuricacid.

The e.m.f. of each cell when fullycharged is about 2.2 volts (V). This valuefalls to a steady 2 V when current is drawn;a typical battery has six cells, giving anoverall voltage of about 12 volts. As thebattery begins to run down, the e.m.f. fallsfurther. During discharge the electrolytebecomes more dilute and its relative den-sity falls. To recharge the battery, current ispassed through it in the opposite directionto the direction of current supply. Thisprocess reverses the cell reactions and in-creases the relative density of the elec-trolyte, which should be about 1.26–1.29for a fully charged battery.

The electrolyte contains hydrogen ions(H+) and sulfate ions (SO4

2–). During dis-charge, H+ ions react with the lead(IV)oxide to give lead(II) oxide and water

PbO2 + 2H+ + 2e– → PbO + H2OThis reaction takes electrons from thelead(IV) oxide plates, causing the positivecharge. A further reaction follows, whichyields soft lead sulfate:

PbO + SO42– + 2H+ → PbSO4 + H2O +

2e–

Electrons are thus released to the negativeelectrode, producing the negative charge.During charging the reactions are reversed:

PbSO4 + 2e– → Pb + SO42–

PbSO4 + 2H2O → PbO2 + 4H+ + SO42–

+ 2e–

lead(II) carbonate (PbCO3) A whitepoisonous powder that occurs naturally asthe mineral CERUSSITE. It forms rhombiccrystals and can be precipitated by reactinga cold aqueous solution of a solublelead(II) salt (e.g. lead(II) nitrate) with am-monium carbonate. It decomposes tolead(II) oxide and carbon dioxide above315°C.

lead(II) carbonate hydroxide (whitelead; 2PbCO3.Pb(OH)2) The most im-portant basic lead carbonate. It is manu-factured electrolytically using high-puritylead anodes. It was once widely used as apigment in white and colored paints and inceramics. Because the compound is poiso-nous, its use for these purposes has beenlargely discontinued.

lead-chamber process A process for-merly used to manufacture sulfuric acid byoxidizing sulfur(IV) oxide with nitrogenmonoxide. The reaction, which was car-ried out in expensive large lead chambers,has now been replaced by the CONTACT

PROCESS.

lead dioxide See lead(IV) oxide.

lead(II) ethanoate (lead(II) acetate;Pb(CH3CO2)2) A poisonous compoundusually occurring as the hydratePb(CH3CO2)2.3H2O, forming monocliniccrystals. At 100°C it loses ethanoic acid

lead(II) acetate

128

and water, forming a basic lead(II)ethanoate. It is extremely soluble in water– 50 g per 100 g of water at 25°C. Lead(II)ethanoate can be obtained as an anhydroussalt. Its chief asset is that it is one of the fewcommon lead(II) salts that are soluble inwater. It has been used to make varnishesand enamels.

lead(IV) hydride See plumbane.

lead monoxide See lead(II) oxide.

lead(II) oxide (lead monoxide; PbO) Ayellow crystalline poisonous powderformed by roasting molten lead in air.Litharge, the most common form, is ob-tained when lead(II) oxide is heated aboveits melting point. If prepared below itsmelting point, another form, called massi-cot, is obtained. Litharge is used in the rub-ber industry, in the manufacture of paintsand varnishes, and in the manufacture oflead glazes for pottery.

lead(IV) oxide (lead dioxide; PbO2) Apoisonous dark-brown solid forminghexagonal crystals. On heating to 310°C itdecomposes into lead(II) oxide and oxy-gen. It can be prepared electrolytically orby reacting lead(II) oxide with potassiumchlorate. Lead(IV) oxide is an oxidizingagent and has been used in the manufac-ture of matches.

lead(II) sulfate (PbSO4) A poisonouswhite crystalline solid that occurs naturallyas the mineral anglesite. It is almost insolu-ble in water and can be precipitated by re-acting an aqueous solution containingsulfate ions with a solution of a solublelead(II) salt (e.g. lead(II) ethanoate). Itforms basic lead(II) sulfates when shakentogether with lead(II) hydroxide andwater. Due to their toxicity, basic lead(II)sulfates have been largely discontinued intheir former use as pigments.

lead(II) sulfide (PbS) A black solid thatoccurs naturally as the mineral galena. Itcan be prepared as a precipitate by reactinga solution of a soluble lead(II) salt with hy-drogen sulfide or with a solution of a solu-

ble sulfide. Lead(II) sulfide has the capacityto rectify alternating current.

lead tetraethyl (tetraethyl lead;Pb(CH2CH3)4) A poisonous liquid that isinsoluble in water but soluble in organicsolvents. It is manufactured by the reactionof an alloy of sodium and lead withchloroethane (CH2ClCH3). The product isobtained by steam distillation. Leadtetraethyl was once used as an additive ininternal-combustion engine fuel to increaseits octane number and thus prevent preig-nition (knocking). Lead-free compoundsare now prescribed or encouraged for thispurpose in many countries.

Leblanc process An obsolete processfor the manufacture of sodium carbonate.Sodium chloride is converted to sodiumsulfate by heating with sulfuric acid. Thissulfate is then roasted in a rotary furnacewhere it is first reduced to the sulfide usingcarbon, and then immediately converted tothe carbonate by the action of limestone.Sodium carbonate solution is obtained byleaching the product with water and thissolution is subsequently dried and calcinedto obtain the solid. The process was in-vented by French chemist Nicolas LeBlanc(1742–1806) in 1783. The Solvay processis now used instead.

Le Chatelier’s principle If a system isat equilibrium and a change is made in theconditions, the equilibrium adjusts so as tooppose the change. The principle was firststated by French chemist Henri Le Chate-lier (1850–1936) in 1888 and can be ap-plied to the effect of temperature andpressure on chemical reactions. A good ex-ample is the Haber process for synthesis ofammonia:

N2 + 3H2 ˆ 2NH3The ‘forward’ reaction

N2 + 3H2 → 2NH3is exothermic. Thus, reducing the tempera-ture displaces the equilibrium toward pro-duction of NH3 (because this tends toincrease temperature). Increasing the pres-sure also favors formation of NH3, becausethis leads to a reduction in the total num-ber of molecules (and hence pressure).

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Le Chatelier’s principle

Leclanché cell A primary voltaic cellconsisting, in its ‘wet’ form, of a carbon-rod anode and a zinc cathode, with a10–20% solution of ammonium chlorideas electrolyte. Manganese(IV) oxide mixedwith crushed carbon in a porous bag or potsurrounding the anode acts as a depolariz-ing agent. The dry form (dry cell) is widelyused for flashlight batteries, transistor ra-dios, etc. It has a mixture of ammoniumchloride, zinc chloride, flour, and gumforming an electrolyte paste. Sometimesthe dry cell is arranged in layers to form arectangular battery, which has a longer lifethan the cylinder type.

LEED (low-energy electron diffraction)A method of electron diffraction that isused to investigate surfaces. A surface isbombarded with a beam of low-energyelectrons, with the diffracted electrons hit-ting a fluorescent screen. The electronbeam has a low energy so that it interactswith the surface rather than the bulk of thematerial. The technique gives a great dealof useful information about surfaces andprocesses occurring at surfaces.

Lennard-Jones, Sir John Edward(1894–1954) British chemist. Lennard-Jones was a theoretical chemist who did agreat deal of work on intermolecular forces.Starting in the late 1920s he helped developand promote molecular orbital theory. Inparticular, in 1929 he showed that thetheory readily explains the paramagnetismof molecular oxygen. He subsequentlyshowed how molecular orbital theory canalso explain the directionability of chemi-cal bonds.

Lennard-Jones potential An expres-sion used to represent the potential energyV of intermolecular interactions as a func-tion of the intermolecular distance r. V canbe written in the form V = –A/r6 + B/r12,where A and B are constants. The Lennard-Jones potential has been used extensivelyto study intermolecular interactions. It wasproposed by the British theoretical chemistSir John Lennard-Jones in 1924.

levo-form See optical activity.

levorotatory See optical activity.

Lewis, Gilbert Newton (1875–1946)American physical chemist. Lewis pro-posed the idea that valence and bonding ofatoms is associated with electrons. He firstput forward this idea in 1916, introducingthe concept of a covalent bond in which thebonding between two atoms occurs be-cause the atoms share a pair of electrons,with each atom supplying one electron. Healso suggested that the outer electrons inatoms form octets. He developed his ideasmore fully in the book Valence and theStructure of Atoms and Molecules (1923).Lewis also performed important work inchemical thermodynamics, with his bookwith Merle Randall entitled Thermody-namics and the Free Energy of ChemicalSubstances (1923) being very influential.

Lewis acid A substance that can acceptan electron pair to form a coordinate bond;a Lewis base is an electron-pair donor. Inthis model, the neutralization reaction isseen as the acquisition of a stable octet bythe acid, for example:

Cl3B + :NH3 → Cl3B:NH3Metal ions in coordination compounds

are also electron-pair acceptors and there-fore Lewis acids. The definition includesthe ‘traditional’ Brønsted acids since H+ isan electron acceptor, but in common usagethe terms Lewis acid and Lewis base are re-served for systems without acidic hydrogenatoms.

Lewis base See Lewis acid.

Lewis formula See Lewis structure.

Lewis octet theory See octet.

Lewis structure (Lewis formula) Atwo-dimensional depiction, by means ofchemical element and electron dot sym-bols, of the possible structure of a moleculeor ion, showing each atom in relation to itsneighbors, the bonds that hold the atomstogether, and the lone pairs in each atom’souter shell.

L-form See optical activity.

Leclanché cell

130

Liebig condenser A simple type of lab-oratory condenser. It consists of a straightglass tube, in which the vapor is con-densed, with a surrounding glass jacketthrough which cooling water flows. It isnamed for German chemist Justus vonLiebig (1803–73).

ligand See complex; chelate.

ligand-field theory A theory that de-scribes the properties of complexes. Lig-and-field theory is an extension of CRYSTAL

FIELD THEORY in which covalent bondingbetween the central atom or ion and theligands is taken into account.

light (visible radiation) A form of elec-tromagnetic radiation able to be detectedby the human eye. Its wavelength range isbetween about 400 nanometers (nm) (farred) and about 700 nm (far violet). Theboundaries are not precise, because indi-viduals vary in their ability to detect ex-treme wavelengths; this ability alsodeclines with age.

lignite (brown coal) The poorest gradeof coal, containing up to 60% carbon andhaving a high moisture content.

lime See calcium hydroxide; calciumoxide.

limestone A natural form of CALCIUM

CARBONATE. It is used in making calciumcompounds, carbon dioxide, and cement.

limewater A solution of calcium hy-droxide in water. If carbon dioxide is bub-bled through lime water a milky precipitateof calcium carbonate forms. Prolongedbubbling of carbon dioxide turns the solu-tion clear again as a result of the formationof soluble calcium hydrogencarbonate(Ca(HCO3)2).

limonite (bog iron ore) A geologicalfield term for a group of yellowish-brownto dark-colored minerals consisting mainlyof iron(III) hydroxide and hydrous oxidesof iron. Limonite is a major iron ore. It isalso used as a pigment.

Linde process A method of liquefyinggases by compression followed by expan-sion through a nozzle. The process is usedfor producing liquid oxygen and nitrogenby liquefying air and then fractionating it.

line defect See defect.

line spectrum A SPECTRUM composed ofa number of discrete lines corresponding tosingle wavelengths of emitted or absorbedradiation. Line spectra are produced byatoms or simple (monatomic) ions in gases.Each line corresponds to a change in elec-tron orbit, with emission or absorption ofradiation.

linkage isomerism See isomerism.

Lipscomb, William Nunn (1919– )American inorganic chemist. Lipscomb isnoted for his work on boranes – hydridesof boron first investigated by Alfred STOCK

in the early part of the 20th century. Usinglow-temperature x-ray diffraction analysis,Lipscomb tackled the problem of investi-gating the notoriously unstable boranes,producing evidence of some remarkablestructures, totally original and completelyunsuspected by earlier chemists. The basicconcept of a multi-center bond was derivedfrom a structure for diborane proposed byLONGUET-HIGGINS. Lipscomb was awardedthe Nobel Prize for chemistry in 1976.

liquid The state of matter in which theparticles of a substance are loosely boundby intermolecular forces. The weakness ofthese forces permits movement of the par-ticles and consequently liquids can changetheir shape within a fixed volume. The liq-uid state lacks the order of the solid state.Thus, amorphous materials, such as glass,in which the particles are disordered andcan move relative to each other, can beclassed as liquids.

liquid air A pale blue liquid. It is a mix-ture of mainly liquid oxygen (boiling at–183°C) and liquid nitrogen (boiling at–196°C).

liquid crystal A substance that can flow

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liquid crystal

like a viscous liquid but has a considerableamount of molecular order. There arethree types of liquid crystal. In a smecticliquid crystal there are layers of alignedmolecules, with the long axes of the mol-ecules being perpendicular to the layers. Ina cholesteric liquid crystal there are alsolayers of aligned molecules, but with theaxes of the molecules being parallel to theplanes of the layers. In a nematic liquidcrystal the molecules are all aligned in thesame direction but not arranged in layers.

liter Symbol: L or l A metric unit of vol-ume equal in SI units to one cubic decime-ter (dm3) or 10–3 meter3, or in c.g.s. units to1000 cm3. One milliliter (mL or ml) is thusthe same volume as one cubic centimeter(cm3). However, the liter is not recom-mended for precise measurements since itwas formerly defined as the volume of onekilogram of pure water at 4°C and stan-dard pressure. By this older definition, oneliter was the same as 1000.028 cm3.

litharge See lead(II) oxide.

lithia See lithium oxide.

lithia water See lithium hydrogencar-bonate.

lithium A light silvery moderately reac-tive alkali metal; the first element of group1 (formerly IA) of the periodic table. It oc-curs in a number of complex silicates, suchas spodumene, lepidolite, and petalite, anda mixed phosphate, tryphilite. It is a rareelement, accounting for 0.0065% of theEarth’s crust, and the lightest of the metals.Lithium ores are treated with concentratedsulfuric acid and lithium sulfate subse-quently separated by crystallization. Theelement can be obtained by conversion tothe chloride and electrolysis of the fusedchloride.

Lithium has the electronic configura-tion 1s22s1 and is entirely monovalent in itschemistry. The lithium ion is, however,much smaller than the ions of the other al-kali metals; consequently it is polarizingand a certain degree of covalence occurs in

its bonds. Lithium also has the highest ion-ization potential of the alkali metals.

The element reacts with hydrogen toform lithium hydride (LiH) a white high-melting solid that releases hydrogen at theanode during electrolysis (confirming theionic nature Li+H–). Lithium reacts withoxygen to give Li2O (sodium gives the per-oxide) and with nitrogen to form Li3N onfairly gentle warming. The metal itself re-acts only slowly with water, giving the hy-droxide (LiOH) but lithium oxide reactsmuch more vigorously to give again the hy-droxide; the nitride is hydrolyzed to am-monia. With halogens the metal reacts toform halides (LiX).

Apart from the fluoride the halides arereadily soluble both in water and in oxy-gen-containing organic solvents. In thisproperty lithium partly resembles magne-sium, which has a similar charge/size ratio.Compared to the other carbonates ofgroup 1, lithium carbonate (Li2CO3) isthermally unstable, decomposing to Li2Oand CO2. This is because the small Li+ ionleads to particularly high lattice energies,favouring the formation of Li2O. Lithiumcompounds impart a characteristic purplecolor to flames.

Lithium is used to make alloys for air-craft parts and other components wherelightness is an important quality. It is alsoused to make lithium batteries and lithiumgrease, and to scavenge oxygen in metal-lurgy. Certain lithium salts are used to treatdepression. Lithium is also used to certainglasses and ceramic glazes.

Symbol: Li; m.p. 180.54°C; b.p.1347°C; r.d. 0.534 (20°C); p.n. 3; mostcommon isotope 7Li; r.a.m. 6.941.

lithium aluminum hydride See lithiumtetrahydroaluminate(III).

lithium carbonate (Li2CO3) A whitesolid obtained by the addition of excesssodium carbonate solution to a solution ofa lithium salt. It is soluble in the presenceof excess carbon dioxide, forming lithiumhydrogencarbonate. If heated (to 780°C) ina stream of hydrogen it undergoes thermaldecomposition to give lithium oxide andcarbon dioxide. Lithium carbonate forms

liter

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monoclinic crystals and differs from theother group 1 carbonates in being onlysparingly soluble in water. It has a close re-semblance to the carbonates of magnesiumand calcium – an example of a diagonal re-lationship in the periodic table. Lithiumcarbonate is used in the preparation ofother lithium compounds and in the treat-ment of depression.

lithium chloride (LiCl) A white solidproduced by dissolving lithium carbonateor oxide in dilute hydrochloric acid andcrystallizing the product. Below 19°C thedihydrate (LiCl.2H2O) is obtained; at19°C the chloride loses one molecule ofwater. It becomes anhydrous at 93.5°C.The compound forms cubic crystals; theanhydrous salt is isomorphous withsodium chloride. Lithium chloride is prob-ably the most deliquescent substanceknown and forms hydrates with 1, 2, and 3molecules of water. It is used as a flux inwelding aluminum.

lithium hydride (LiH) A white crys-talline solid produced by direct combina-tion of the elements at a temperature above500°C. Lithium hydride has a cubic struc-ture and is more stable than the othergroup 1 hydrides. Electrolysis of the fusedsalt yields hydrogen at the anode. Lithiumhydride reacts violently with water to givelithium hydroxide and hydrogen; the reac-tion is highly exothermic. The compoundis used as a reducing agent and in thepreparation of other hydrides.

lithium hydrogencarbonate (LiHCO3)A compound known only in solution, pro-duced by the reaction between carbondioxide, water, and lithium carbonate.When a solution of the hydrogencarbonateis heated, carbon dioxide and lithium car-bonate are produced. Solutions of the hy-drogencarbonate are sold and used inmedicine under the name of lithia water.

lithium hydroxide (LiOH) A whitesolid made industrially as the monohydrate(LiOH.H2O) by reacting calcium hydrox-ide with a lithium ore or with a salt madefrom the ore. Lithium hydroxide has a

closer resemblance to the group 2 hydrox-ides than to the group 1 hydroxides. It isused in lubricants and batteries, and to ab-sorb carbon dioxide.

lithium oxide (lithia; Li2O) A whitesolid produced by burning metallic lithiumin air above its melting point. It can also beproduced by the thermal decomposition oflithium carbonate or hydroxide. Lithiumoxide reacts slowly with water to form asolution of lithium hydroxide; the reactionis exothermic. The compound has a cal-cium-fluoride structure. Lithium oxide isused in lubricants, glass, ceramics, andwelding and brazing fluxes.

lithium sulfate (Li2SO4) A white solidprepared by the addition of excess lithiumoxide or carbonate to a solution of sulfuricacid. It is readily soluble in water fromwhich it crystallizes as the monohydrate. Incontrast to the other group 1 sulfates itdoes not form alums and it is not isomor-phous with these other sulfates.

lithium tetrahydroaluminate(III) (lith-ium aluminum hydride; LiAlH4) A whitesolid produced by the action of lithium hy-dride on aluminum chloride, the hydridebeing in excess. Lithium tetrahydroalumi-nate reacts violently with water, releasinghydrogen. It is a powerful reducing agent,much used in organic chemistry. In inor-ganic chemistry it is used in the preparationof hydrides.

litmus A natural pigment that changescolor when in contact with acids and alka-lis; above a pH of 8.3 it is blue and belowa pH of 4.5 it is red. Thus it gives a roughindication of the acidity or basicity of a so-lution; because of the rather broad rangeover which it changes it is not used for pre-cise work. Litmus is used both in solutionand as litmus paper.

lixiviation The process of separatingsoluble components from a mixture bywashing them out with water.

localized bond A molecular bond inwhich the electrons contributing to the

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localized bond

bond remain between the two atoms con-cerned, i.e. the bonding orbital is localized.The majority of bonds are of this type.Compare delocalized bond.

lodestone A naturally occurring mag-netic oxide of iron, a form of MAGNETITE

(Fe3O4). A piece of lodestone is a naturalmagnet and lodestones were used as mag-netic compasses in ancient times. See mag-netite.

lone pair A pair of valence electronshaving opposite spin that are located to-gether on one atom, i.e. are not shared asin a covalent bond. Lone pairs occupy sim-ilar positions in space to bond pairs andaccount for the shapes of molecules. Amolecule with a lone pair can donate thispair to an electron acceptor, such as H+ ora metal ion, to form coordinate bonds. Seecomplex.

long period See period.

Longuet-Higgins, Hugh Christopher(1923– ) British theoretical chemist. Oneof his first contributions was to explain theproperties of boron hydrides in terms ofbridged structures. In the second half of the1940s he performed calculations with

Charles COULSON on conjugated molecules.Longuet-Higgins also applied statisticalmechanics to chemical problems, particu-larly mixtures and polymer solutions.

Lotka–Volterra mechanism A possi-ble mechanism for OSCILLATING REACTIONS.A reactant R is converted into a product P.The chemical reaction is in a steady statebut is not in chemical equilibrium. Thereare three steps in this mechanism:

R + X → 2X,X + Y → 2Y,

Y → P.Autocatalysis is involved in the first twosteps of this process. It appears that oscil-lating chemical reactions have mechanismsthat are different from the Lotka–Volterramechanism. This type of mechanism doesoccur in certain types of complex systemsuch as predator–prey relationships in biol-ogy. It was in the biological context thatthe mechanism was investigated by the Ital-ian mathematician Vito Volterra (1860–1940).

lowering of vapor pressure A colliga-tive property of solutions in which thevapor pressure of a solvent is lowered as asolute is introduced. When both solventand solute are volatile the effect of increas-

lodestone

134

S

104.5°

Water Sulfur tetrafluoride

C

H HH H H H

N

H H

H H

O

H H

F

F

F

F

Methane Ammonia

109.5° 107°

H

Lone pair

ing the solute concentration is to lower thepartial vapor pressure of each component.When the solute is a solid of negligiblevapor pressure the lowering of the vaporpressure of the solution is directly propor-tional to the number of species introducedrather than to their nature and the propor-tionality constant is regarded as a generalsolvent property. Thus the introduction ofthe same number of moles of any solutecauses the same lowering of vapor pres-sure, if dissociation does not occur. If thesolute dissociates into two species on dis-solution the effect is doubled. The kineticmodel for the lowering of vapor pressuretreats the solute molecules as occupyingpart of the surface of the liquid phase andthereby restricting the escape of solventmolecules. The effect can be used in themeasurement of relative molecular masses,particularly for large molecules, such aspolymers. See also depression of freezingpoint; Raoult’s law.

Lowry–Brønsted theory See acid; base.

lumen Symbol: lm The SI derived unit ofluminous flux, equal to the luminous fluxemitted in a solid angle of one steradian(Sr) by a point source of one candela (cd),radiating equally in all directions. 1 lm = 1cd sr.

luminescence The emission of radiationfrom a substance in which the particleshave absorbed energy and gone intoexcited states. They then return to lowerenergy states with the emission of electro-magnetic radiation. If the luminescencepersists after the source of excitation is re-moved it is called phosphorescence: if not,it is called fluorescence.

LUMO See frontier orbital theory.

lutetium A silvery element of the lan-thanoid series of metals. It occurs in asso-

ciation with other lanthanoids, especiallyin the mineral monazite. Lutetium is therarest of the naturally occurring elements.It is used as a catlyst.

Symbol: Lu; m.p. 1663°C; b.p. 3395°C;r.d. 9.84 (25°C); p.n. 71; most commonisotope 175Lu; r.a.m. 174.967.

lux Symbol: lx The SI derived unit of il-lumination, equal to the illumination pro-duced by a luminous flux of one lumenfalling on a surface of one square meter. 1lx = 1 lm m–2.

Lyman series See hydrogen atom spec-trum.

lyophilic Solvent attracting. When thesolvent is water, the word hydrophilic isoften used. The terms are applied to:1. Ions or groups on a molecule. In aque-

ous or other polar solutions ions orpolar groups are lyophilic. For example,the –COO– group on a soap is thelyophilic (hydrophilic) part of the mol-ecule.

2. The disperse phase in colloids. Inlyophilic colloids the dispersed particleshave an affinity for the solvent, and thecolloids are generally stable. Comparelyophobic.

lyophobic Solvent repelling. When thesolvent is water, the word hydrophobic isused. The terms are applied to:1. Ions or groups on a molecule. In aque-

ous or other polar solvents, the lyopho-bic group is nonpolar. For example, thehydrocarbon group on a soap moleculeis the lyophobic (hydrophobic) part.

2. The disperse phase in colloids. Inlyophobic colloids the dispersed parti-cles are not solvated and the colloid iseasily solvated. Gold and sulfur sols areexamples.

Compare lyophilic.

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lyophobic

macromolecular crystal A crystalcomposed of atoms joined together by co-valent bonds, which form giant three-di-mensional or two-dimensional networks.Diamond and silicates are examples ofmacromolecular crystals.

macromolecule A large molecule; e.g. anatural or synthetic polymer.

Madelung constant A constant, de-noted α, which appears in calculations ofthe cohesive energy of ionic crystals whenthe electrostatic interactions between ionsin a lattice are summed. The Madelungconstant is dimensionless and is a charac-teristic of the specific crystal structure. It isnamed for E. Madelung who introduced itin 1918.

magnesia See magnesium oxide.

magnesite See magnesium carbonate.

magnesium A light ductile malleable sil-ver-white alkaline-earth metal; the secondelement of group 2 (formerly group IIA) ofthe periodic table. It has the electronic con-figuration of neon with two additionalouter 3s electrons. The element accountsfor 2.09% of the Earth’s crust and is eighthin order of abundance. It occurs in a widevariety of minerals such as brucite(Mg(OH)2), carnallite (KCl.MgCl2.6H2O),epsomite (MgSO4.7H2O), magnesite(MgCO3), and dolomite (CaCO3.MgCO3);and also as magnesium chloride (MgCl2) insea water. The metal is obtained by severalroutes, depending on the mineral used, alleventually leading to the chloride which isthen fused and subjected to electrolysis.

The element has a fairly low ionizationpotential and is electropositive. It reacts di-

rectly with oxygen, nitrogen, and sulfur onheating, to form the oxide, MgO, the ni-tride, Mg3N2, and the sulfide MgS, all ofwhich are hydrolyzed by water to give thehydroxide. It burns in air with an intensewhite flame.

Magnesium also reacts directly with thehalogens to form halides. These show aslight deviation in the general behavior ofhalides by dissolving normally in waterand undergoing a significant amount of hy-drolysis only when the solutions are evap-orated:

MgX2 + 2H2O → Mg(OH)2 + 2HXMagnesium hydroxide is considerably

less soluble than the hydroxides of theheavier elements in group 2 and is a weakerbase. Magnesium forms a soluble sulfate,chlorate, and nitrate. Like the heavier alka-line earths, it also forms an insoluble car-bonate, but MgCO3 is the least thermallystable of the group. Unlike calcium, mag-nesium does not form a carbide.

Magnesium metal is industrially impor-tant as a major component, along with alu-minium and zinc, of lightweight alloys.The alloy surfaces develop an imperviousoxide film which protects them from pro-gressive deterioration. The metal is alsoused in flash bulbs, fireworks, and flares.

Because of its ionic nature magnesiumforms very few coordination compounds.Donor–acceptor species in aqueous solu-tions are short-lived, but magnesium bro-mide and iodide have sufficient acceptorproperties to dissolve in donor solventssuch as alcohols. Magnesium in traceamounts is essential for living things, beingrequired by animals for proper metabolismand by green plants as a component ofchlorophyll.

The element crystallizes in the close-packed hexagonal structure.

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M

Symbol: Mg; m.p. 649°C; b.p. 1090°C;r.d. 1.738 (20°C); p.n. 12; most commonisotope 24Mg; r.a.m. 24.3050.

magnesium bicarbonate See magne-sium hydrogencarbonate.

magnesium carbonate (MgCO3) Awhite solid that occurs naturally as themineral magnesite and in association withcalcium carbonate in dolomite. Magne-sium carbonate is sparingly soluble inwater but reacts with dilute acids to givethe salt, carbon dioxide, and water. It thusfinds use in medicine as a mild antacid.Magnesium carbonate also decomposeseasily on heating to give magnesium oxide,which is an important refractory material.

magnesium chloride (MgCl2) A solidthat exists in a variety of hydrated forms,most commonly as the hexahydrate,MgCl2.6H2O. When heated, this hydratedform is hydrolyzed by its water of crystal-lization, with the evolution of hydrogenchloride and the formation of the oxide:

MgCl2 + H2O → MgO + 2HClThe anhydrous chloride must therefore beprepared by evaporating an aqueous solu-tion in an atmosphere of hydrogen chlo-ride. It is used in the preparation of cottonfabrics and artificial leather, as a laxative,and to fireproof wood. Magnesium metalis produced from the fused chloride byelectrolysis.

magnesium hydrogencarbonate (mag-nesium bicarbonate; Mg(HCO3)2) Thesolid hydrogencarbonate is unknown atroom temperature. The compound is pro-duced in solution when water containingcarbon dioxide dissolves magnesium car-bonate:

MgCO3 + H2O + CO2 → Mg(HCO3)2It is a cause of temporary hardness inwater.

magnesium hydroxide (Mg(OH)2) Awhite solid, occurring naturally as the min-eral brucite, that dissolves sparingly inwater to give an alkaline solution. It can beprepared by adding an alkali to a solublemagnesium compound. Magnesium hy-

droxide is used as an antacid, in sugar re-fining, and in uranium processing.

magnesium oxide (magnesia; MgO) Awhite solid that occurs naturally as themineral periclase. It can be prepared byheating magnesium in oxygen or by ther-mal decomposition of magnesium hydrox-ide, carbonate, or nitrate. Magnesiumoxide is weakly basic because of the attrac-tion of the oxide ions for protons fromwater molecules:

O2– + H2O → 2OH–

Magnesium oxide melts at 2800°C, mak-ing it useful as a refractory lining for fur-naces. It is also used as an antacid and tomake reflective coatings for optics andwindshields.

magnesium peroxide (MgO2) A whiteinsoluble solid prepared by reactingsodium or barium peroxide with a concen-trated solution of a magnesium salt. Mag-nesium peroxide is used as a bleach fordyestuffs and silk.

magnesium sulfate (MgSO4) A solidthat occurs naturally in many salt depositsin combination with other minerals. Onehydrated form, MgSO4.7H2O, is known asEpsom salt or epsomite. Epsom salt ismade on a commercial scale by reactingmagnesium carbonate with sulfuric acid.Magnesium sulfate is used as a purgativedrug, as a treatment for indigestion, and asan antidote for barium and barbituratepoisoning. It is also used in the dyeing andsizing of textiles, in tanning, in syntheticfiber production, and as a source of mag-nesium in fertilizers.

magnetic constant See magnetic per-meability.

magnetic moment The ratio of thetorque exerted on a magnet, a loop thatcarries an electrical current, or a movingelectrical charge in a magnetic field to thefield strength of that magnetic field. Anelectron has an orbital magnetic momentbecause of the magnetic field generated byits orbital motion around its nucleus and aspin magnetic moment because of its quan-

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magnetic moment

tum mechanical spin. Atomic nuclei canalso have magnetic moments because oftheir spin. The existence of electron andnuclear magnetic moments is made use ofin various branches of spectroscopy and inmagnetochemistry.

magnetic permeability symbol µ. Aquantity that characterizes the response ofa medium to a magnetic field. The perme-ability of free space, which is denoted µ0and is also called the magnetic constant, isa constant that appears in Ampère’s law re-lating an electric current to the magneticfield produced by the current. It has thevalue 4π × 10–7 Hm–1 in S.I. units. Themagnetic permeability of a medium µ re-lates the magnetic flux density B to themagnetic field strength H by the equation:B = µH. The relative magnetic permeabil-ity, µr, of a medium is defined by µ = µrµ0.

magnetic quantum number See atom.

magnetic separation A method of sep-arating crushed mineral mixtures using themagnetic properties that a component maypossess, such as ferromagnetism or dia-magnetism.

magnetic susceptibility Symbol χ. Adimensionless quantity, related to the MAG-NETIC PERMEABILITY of a medium, that char-acterizes the magnetic nature of thatmedium. The relation between the mag-netic susceptibility and the relative mag-netic permeability µr is given by χ = µr

–1. Ifthe material is diamagnetic χ has a smallnegative value. If the material is paramag-netic χ has a small positive value. If the ma-terial is ferromagnetic χ has a large positivevalue.

magnetism The study of the nature andcause of magnetic force fields, and how dif-ferent substances are affected by them.Magnetic fields are produced by movingcharge – both on a large scale (as with cur-rent in a wire coil, forming an electromag-net), or on the small scale of the movingcharges in atoms. It is generally assumedthat the Earth’s magnetism and that of

other extraterrestrial bodies have the samecause.

Substances may be classified on thebasis of how samples interact with fields.Different types of magnetic behavior resultfrom the type of atom. Diamagnetism,which is common to all substances, is dueto the orbital motion of electrons. Para-magnetism is due to electron spin, and isthus a property of materials containing un-paired electrons. It is particularly impor-tant in transition-metal chemistry, inwhich the complexes often contain un-paired electrons. Magnetic measurementscan give information about the bonding inthese complexes. Ferromagnetism, thestrongest effect, also involves electron spinand the alignment of magnetic moments indomains.

magnetite A black mineral form ofmixed iron(II)–iron(III) oxide, Fe3O4, amajor type of iron ore. It is also used as aflux in making ceramics. See also lode-stone; triiron tetroxide.

magnetochemistry The branch ofchemistry concerned with the magneticproperties of molecules. Magnetochem-istry is used extensively in the study oftransition-metal complexes because meas-uring their MAGNETIC SUSCEPTIBILITY en-ables information about chemical bondingin the complex to be obtained using LIGAND

FIELD THEORY.

magneton Symbol µ. A unit for measur-ing magnetic moments at the atomic andsubatomic scales. The Bohr magneton µB isgiven by µB = eh/4πme, where e and m arethe charge and mass of an electron respec-tively, and h is the Planck constant. TheBohr magneton is the smallest unit the or-bital magnetic moment of an electron in anatom can have. The nuclear magneton µNis given by replacing the mass of the elec-tron by the mass of the proton and is there-fore about 1840 times smaller than theBohr magneton.

malachite A green mineral consisting ofcopper(II) carbonate and hydroxide

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(CuCO3.Cu(OH)2). It is used as an ore ofcopper, as a pigment, and in jewelry.

manganate(VI) A salt containing theion MnO4

2–.

manganate(VII) (permanganate) A saltcontaining the ion MnO4

–. Manga-nate(VII) salts are purple and strong oxi-dizing agents. In basic solutions the darkgreen manganate(VI) ion is formed.

manganese A transition metal, the firstelement of group 7 (formerly VIIB) of theperiodic table. It occurs naturally as ox-ides, e.g. pyrolusite (MnO2) and hausman-nite (trimanganese tetraoxide, Mn3O4).Nodules found at various locations on theocean floor are about 25% manganese.The metal is recovered from its oxides afterroasting by reduction with aluminum,carbon, or magnesium, followed by elec-trolysis. Managanese is moderately elec-tropositive and combines with the non-metals, excluding hydrogen, when heated.Its main use is in alloy steels made byadding pyrolusite to iron ore in an electricfurnace. It decomposes cold water and di-lute acids to give hydrogen. Manganese ex-hibits all possible positive oxidation states,with +7, +4, and (the most stable) +2 beingthe most common. Manganese(II) salts arepale pink. With alkali their solutions pre-cipitate manganese(II) hydroxide, whichrapidly oxidizes in air to brown man-ganese(III) oxide.

Symbol: Mn; m.p. 1244°C; b.p.1962°C; r.d. 7.44 (20%C); p.n. 25; mostcommon isotope 55Mn; r.a.m. 54.93805.

manganese dioxide See manganese(IV)oxide.

manganese(II) oxide (manganous oxide;MnO) A green powder prepared by heat-ing manganese(II) carbonate or oxalate inthe absence of air. Alternatively it may beprepared by heating the higher manganeseoxides in a stream of hydrogen. Man-ganese(II) oxide is a basic oxide and is al-most insoluble in water. At hightemperatures it is reduced by hydrogen tomanganese. On exposure to air, man-

ganese(II) oxide rapidly oxidizes. The com-pound has a crystal lattice similar to that ofsodium chloride.

manganese(III) oxide (manganic oxide;manganese sesquioxide; Mn2O3) A blackpowder obtained by igniting manga-nese(IV) oxide or a manganese(II) salt inair at 800°C. The powder reacts slowlywith cold dilute acids to form man-ganese(III) salts. Manganese(III) oxide oc-curs in nature as braunite(3Mn2O3.MnSiO3) and as the monohy-drate mineral manganite (Mn2O3.H2O).The manganese(III) oxide crystal latticecontains Mn3+ and O2– ions. With concen-trated alkalis, it undergoes disproportiona-tion to give manganese(II) andmanganese(IV) ions.

manganese(IV) oxide (manganese diox-ide; MnO2) A black powder prepared bythe action of heat on manganese(II) nitrate.A hydrated form occurs naturally as themineral pyrolusite. Manganese(IV) oxide isinsoluble in water. It is a powerful oxidiz-ing agent: it reacts with hot concentratedhydrochloric acid to produce chlorine andwith warm sulfuric acid to give oxygen.The dihydrate (MnO2.2H2O) is formedwhen potassium manganate(VII) is re-duced in alkaline solution. It is used as acatalyst in the laboratory preparation ofchlorine, as a depolarizer in electric drycells, and in the glass industry. At 500–600°C, manganese(IV) oxide decomposesto give manganese(III) oxide (Mn2O3) andtrimanganese tetroxide (Mn3O4). It hasgood electrical conductivity.

manganese sesquioxide See man-ganese(III) oxide.

manganic oxide See manganese(III)oxide.

manganin An alloy of copper, man-ganese, and nickel. It has a high electricalresistivity over a wide range of tempera-tures and is used to make resistors.

manganite See manganese(III) oxide.

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manganite

manganous oxide See manganese(II)oxide.

manometer A device for measuringpressure. A simple type is a U-shaped glasstube containing mercury or other liquid.The pressure difference between the armsof the tube is indicated by the difference inheights of the liquid.

marble A dense metamorphic rock com-posed of recrystallized limestone ordolomite. It is used in construction and asa decorative stone.

Marcus, Rudolph Arthur (1923– )Canadian–American chemist. In the 1950sMarcus began to work on electron-transferreactions. The addition, removal, andtransfer of electrons is the driving force be-hind many basic chemical processes includ-ing photosynthesis, respiration, and theproduction of solar energy. Such reactionsare, in principle, very simple, involving themovement of an electron from one ion toform another ion. The rates of such reac-tions can, however, vary widely. Marcuswas able to explain electron-transfer reac-tion rates in terms of the way in which thesolvent molecules, initially configured tosolvate the reactant, reorganize to solvatethe products. He was awarded the 1992Nobel Prize for chemistry for his work inthis field.

Marsh’s test See arsine.

mass Symbol: m A measure of the quan-tity of matter in an object. Mass is deter-mined in two ways: the inertial mass of anobject determines its tendency to resistchange in motion; the gravitational massdetermines its gravitational attraction forother masses. The SI base unit of mass isthe kilogram.

mass action, law of At constant tem-perature, the rate of a chemical reaction isdirectly proportional to the active mass ofthe reactants, the active mass being takenas the concentration. In general, the rate ofreaction decreases steadily as the reactionproceeds; a measure of the concentration

of any of the reactants will give a measureof the rate of reaction.

For the reaction A + B → products, thelaw of mass action states that

rate = k[A][B]where [A] and [B] represent the concentra-tion of the reactants in mol dm–3 and k is aconstant dependent on the reaction. Theactive mass can equal the concentration inmol dm–3 only if there is no interaction orinterference between the reacting mol-ecules. Many systems exhibit such interac-tions and interference and consequentlythe concentration has to be multiplied byan activity coefficient in order to obtain theeffective active mass. See activity coeffi-cient.

mass concentration See concentration.

mass density See concentration.

mass-energy equation The equation E= mc2, where E is the total energy (rest-mass energy + kinetic energy + potentialenergy) of a mass m, c being the speed oflight. The equation is a consequence of Ein-stein’s special theory of relativity; mass is aform of energy and energy also has mass.Conversion of rest-mass energy into kineticenergy (and thus heat) is the source ofpower in radioactive substances and thebasis of nuclear-power generation.

massicot See lead(II) oxide.

mass number See nucleon number.

mass spectrograph See mass spectrom-eter.

mass spectrometer An instrument forproducing ions in a gas and analyzing themaccording to their charge/mass ratio. Theearliest experiments by J. J. Thomson(1856–1940) used a stream of positive ionsfrom a discharge tube, which were de-flected by parallel electric and magneticfields at right angles to the beam. Each typeof ion formed a parabolic trace on a pho-tographic plate (a mass spectrograph).

In modern instruments, as originated byFrancis Aston (1877–1945), the ions are

manganous oxide

140

produced by ionizing the gas with elec-trons. The positive ions are accelerated outof this ion source into a high-vacuum re-gion. Here, the stream of ions is deflectedand focused by a combination of electricand magnetic fields, which can be varied sothat different types of ion fall on a detector.In this way, the ions can be analyzed ac-cording to their mass, giving a mass spec-trum of the material. Mass spectrometersare used for accurate measurements of rel-ative atomic mass and for analysis of iso-tope abundance. They can also be used toidentify compounds and analyze mixtures.The relative proportions of different typesof ions in a mixture is used to find thestructure of new compounds. The charac-teristic spectrum can also identify com-pounds by comparison with standardspectra.

mass spectrum See mass spectrometer.

matrix 1. A continuous solid phase inwhich particles of a different solid phaseare embedded.2. (plural matrices) A rectangular array ofquantities which are frequently real orcomplex numbers arranged in rows orcolumns. A matrix is usually indicated byenclosing the array in square brackets. Ma-trices are used extensively in mathematicsand its physical applications. If the sizes ofthe matrices are right it is possible to addand multiply matrices together. An impor-tant feature of the multiplication of twomatrices A and B is that, in general,AB≠BA; i.e. matrix multiplication is non-commutative.

matrix mechanics A formulation ofquantum mechanics in which the funda-mental equations are stated in terms of ma-trices. This formulation was stated by theGerman physicist Werner Heisenberg in1925 as a set of mathematical rules forphysical quantities of interest. The Germanphysicist Max Born recognized that therules for quantum mechanics correspondedto matrix algebra. Born, Heisenberg, andanother German physicist Pascual Jordandeveloped matrix mechanics further. Ma-trix mechanics is historically important be-

cause it was the first formulation of quan-tum mechanics. In 1926 the Austrianphysicist Erwin Schrödinger and othersshowed that matric mechanics and WAVE

MECHANICS are equivalent, and so give thesame answers to physical and chemicalproblems. However, matrix mechanics isused less frequently to solve problems thanis wave mechanics.

matte A mixture of iron and copper sul-fides obtained at an intermediate stage insmelting copper ores.

maxwell Symbol: Mx A unit of mag-netic flux used in the c.g.s. system. It isequal to 10–8 Wb in SI units.

Maxwell–Boltzmann distribution Thestatistical distribution of the speeds of mol-ecules in a gas as a function of both the ab-solute temperature and the mass of themolecules. It was derived by the Britishphysicist James Clerk Maxwell (1831–79)and the Austrian physicist Ludwig Boltz-mann (1844–1906) in the second half ofthe 19th century using statistical methods.

mean bond energy See bond energy.

mechanism A step-by-step descriptionof the events taking place in a chemical re-action. It is a theoretical framework ac-counting for the fate of bonding electronsand illustrates which bonds are broken andwhich are formed. For example, in thechlorination of methane to give chloro-methane:step 1

Cl:Cl → 2Cl•step 2

Cl• + CH4 → HCl + CH3•step 3

CH3• + Cl:Cl → CH3Cl + Cl•

mega- Symbol: M A prefix used with SIunits, denoting 106. For example, 1 mega-hertz (MHz) = 106 hertz (Hz).

meitnerium A radioactive synthetictransactinide metallic element created bybombarding bismuth-209 nuclei with iron-58 nuclei in a particle accelerator. Only a

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meitnerium

few short-lived atoms of the element haveever been created.

Symbol: Mt; m.p., b.p., and r.d. un-known; p.n. 109; only known isotope266Mt (half-life about 3.4 × 10–3s).

melting (fusion) The process by which asolid is converted into a liquid by heat orpressure.

melting point The temperature atwhich a solid is in equilibrium with its liq-uid phase at standard pressure and abovewhich the solid melts. This temperature isalways the same for a particular pure solid.Ionically bonded solids generally havemuch higher melting points than those inwhich the forces are covalent or intermole-cular.

membrane A thin pliable sheet of tissueor other material acting as a boundary. Themembrane may be either natural (as incells, skin, etc.) or synthetic modificationsof natural materials (cellulose derivativesor rubbers). In many physicochemicalstudies membranes are supported onporous materials, such as porcelain, to pro-vide mechanical strength. Membranes aregenerally permeable to some degree.

Membranes can be prepared to permitthe passage of other molecules and micro-molecular material. Because of permeabil-ity effects, concentration diffferences at amembrane give rise to a whole range ofmembrane-equilibrium studies, of whichosmosis, dialysis, and ultrafiltration are ex-amples. See also semipermeable mem-brane.

Mendeleev, Dmitri Ivanovich (1834–1907) Russian chemist. Mendeleev is re-membered for developing the periodictable of chemical elements in a classicpaper published in 1869 entitled On theRelation of the Properties to the AtomicWeights of Elements. Other scientists suchas Julius Lothar Meyer and John Newlandshad similar ideas at about the same timebut Mendeleev developed his ideas muchmore fully, including the predictions of theexistence and properties of hitherto un-known elements such as gallium, scan-

dium, and germanium, which were discov-ered subsequently and found to have theproperties Mendeleev had predicted.

mendelevium A synthetic silver-whiteradioactive transuranic element of the acti-noid series, first created by bombardingeinsteinium-253 isotope with alpha parti-cles. Several short-lived isotopes have beensynthesized.

Symbol: Md; m.p. 1021°C; b.p.3074°C; r.d. unknown, p.n. 101; most sta-ble isotope 258Md (half-life 57 minutes).

mercuric chloride See mercury(II) chlo-ride.

mercuric oxide See mercury(II) oxide.

mercuric sulfide See mercury(II) sul-fide.

mercurous chloride See mercury(I)chloride.

mercurous oxide See mercury(I) oxide.

mercurous sulfide See mercury(I) sul-fide.

mercury A heavy tin-silver white transi-tion metal, the third element of group 12(fromerly IIB) of the periodic table. It is theonly metal that is a liquid at room temper-ature (20–22°C). It occurs naturally as themineral cinnabar (HgS, mercury(II) sul-fide); small drops of metallic mercury alsooccur native in cinnabar and in some vol-canic rocks. Elemental mercury vapor isvery poisonous as are many mercury com-pounds. Mercury is used in thermometersand barometers, special AMALGAMS, scien-tific apparatus, electrical switches, mer-cury-vapor lamps, and in mercury cells.Mercury compounds are used as fungi-cides,timber preservatives, and detonators.

Symbol: Hg; m.p. –38.87°C; b.p.356.58°C; r.d. 13.546 (20°C); p.n. 80;most common isotope 202Hg; r.a.m.200.59. See also zinc group.

mercury cell A voltaic or electrolyticcell in which one or both of the electrodes

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142

consists of mercury or an amalgam. Amal-gam electrodes are used in the DANIELL CELL

and the WESTON CADMIUM CELL. Flowingmercury electrodes are also used in certainelectrolytic cells. In the production of chlo-rine, sodium chloride solution is elec-trolyzed in a cell with carbon anodes and aflowing mercury cathode. At the cathode,sodium metal is formed, which forms anamalgam with the mercury. See also po-larography.

mercury(I) chloride (mercurous chloride;calomel; Hg2Cl2) A white precipitateprepared by adding dilute hydrochloricacid to a mercury(I) salt solution or by sub-liming mercury(II) chloride with mercury.Mercury(I) chloride is sparingly soluble inwater and is blackened by both ammoniagas, which it absorbs, and by alkalis. It isused in the calomel electrode and was for-merly used as a purgative.

mercury(II) chloride (mercuric chloride;HgCl2) A colorless crystalline compoundprepared by direct combination of mercurywith cold dry chlorine. Mercury(II) chlo-ride is soluble in water; it also dissolves inconcentrated hydrochloric acid because ofthe formation of complex ions, HgCl42–

and HgCl3–. On heating, mercury(II) chlo-ride sublimes forming a white translucentmass. It is extremely poisonous, but in di-lute solution (1:1000) it is used as an anti-septic. It is also used as a fungicide.

mercury(I) oxide (mercurous oxide;Hg2O) The black precipitate formed onaddition of sodium hydroxide solution to asolution of mercury(I) nitrate is thought bysome to be mercury(I) oxide; others, whodoubt its existence, think that the black-ness of the precipitate is due to some freemercury. X-ray examination of this blackcompound has shown it to be an intimatemixture of mercury(II) oxide and mercury.

mercury(II) oxide (mercuric oxide;HgO) A poisonous compound formed asa yellow powder by the addition ofsodium(I) hydroxide to a solution of mer-cury(II) nitrate or as a red solid by heatingmercury at 350°C for a long time. The dif-

ference in color is simply due to particlesize. If the oxide is strongly heated it de-composes to give mercury and oxygen.

mercury(I) sulfide (mercurous sulfide;Hg2S) The brownish-black precipitateoriginally formed when mercury is treatedwith cold concentrated sulfuric acid overan extended period is thought to be mer-cury(I) sulfide. Alternatively it may be pre-pared by the action of hydrogen sulfide oran alkaline sulfide on a mercury(I) salt so-lution. As soon as the mercury(I) sulfide isformed it disproportionates to give mer-cury(II) sulfide and mercury.

mercury(II) sulfide (mercuric sulfide;HgS) A compound that occurs in natureas the minerals CINNABAR (a red solid) andmetacinnabarite (a black solid). It is pre-pared as a black precipitate by the action ofhydrogen sulfide on a soluble mercury(II)salt solution. On heating, mercury(II) sul-fide sublimes and becomes red. It is insolu-ble in dilute hydrochloric and nitric acidsbut will dissolve in concentrated nitric acidand aqua regia. Mercury(II) sulfide is usedas the pigment vermilion.

mer-isomer See isomerism.

meso-form See optical activity.

mesomerism See resonance.

meta- Certain acids regarded as formedfrom an anhydride and water are namedmeta acids to distinguish them from themore hydrated ortho acids. For example,H2SiO3 (SiO2 + H2O) is metasilicic acid;H4SiO4 (SiO2 + 2H2O) is orthosilicic acid.

See also para-.

metacinnabarite See mercury(II) sul-fide.

metaiodic(VII) acid See iodic(VII)acid.

metal See metals.

metal carbonyl A coordination com-pound formed between a metal and CAR-

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metal carbonyl

BONYL GROUPS. Transition metals formmany such compounds.

metallic bond A bond formed betweenatoms of a metallic element in its zero oxi-dation state and in an array of similaratoms. The outer electrons of each atomare regarded as contributing to a free elec-tron ‘gas’, which occupies the whole crys-tal of the metal. It is the attraction of thepositive atomic cores for the negative elec-tron gas that provides the strength of themetallic bond.

Quantum mechanical treatment of theelectron gas restricts its energy to a series of‘bands’. It is the behavior of electrons inthese bands that gives rise to semiconduc-tors. See also delocalization.

metallic crystal A crystal formed bymetal atoms in the solid state. Each atomcontributes its valence (outer) electrons toa free electron ‘gas’. These electrons arefree to migrate through the solid while theremaining ions are arranged in a lattice.The ability of electrons to move throughthe lattice accounts for the electrical andthermal conductivity of metals.

metallocene A sandwich compound inwhich a metal atom or ion is coordinatedto two cyclopentadienyl ions. Ferrocene(Fe(C5H5)2) is the commonest example.

metalloid Any of a class of chemical el-ements that are intermediate in propertiesbetween METALS and NONMETALS. Exam-ples are germanium, arsenic, and tellurium,all of which are semiconductors.

There is, in fact, no clear-cut distinctionbetween metals and nonmetals. In the peri-odic table, there is a change from metallicto nonmetallic properties across the table,and an increase in metallic propertiesdown a group. Consequently there are el-ements (boron, silicon, germaium, arsenic,antimony, and telluvium) that form a diag-onal running to the right down the tablenear its right edge and which exhibit theseintermediate properties. Other elementsare also sometimes considered to be metal-loids, according to their chemical proper-ties. Tin, for instance, forms salts with

acids but also forms stannates with alkalis.Its oxide is amphoteric. Tin also has metal-lic (white tin) and nonmetallic (gray tin) al-lotropes.

metallurgy The study of metals, espe-cially methods of extracting metals fromtheir ores and the formation and propertiesof alloys.

metals Any of a class of chemical el-ements with certain characteristic proper-ties. In everyday usage, metals are elements(and alloys) such as iron, aluminum, andcopper, which are typically lustrous mal-leable solids and good conductors of heatand electricity. Such properties do not al-ways apply because some metals, e.g. bis-muth, are poor conductors, mercury is aliquid at room temperature, etc.

In chemistry, metals are distinguishedby their chemical properties into two maingroups. Reactive metals, such as the alkalimetals and alkaline-earth metals, are elec-tropositive elements. They are high in theelectromotive series and tend to form com-pounds by losing electrons to give positiveions. They have basic oxides and hydrox-ides. This typical metallic behavior de-creases across the periodic table andincreases down a group in the table.

The other group of metals is the transi-tion elements, which are less reactive, havevariable valences, and tend to form com-plexes. In the solid and liquid states metalshave METALLIC BONDS, formed by positiveions with free electrons. See also metalloid;nonmetal.

metaphosphoric acid See phosphoric(V)acid.

metastable species An excited state ofan atom, ion, or molecule, that has a rela-tively long lifetime before reverting to theground state. Metastable species are inter-mediates in some reactions.

metastable state A condition of a sys-tem or object in which it appears to be instable equilibrium but, if disturbed, cansettle into a lower energy state. For exam-ple, supercooled water is liquid below 0°C

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144

(at standard pressure). When a small crys-tal of ice or dust (for example) is intro-duced, rapid freezing occurs.

metathesis See double decomposition.

meter (metre) Symbol: m The SI baseunit of length, defined as the distance trav-eled by light in vacuum in 1/299 792 458second. This definition was adopted in1983 to replace the 1960 definition of alength equal to 1 650 763.73 wavelengthsin vacuum corresponding to the transitionbetween the levels 2p10 and 5d5 of thekrypton-86 atom.

methane The simplest hydrocarbon,CH4. It is the main component of naturalgas.

methanide See carbide.

methyl orange An acid–base indicatorthat is red in solutions below a pH of 3.1and yellow above a pH of 4.4. Because thetransition range is clearly on the acid side,methyl orange is suitable for the titrationof an acid with a moderately weak base,such as sodium carbonate.

methyl red An acid–base indicator thatis red in solutions below a pH of 4.4 andyellow above a pH of 6.0. It is often usedfor the same types of titration as methyl or-ange but the transition range of methyl redis nearer neutral (pH 7) than that of methylorange. The two molecules are structurallysimilar.

metre See meter.

metric system A decimal system ofunits based on the meter or centimeter asthe base unit of length, the gram or kilo-gram as the base unit of mass, and the sec-ond as the base unit of time.

metric ton See tonne.

mho See siemens.

mica A member of an important groupof aluminosilicate minerals that have a

characteristic three-layered structure. Thethree main types are biotite, lepidolite, andmuscovite, which differ in their content ofother elements (such as potassium, magne-sium, and iron). Mica flakes are used aselectrical insulators, dielectrics, and smallheat-proof windows.

micro- Symbol: µ A prefix used with SIunits, denoting 10–6. For example, 1 mi-crometer (µm) = 10–6 meter (m).

micron Symbol: µ A m.k.s.a. unit oflength equal to 10–6 meter in SI units.

microwaves A form of electromagneticradiation, ranging in wavelength fromabout 1 mm (where it merges with in-frared) to about 120 mm (bordering onradio waves). Microwaves are produced byvarious electronic devices including theklyston; they are often carried over shortdistances in tubes of rectangular sectioncalled waveguides.

Spectra in the microwave region cangive information on the rotational energylevels of certain molecules. See also electro-magnetic radiation.

milk of lime See calcium hydroxide.

milli- Symbol: m A prefix used with SIunits, denoting 10–3. For example, 1 mil-limeter (mm) = 10–3 meter (m).

millimeter of mercury See mmHg.

mineral A naturally occurring inorganiccompound, having a characteristic crys-talline structure and definite chemical com-position; a mineral’s physical propertiesare also more or less constant. In contrast,rocks are composed of mixtures of individ-ual minerals. See rock.

mineral acid An inorganic acid, espe-cially an acid used commercially in largequantities. Examples are hydrochloric, ni-tric, and sulfuric acids.

mirror image A shape that is identicalto another except that its structure is re-versed as if viewed in a mirror. If an object

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is not symmetrical it cannot be superim-posed on its mirror image. For example,the left hand is the mirror image of theright hand.

misch metal A pyrophoric alloy ofcerium and other lanthanoids, used inlighter flints.

miscible Denoting combinations of sub-stances that, when mixed, give rise to onlyone phase; i.e. substances that dissolve ineach other. See solid solution; solution.

Mitscherlich’s law (law of isomor-phism) The law stating that substancesthat crystallize in isomorphous forms (i.e.have identical crystalline forms and formmixed crystals) have similar chemical com-positions. The law can be used to indicatethe formulae of compounds. For instance,the fact that chromium(III) oxide is iso-morphous with Fe2O3 and Al2O3 impliesthat its formula is Cr2O3.

mixed indicator A mixture of two ormore indicators so as to decrease the pHrange, heighten the color, etc. over or atwhich the change occurs.

mixed oxides See oxygen.

mixture Two or more substances form-ing a system in which there is no chemicalbonding between the two. In homogeneousmixtures (e.g. solutions or mixtures ofgases) the molecules of the substances aremixed, and there is only one PHASE. In het-erogeneous mixtures (e.g. certain alloys)different phases can be distinguished. Mix-tures differ from chemical COMPOUNDS inthat:1. The chemical properties of the compo-

nents of a mixture are the same as thoseof the pure substances.

2. The mixture can be separated by physi-cal means (e.g. distillation or crystalliza-tion) or mechanically.

3. The proportions of the components canvary. Some mixtures (e.g. certain solu-tions) can vary in proportions only be-tween definite limits.

m.k.s.a. system A coherent metric sys-tem of units for mechanics and electromag-netics based on the meter, the kilogram, thesecond, and the ampere. It superseded them.k.s. system and was in turn supersededby the SI in 1960.

m.k.s. system A metric system of unitsfor mechanics based on the meter, the kilo-gram, and the second. It superseded thec.g.s. system and was in turn superseded bythe m.k.s.a. system in 1946.

mmHg (millimeter of mercury) A c.g.s.unit of pressure defined as the pressure thatwill support a column of mercury one mil-limeter high under specified conditions. Itis equal to 133.322 4 Pa in SI units. It con-tinues to be used in health care.

mobile phase See chromatography.

molal concentration See concentra-tion.

molality See concentration.

molar 1. Denoting a physical quantitydivided by the amount of substance. In al-most all cases the amount of substance willbe in moles. For example, volume (V) di-vided by the number of moles (n) is molarvolume Vm = V/n.2. A molar solution contains one mole ofsolute per cubic decimeter of solvent.

molar heat capacity See Dulong andPetit’s law.

molarity See concentration.

mole Symbol: mol The SI base unit ofamount of substance, defined as theamount of substance that contains as manyelementary entities as there are atoms in0.012 kilogram of 12C. The elementary en-tities may be atoms, molecules, ions, elec-trons, photons, etc., and they must bespecified. The amount of substance is pro-portional to the number of entities, theconstant of proportionality being the AVO-GADRO CONSTANT. One mole contains6.022 192 × 1023 entities. One mole of an

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element with relative atomic mass A has amass of A grams (this mass was formerlycalled one gram-atom of the element).

molecular beam A beam of molecules(or atoms or ions). Molecular beams areused in spectroscopy and in the study of in-termolecular forces, chemical reactions,and surfaces.

molecular crystal A crystal in whichmolecules, as opposed to atoms, occupylattice points. Examples include iodine andsolid carbon dioxide (dry ice). Because theforces holding the molecules together areweak, molecular crystals have low meltingpoints. When the molecules are small, thecrystal structure approximates to a close-packed arrangement. See close packing.

molecular dipole moment See dipolemoment.

molecular formula The formula of acompound showing the number and typesof the atoms present in a molecule, but notthe arrangement of the atoms. For exam-ple, H2O2 represents the molecular for-mula of hydrogen peroxide but not itsempirical formula (HO), or the arrange-ment of its atoms (HOOH). The molecularformula can be determined only if the mo-lecular mass is known. Compare empiricalformula; general formula; structural for-mula.

molecularity The total number of react-ing molecules in the individual steps of achemical reaction. Thus, a unimolecularstep has molecularity 1, a bimolecular step2, etc. Molecularity is always an integer,whereas the order of a reaction need notnecessarily be so. The molecularity of a re-action gives no information about themechanism by which it takes place.

molecular orbital See orbital.

molecular sieve A substance throughwhich molecules of a limited range of sizescan pass, enabling volatile mixtures to beseparated. Zeolites and other metal alu-minum silicates can be manufactured with

pores of constant dimensions in their mo-lecular structure. When a sample is passedthrough a column packed with granules ofthis material, some of the molecules enterthese pores and become trapped. The re-mainder of the mixture passes through theinterstices in the column. The trapped mol-ecules can be recovered by heating. Mo-lecular-sieve chromatography is widelyused in chemistry and biochemistry labora-tories. A modified form of molecular sieveis used in gel filtration. The sieve is a con-tinuous gel made from a polysaccharide. Inthis case, molecules larger than the largestpore size are totally excluded from the col-umn.

molecular spectrum The absorption oremission spectrum that is characteristic ofa molecule. Molecular spectra are usuallyband spectra.

molecular symmetry The set of sym-metry operations (rotations, reflections,etc.) that can be applied to a molecule. Thisset forms the point group of the molecule.Molecular symmetry is analyzed systemat-ically using GROUP THEORY.

molecular weight See relative molecu-lar mass.

molecule A particle formed by the com-bination of atoms in a whole-number ratio.A molecule of an element (combiningatoms are the same, e.g. O2) or of a com-pound (different combining atoms, e.g.HCl) retains the properties of that elementor compound. Thus, any quantity of acompound is a collection of many identicalmolecules. Molecular sizes are characteris-tically 10–10 to 10–9 m.

Many molecules of natural products areso large that they are regarded as giantmolecules (MACROMOLECULES); they maycontain thousands of atoms and have com-plex structural formulae that require veryadvanced techniques to identify. See alsoformula; relative molecular mass.

mole fraction The number of moles of agiven component in a mixture divided bythe total number of moles present of all the

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components. The mole fraction of compo-nent A is

nA/(nA + nB + nC + …)where nA is the number of moles of A, etc.

molybdenum A hard brittle silver-white transition metal, the second elementof group 6 (formerly VIB) of the periodictable. It occurs naturally in the mineralsmolybdenite (MoS2) and wulfenite(PbMoO4). Molybdenite, the main ore, isconverted to the oxide by roasting, whichis then reduced to the metal with hydrogen.Molybdenum is used in alloy steels, incan-descent bulbs, and catalysts. In traceamounts it is also an essential dietary el-ement for animals. The compound ammo-nium molybdate, dissolved in nitric acid, isused as a test for phosphates(V). Purifiedmolybdenum(IV) sulfide (MoS2) is used inlubricants to enhance viscosity.

Symbol: Mo; m.p. 2620°C; b.p.4610°C; r.d. 10.22 (20°C); p.n. 42; mostcommon isotope 98Mo; r.a.m. 95.94.

monatomic Denoting a molecule, radi-cal, or ion consisting of only one atom. Forexample, helium is a monatomic gas andH• is a monatomic radical.

Mond process See nickel.

monobasic acid An acid that has onlyone active proton, such as hydrochloricacid. Compare dibasic acid.

monoclinic crystal See crystal system.

monohydrate A salt that has a singlemolecule of water of crystallization, suchas sodium carbonate monohydrate,Na2CO3.H2O. See water of crystallization.

monomer The molecule, group, or com-pound from which a dimer, trimer, or poly-mer is formed.

monotropy The existence of a single al-lotrope of an element that is always morestable than the other allotrope(s) regardlessof temperature; phosphorus, for example,exhibits monotropy. The phase diagramfor a monotropic element shows that one

allotrope always has a lower vapor pres-sure than the other(s) at all temperatures;this is the stable allotrope. See also al-lotropy; enantiotropy.

monovalent (univalent) Having a va-lence of one.

mordant An inorganic compound usedto fix dye in cloth. The mordant (e.g. alu-minum hydroxide or chromium salts) isprecipitated in the fibers of the cloth, andthe dye then absorbs in the particles.

Moseley’s law Lines in the x-ray spec-tra of elements have frequencies that de-pend on the proton number of the element.For a set of elements, a graph of the squareroot of the frequency of x-ray emissionagainst proton number is a straight line(for spectral lines corresponding to thesame transition).

Mössbauer effect The emission ofgamma rays with a very narrow spread ofwavelengths from a solid. Usually, whengamma rays are emitted by a radioactivenucleus they have a broad spread of wave-lengths due to the recoil of the emitting nu-clei. However, if the emitting nuclei areembedded in a crystal the whole lattice re-coils, meaning that there is very little effecton the wavelength of the emitted gammarays.

In Mössbauer spectroscopy, a source ofgamma rays is mounted on a moving sup-port close to a sample. Detectors monitorgamma rays scattered by the sample. Thewavelength of the gamma rays from thesource can be changed by varying the speedat which the source is moved toward thesample (the Doppler effect). The spectrumis a plot of detector response against speedof source. A sharp decrease in response ata particular speed indicates resonance ab-sorption of gamma ray by a nuclide in thesample. The wavelength (energy) at whichthis occurs depends on the energy levels inthe sample nucleus, but these are affectedto some extent by the surrounding elec-trons (i.e. there are chemical shifts).

molybdenum

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mother liquor The solution remainingafter the formation of crystals.

Mulliken, Robert Sanderson (1896–1986) American chemist and physicist.Mulliken devoted most of his career to un-derstanding the electronic structure of mol-ecules and molecular spectra in terms ofquantum mechanics. He was awarded the1966 Nobel Prize for chemistry for his‘fundamental work concerning chemicalbonds and the electronic structure of mol-ecules by the molecular-orbital method’.

multicenter bond A two-electron bondformed by the overlap of orbitals frommore than two atoms (usually 3). Thebridging in diborane, B2H6, is believed totake place by overlap of an sp3 hybrid or-bital from each boron atom with the 1s or-bital on the hydrogen atom. Thismulticenter bond is called a three-centertwo-electron bond. The molecule is elec-tron-deficient. See also boron hydride;electron-deficient compounds.

multidecker sandwich compound Seesandwich compound.

multidentate ligand A ligand that pos-sesses at least two sites at which it can co-ordinate.

multiple bond A bond between twoatoms involving more than one pair of elec-trons, e.g. a double bond or a triple bond.This additional bonding arises from over-lap of atomic ORBITALS that are perpendic-ular to the internuclear axis and gives rise

to an increase in electron density above andbelow the internuclear axis. Such bondsare called pi bonds. If the sigma bond axisis taken as the z-axis (i.e. the internuclearaxis) then overlap of orbitals along the x-axis gives rise to a pi bond in the xz plane.Similarly orbitals on the y-axis form a pibond in the yz plane.

multiple proportions, law of Pro-posed by John Dalton in 1804, the lawstates that when two elements A and Bcombine to form more than one com-pound, the masses of B that combine witha fixed mass of A are in small, whole-num-ber ratios. For example, in dinitrogenoxide, N2O, nitrogen monoxide, NO, anddinitrogen tetroxide, N2O4, the amounts ofnitrogen combined with a fixed mass ofoxygen are in the ratio 4:2:1.

multiple-range indicator See universalindicator.

Mumetal (Trademark) A magneticalloy containing about 75% nickel, the re-mainder being iron, copper, and chromiumand sometimes molybdenum. It is easilymagnetized and demagnetized, has a highpermeability, and is used in transformercores and electromechanical equipment.

muriate An obsolete name for a chlo-ride; e.g. muriate of potash (KCl), muri-atric acid (HCl).

muriatic acid See muriate.

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nano- Symbol: n A prefix used with SIunits, denoting 10–9. For example, 1nanometer (nm) = 10–9 meter (m).

nanotechnology The technology of de-vices at the nanometer scale. In such smalldevices the quantum mechanical nature ofelectrons has to be taken into account fully.Instruments such as the ATOMIC FORCE

MICROSCOPE which can identify and manip-ulate individual atoms are very useful innanotechnology.

nanotubes Tubular structures with di-ameters of a few nanometers. Examples ofnanotubes are the carbon structuresknown as bucky tubes, which have a struc-ture similar to that of BUCKMINSTER-FULLERENE. Interest has been shown innanotubes as possible microscopic probesin experiments, as semiconductor materi-als, and as a component of composite ma-terials.

nascent hydrogen A particularly reac-tive form of hydrogen, which is believed toexist briefly between its generation (e.g. bythe action of dilute acid on magnesium)and its appearance as bubbles of normalhydrogen gas. It is thought that part of thefree energy of the production reaction re-mains with the hydrogen molecules for ashort time. Nascent hydrogen may be usedto produce the hydrides of phosphorus, ar-senic, and antimony, which are not readilyformed from ordinary hydrogen.

natron A naturally occurring mineralform of hydrated SODIUM CARBONATE

(Na2CO3.10H2O), found on the beds ofdried-out soda lakes.

natural abundance See abundance.

negative catalyst See catalyst.

neodymium A soft toxic silvery yellowelement belonging to the lanthanoid seriesof metals. It occurs in association withother lanthanoids in such minerals as al-lanite, bastnaesite, cerite, and monazite.Neodymium is used in various alloys, as acatalyst, in compound form in carbon-arcsearchlights, etc., and in the glass industry.

Symbol: Nd; m.p. 1021°C; b.p.3068°C; r.d. 7.0 (20°C); p.n. 60; mostcommon isotope 140Nd;r.a.m. 144.24.

neon An inert colorless odorlessmonatomic gas the second member of therare gases; i.e. group 18 (formerly VIIIA or0) of the periodic table. It has a very highionization potential and forms no com-pounds. It occurs in minute quantities(0.0018% by volume) in air and is ob-tained from liquid air by fractional distilla-tion. It is used in neon signs and lamps(emitting a characteristic orange-red light),electrical equipment, Geiger counters, andgas lasers.

Symbol: Ne; m.p. –248.67°C; b.p.–246.05°C; mass density 0.9 kg m–3 (0°C);p.n. 10; most common isotope 20Ne; r.a.m.20.18.

neptunium A toxic radioactive silveryelement of the actinoid series of metals thatwas the first transuranic element to be syn-thesized (1940). Found on Earth only inminute quantities in uranium ores, it is alsoobtained as a by-product from uraniumfuel elements bombarded by neutrons innuclear reactors. It can be converted to plu-tonium-238, which can be used as a nu-clear power source.

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Symbol: Np; m.p. 640°C; b.p. 3902°C;r.d. 20.25 (20°C); p.n. 93; most stable iso-tope 237Np (half-life 2.14 × 106 years).

Nernst, Hermann Walther (1864–1941) German physical chemist. Nernst isbest remembered for his contributions toelectrochemistry and for discovering thethird law of thermodynamics. His work onelectrochemistry included the concept ofthe solubility product and the use of buffersolutions. In 1906 he stated a theorem con-cerning the entropy of crystals at absolutezero which, in slightly different form, be-came known as the third law of thermody-namics. He also studied photochemistryand wrote an influential book entitledTheoretical Chemistry (1893). He wasawarded the 1920 Nobel Prize for chem-istry.

neutralization The stoichiometric reac-tion of an acid and a base in volumetricanalysis. The neutralization point or endpoint is detected with indicators.

neutral oxide An oxide that forms nei-ther an acid nor a base on reaction withwater. Examples include carbon monoxide(CO), dinitrogen oxide (N2O), and water(H2O). Compare acidic oxide; amphoteric;basic oxide.

neutron diffraction A method of struc-ture determination used for solids, liquids,and gases that makes use of the quantummechanical wave nature of neutrons. Ther-mal neutrons with average kinetic energiesof about 0.025 eV have a wavelength ofabout 0.1 nanometer, making them suit-able for investigating the structure of mat-ter at the atomic level. Neutron diffractionis particularly useful for identifying the po-sitions of hydrogen atoms. These are diffi-cult to establish using x-rays since x-raysinteract with electrons, and hence are scat-tered weakly by hydrogen atoms, whichhave only one electron. Protons scatterneutrons strongly, meaning that the posi-tions of protons can readily be determinedusing neutron scattering. Neutron scatter-ing has also been used extensively in inves-tigations of the magnetic ordering in solids

because there is an interaction between themagnetic moments of the neutrons and themagnetic moments of atoms in the sample.

neutron number Symbol: N The num-ber of neutrons in the nucleus of an atom;i.e. the nucleon number (A) minus the pro-ton number (Z).

Newlands’ law (law of octaves) Theobservation that, when the elements arearranged in order of increasing relativeatomic mass, there is a similarity betweenmembers that are eight elements apart (in-clusive). For example, lithium has similari-ties to sodium, beryllium to magnesium,and so on. Newlands discovered this rela-tionship in 1863 before the announcementof Mendeléev’s famous periodic law(1869). It is now recognized that New-lands’ law arises from there being eightmembers in each short period. It is namedfor the British chemist John Newlands(1837–98).

newton Symbol: N The SI derived unitof force, equal to the force needed to accel-erate one kilogram by one meter second–2.1 N = 1 kg m s–2.

Nichrome (Trademark) Any of a groupof nickel–chromium–iron alloys, contain-ing 60–80% nickel and about 16%chromium; small amounts of other el-ements, such as carbon or silicon, may beadded. They can withstand very high tem-peratures and their high electrical resistiv-ity makes them suitable for use in heatingelements.

nickel A ductile malleable relaativelyhard corrosion-resistant transition metal,the first member of group 10 (formerlypart of subgroup VIIIB) of the periodictable. Nickel occurs in nature, most impor-tantly, as the sulfides millerite (NiS) andpentlandite (Ni,Fe)9S8. It is extracted bythe Mond process, which involves roastingthe ore to obtain the oxide. The oxide isthen reduced using carbon monoxide, fol-lowed by the formation and subsequent de-composition of volatile NICKEL CARBONYL

Ni(CO)4. Nickel is used as a catalyst, in

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coinage alloys, and in stainless steel. Itscompounds in its main oxidation state (+2)are usually green.

Symbol: Ni; m.p. 1453°C; b.p. 2732°C;r.d. 8.902 (25°C); p.n. 28; most commonisotope 58Ni; r.a.m. 58.6934.

nickel carbonyl (tetracarbonyl nickel(0);Ni(CO)4) A colorless liquid with a highlytoxic vapor. It is a typical example of atransition metal complex, prepared bypassing carbon monoxide over finely di-vided nickel at a temperature of approxi-mately 60°C. The product is purified byfractional distillation. Nickel carbonyl de-composes on heating to give pure nickel. Itis used as a catalyst and in the preparationof nickel by the Mond process.

nickelic oxide See nickel(III) oxide.

nickel–iron accumulator (Edison cell;Nife or NIFE cell) A type of secondarycell in which the electrodes are formed bysteel grids. The positive electrode is im-pregnated with a nickel–nickel hydridemixture and the negative electrode is im-pregnated with iron oxide. Potassium hy-droxide solution forms the electrolyte. Anickel–iron cell is lighter and more durablethan a lead accumulator, and it can workwith higher currents. Its e.m.f. is approxi-mately 1.3 volts.

nickelous oxide See nickel(II) oxide.

nickel(II) oxide (nickelous oxide; NiO)A pale green powder formed by heatingnickel(II) hydroxide, nitrate, or carbonatein the absence of air. It is a basic oxide, dis-solving in dilute acids to give green solu-tions of nickel(II) salts. Nickel(II) oxidemay be reduced to the metal by carbon,carbon monoxide, or hydrogen.

nickel(III) oxide (nickelic oxide; Ni2O3)A black powder formed by the action ofheat on nickel(II) oxide in air. It can also beprepared by igniting nickel(II) nitrate orcarbonate. It exists also as the dihydrate,Ni2O3.2H2O.

nickel-silver (German silver) Any of a

group of alloys containing nickel, copper,and zinc (but no silver) in various propor-tions. They are white or silvery in color,can be highly polished, and have good cor-rosion-resistance. They are used, for exam-ple, as base metals in silver plating,chromium plating, and enameling.

nielsbohrium Symbol: Ns A name for-merly suggested for element-107, nowknown as bohrium (Bh). See element.

NIFE cell (Nife cell) See nickel–iron ac-cumulator.

niobium A soft silvery malleable ductiletransition metal, the second member ofgroup 5 (formerly VB) of the periodictable. It occurs naturally chiefly in the min-erals columbite ((Fe,Mn) (Nb,Ta)2 O6) andtantalite ((Fe,Mn)(Ta,Nb)2O6). It is used inwelding, special steels for high temperatureenvironments, and in superconductor al-loys. It was formerly known as colum-bium.

Symbol: Nb; m.p. 2468°C; b.p.4742°C; r.d. 8.570 (20°C); p.n. 41; mostcommon isotope 93Nb; r.a.m. 92.90638.

niter See potassium nitrate.

nitrate A salt of nitric acid.

nitration A reaction introducing thenitro (–NO2) group into a compound. Ni-tration is usually carried out using a mix-ture of concentrated nitric and sulfuricacids, although the precise conditions dif-fer from compound to compound. The at-tacking species is NO2

+ (the nitryl ion).

nitric acid (HNO3) A colorless fumingcorrosive liquid that is a strong acid. Nitricacid can be made in a laboratory by the dis-tillation of a mixture of an alkali metal ni-trate and concentrated sulfuric acid.Commercially it is prepared by the cat-alytic oxidation of ammonia and is sup-plied as concentrated nitric acid, whichcontains 68% of the acid and is often col-ored yellow by dissolved oxides of nitro-gen.

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Nitric acid is a strong oxidizing agent.Most metals are converted to their nitrateswith the evolution of oxides of nitrogen(the composition of the mixture of the ox-ides depends on the temperature and on theconcentration of the nitric acid used).Some nonmetals (e.g. sulfur and phospho-rus) react to produce oxyacids.

nitric oxide See nitrogen monoxide.

nitride A compound of nitrogen with amore electropositive element such as mag-nesium or calcium.

nitriding See case hardening.

nitrification The action of nitrifyingbacteria in converting ammonia and ni-trites to nitrates. It is an important stage inthe nitrogen cycle. See also nitrogen cycle;nitrogen fixation.

nitrite A salt of nitrous acid.

nitrogen A colorless odorless gaseouselement, the first member of group 15 (for-merly VA) of the periodic table. It is a veryelectronegative element existing in the un-combined state as gaseous diatomic N2molecules. The nitrogen atom has the elec-tronic configuration [He]2s22p3. It is typi-cally nonmetallic and its bonding isprimarily by polarized covalent bonds.With electropositive elements the nitrideion N3– may be formed.

Nitrogen accounts for about 78% ofthe atmosphere (by volume) and it also oc-curs as sodium nitrate in various mineraldeposits. It is separated for industrial useby the fractional distillation of liquid air.Pure nitrogen is prepared by thermal de-composition of azides:

2NaN3 → 2Na + 3N2The ‘active nitrogen’ gas obtained by

passing an electric discharge through nitro-gen is a mixture of ordinary nitrogen mol-ecules and excited single nitrogen atoms.

As a diatomic molecule nitrogen is ef-fectively triple-bonded and has a high dis-sociation energy (940 kJ mol–1). It istherefore relatively inert at room tempera-ture and reacts readily only with lithium

and other highly electropositive elements.Consequently it is used to provide an inertatmosphere for many metallurgicalprocesses and electrical devices. The directcombination of nitrogen and hydrogen oc-curs at elevated temperatures and pressures(400–600°C, 100 megapascals) and is thebasis of the industrially important Haberprocess for the manufacture of ammonia.Liquid nitrogen is used in cryogenics.

Nitrogen forms a number of oxides:1. Dinitrogen oxide (nitrous oxide, N2O) –

a neutral oxide.2. Nitrogen monoxide (nitric oxide, NO) –

a neutral oxide.3. Nitrogen(III) oxide (nitrogen sesquiox-

ide, N2O3) – an unstable oxide that isthe anhydride of nitrous acid.

4. Nitrogen dioxide (NO2), which gives amixture of nitrous and nitric acids inwater.

5. Dinitrogen tetroxide (N2O4), which is inequilibrium with nitrogen dioxide.

6. Dinitrogen pentoxide (N2O5) – the an-hydride of nitric acid.

7. Nitrogen trioxide (NO3) – an unstablewhite solid formed by the reaction ofdinitrogen pentoxide and ozone (O3).Nitrogen also forms a number of binary

halides such as NF3 and NCl3, and halogenazides such as ClN3. Except for NF3 theseare all very explosive, and even NF3, whichis reputedly stable, has been known to ex-plode.

Transition metals form nitrides that aredifferent from the ionic or covalent ni-trides, the stoichiometries are, for example,ZrN, W2N, Mn4N. They are often called‘interstitial’ nitrides. They are very hardand their formation is the basis of nitriding– surface hardening by exposing hot metalto ammonia gas (see case hardening).

Nitrogen has two isotopes; 14N, thecommon isotope, and 15N (natural abun-dance 0.366%). The latter is used as amarker in mass spectrometric studies.

Symbol: N; m.p. –209.86°C; b.p.–195.8°C; mass density 1.2506 kg m–3

(0°C); p.n. 7; r.a.m. 14.

nitrogen cycle The circulation of nitro-gen and its compounds in the environment,one of the major natural cycles of an el-

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ement. Nitrogen occurs mainly as a di-atomic gas in the atmosphere and as ni-trates in the soil. Denitrification of thesenitrates converts them to nitrogen. Plantsalso take up nitrates and are eaten by ani-mals; the plants and animals convert themto proteins, which after the death of the or-ganisms are broken down to yield ammo-nia, which NITRIFICATION converts back tonitrates. NITROGEN FIXATION by bacteria orlightning also gives rise to nitrates.

nitrogen dioxide (NO2) A brown gasproduced by the dissociation of dinitrogentetroxide N2O4, with which it is in equilib-rium, the dissociation being complete at140°C. Further heating causes dissociationto colorless nitrogen monoxide and oxy-gen:

2NO2(g) = 2NO(g) + O2(g)Nitrogen dioxide can also be made by

the action of heat on metal nitrates, al-though not the nitrates of the alkali metalsor some of the alkaline-earth metals.

nitrogen fixation (fixation of nitrogen)A reaction in which atmospheric nitrogenis converted into nitrogen compounds. Ni-trogen fixation occurs naturally as part ofthe life cycle processes of certain bacteriapresent in the soil and in the roots of legu-minous plants. Small amounts of nitro-gen(II) oxide (NO) are also formed inthunderstorms by direct combination ofthe elements during lightning discharges.Fixation of nitrogen is important becausenitrogen is an essential element for life, andbecause vast amounts of nitrogenous fertil-izers are used for agriculture. Formerprocesses for fixing nitrogen were theBirkeland-Eyde process (reacting N2 andO2 in an electric arc – the equivalent of alightning discharge in nature), and thecyanamide process (heating calcium dicar-bide with nitrogen to give CaCN2). Themajor method employed today is theHaber process for making ammonia.

nitrogen monoxide (nitric oxide; NO)A colorless gas that is insoluble in waterbut dissolves in a solution containingiron(II) ions owing to the formation of thecomplex ion (FeNO)2+: the nitrogen

monoxide can be released by heating. Ni-trogen monoxide can be prepared in thelab by the action of nitric acid on copperturnings and purified by using a solution ofiron(II) ions to absorb the product. Com-mercially, nitrogen monoxide is preparedby the catalytic oxidation of ammonia orby the direct union of nitrogen and oxygenin an electric arc. Nitrogen monoxide is themost heat-stable of the oxides of nitrogen,only decomposing above 1000°C. At ordi-nary temperatures it combines immediatelywith oxygen to give nitrogen dioxide:

2NO(g) + O2(g) → 2NO2(g)

nitrogen oxides (NOx) Any of the vari-ous compounds formed between nitrogenand oxygen. See dinitrogen oxide; nitro-gen; nitrogen dioxide; nitrogen monoxide.

nitrous acid (HNO2) A weak acidknown only in solution or as a gas, ob-tained by acidifying a solution of a nitrite.It readily decomposes on warming or shak-ing to nitrogen monoxide and nitric acid. Itis used extensively in the dyestuffs indus-try. Nitrous acid and the nitrites are nor-mally reducing agents but in certaincircumstances they can behave as oxidizingagents, e.g. with sulfur dioxide and hydro-gen sulfide.

nitrous oxide See dinitrogen oxide.

nitryl ion (nitronium ion) The positiveion NO2

+. See nitration.

NMR See nuclear magnetic resonance.

nobelium A synthetic radioactivetransuranic element of the actinoid series,created by bombarding a curium isotopewith the nuclei of a carbon isotope. Severalvery short-lived isotopes have been synthe-sized.

Symbol: No; m.p. 863°C; b.p., r.d. un-known; p.n. 102; most stable isotope259No (half-life 58 minutes).

noble gases See rare gases.

nonionic detergents See detergents.

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nonlocalized bond See delocalizedbond.

nonmetal Any of a class of chemical el-ements. Nonmetals lie right of the diagonalformed by the METALLOIDS in the periodictable. They are electronegative elementswith a tendency to form covalent com-pounds or negative ions. They have acidicoxides and hydroxides. In the solid state,nonmetals are either covalent volatile crys-tals or macromolecular (giant-covalent)crystals. See also metals.

nonpolar compound A compoundthat has molecules with no permanent di-pole moment. Examples of nonpolar com-pounds are hydrogen and carbon dioxide.

nonpolar solvent See solvent.

nonstoichiometric compound A chemi-cal compound whose molecules containfractional numbers of atoms. For example,titanium(IV) oxide in the form of the min-eral rutile has the chemical formula TiO1.8.

normality The number of gram equiva-lents per cubic decimeter of a given solu-tion.

normal modes of vibration The basicvibrations of a polyatomic molecule. All vi-brational motion of a polyatomic moleculeis a superposition of the normal modes ofvibration of the molecule. If N is the num-ber of atoms in a molecule the number ofmodes of vibration is 3N–5 for a linearmolecule and 3N–6 for a nonlinear mol-ecule. Each of these vibrational modes hasa characteristic frequency, although it ispossible for some of them to be degenerate.For example, in a linear triatomic moleculethere are four normal modes of vibrationsince 3N–5 = 4 for N = 3. These vibrationalmodes are: (a) symmetric stretching(breathing) vibrations; (b) antisymmetricstretching vibrations; and (c) two bendingvibrations, which are degenerate.

normal solution A solution that con-tains one gram equivalent weight per literof solution. Values are designated by the

symbol N, e.g. 0.2N, N/10, etc. Becausethere is not a clear definition of equivalentweight suitable for all reactions, a solutionmay have one value of normality for onereaction and another value in a different re-action. Because of this many workers pre-fer the molar solution notation.

Norrish, Ronald George Wreyford(1897–1978) British physical chemist.Norrish made important contributions tothe study of fast chemical reactions, partic-ularly those initiated by light. Between1949 and 1965 he developed the tech-niques of flash photolysis and kinetic spec-troscopy with George PORTER to study fastchemical reactions. Norrish and Portershared the 1967 Nobel Prize for chemistrywith Manfred EIGEN for this work. In hislater years Norrish studied chain reactionsand the kinetics of polymerization.

NOx See nitrogen oxides.

NTP See STP.

nuclear magnetic resonance (NMR) Amethod of investigating nuclear spin. In anexternal magnetic field the nucleus of anatom can have certain quantized energystates, corresponding to certain orienta-tions of the spin magnetic moment. Hydro-gen nuclei, for instance, can have twoenergy states, and transitions between thetwo occur by absorption of radiofrequencyradiation. In chemistry, this is the basis ofa spectroscopic technique for investigatingthe structure of molecules. Radiofrequencyradiation is fed to a sample and the mag-netic field is changed slowly. Absorption ofthe radiation is detected when the differ-ence between the nuclear levels corre-sponds to absorption of a quantum ofradiation. This difference depends slightlyon the electrons around the nucleus – i.e.the position of the atom in the molecule.Thus a different absorption frequency isseen for each type of hydrogen atom. Inethanol, for example, there are three fre-quencies, corresponding to hydrogenatoms on the CH3, CH2, and OH groups.The intensity of absorption also dependson the number of hydrogen atoms (3:2:1).

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NMR spectroscopy is a powerful methodof finding the structures of complex com-pounds.

nucleon number (mass number) Symbol:A The number of nucleons (protons plusneutrons) in an atomic nucleus.

nucleus The compact positively chargedcenter of an atom, made up of one or morenucleons (protons and neutrons) aroundwhich is a cloud of one or more electrons.The density of nuclei is about 1015 kg m–3.The number of protons in the nucleus de-fines the element, being its proton number(also known as its atomic number). Thenucleon number, or atomic mass number,is the sum of the protons and neutrons. Thesimplest nucleus is that of a hydrogenatom, 1H, being simply one proton (mass1.67 × 10–27 kg). The most massive nucleusthat occurs in any quantity on Earth is 238U

of 92 protons and 146 protons (mass 4 ×10–25 kg, radius 9.54 × 10–15 m). Only cer-tain combinations of protons and neutronsform stable nuclei. Others undergo sponta-neous decay.

A nucleus is depicted by a symbol indi-cating nucleon number (mass number),proton number (atomic number), and el-ement name. For example, 1

21

3Na repre-sents a nucleus of sodium having 11protons and mass 23, hence there are (23 –11) = 12 neutrons.

nuclide A nuclear species with a givennumber of protons and neutrons; for ex-ample, 23Na, 24Na, and 24Mg are all differ-ent nuclides. Thus:

23Na has 11 protons and 12 neutrons24Na has 11 protons and 13 neutrons24Mg has 12 protons and 12 neutronsThe term is applied to the nucleus and

often also to the atom in which it is found.

nucleon number

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occlusion 1. The process in which smallamounts of one substance are trapped inthe crystals of another; for example, pock-ets of liquid occluded during crystallizationfrom a solution.2. Absorption of a gas by a solid; for ex-ample, the occlusion of hydrogen by palla-dium.

ocher A clay or rock containing signifi-cant amounts of iron(III) oxide (Fe2O3),used as a yellow, orange, or brown pig-ment.

octahedral complex See complex.

octahydrate A crystalline compoundcontaining eight molecules of water ofcrystallization per molecule of compound.

octavalent Having a valence of eight.

octaves, law of See Newlands’ law.

octet A stable shell of eight electrons inan atom. The completion of the octet givesrise to particular stability, which is thebasis of the Lewis octet theory: 1. The rare gases have complete octets and

are essentially chemically inert.2. The bonding in small covalent molecules

is frequently achieved by the centralatom completing its octet by sharingelectrons with surrounding atoms, e.g.CH4, H2O.

3. The ions formed by electropositive andelectronegative elements are generallythose with a complete octet, e.g. Na+,Ca2+, O2–, Cl–.

oersted Symbol: Oe A c.g.s. unit of mag-

netic field strength. It is equal in SI units to103/4π A m–1.

ohm Symbol: Ω The SI derived unit ofelectrical resistance, equal to a resistancethat passes a current of one ampere whenthere is an electric potential difference ofone volt across it. 1 Ω = 1 V A–1.

oil of vitriol Sulfuric(VI) acid.

oleum (disulfuric(VI) acid; fuming sulfuricacid; pyrosulfuric acid; H2S2O7) A color-less fuming liquid formed by dissolving sul-fur(VI) oxide in concentrated sulfuric acid.It gives sulfuric acid on dilution withwater. See contact process.

olivine A greenish rock-forming silicatemineral ((Mg,Fe)2SiO4) containing ironand magnesium. Clear, green, gemstonequality olivine is called peridot.

onium ion An ion formed by additionof a proton (H+) to a previously neutralmolecule. The hydronium ion (H3O+) andammonium ion (NH4

+) are examples.

Onsager, Lars (1903–76) Norwegian-born American chemist. Onsager madeseveral important contributions to theoret-ical chemistry and physics. In 1926 he im-proved on the Debye-Hückel theory ofelectrolytes by taking the Brownian motionof ions into account. He subsequently in-vestigated the dielectric constants of matterthat contains polar molecules. In 1931 hepublished fundamental work on nonequi-librium thermodynamics. He was awardedthe 1968 Nobel Prize for chemistry for thiswork.

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O

oolite Any sedimentary rock consistingof an aggregate of small spherical masses,most typically composed of calcium car-bonate (CaCO3), in a fine matrix.

opal A naturally occurring hydratednoncrystalline form of silica (SIO2.nH2O),valued as gemstones. Various layers withinprecious opal can reflect and refract lightto give a play of spectral colors. Varietiesinclude black, fire, and milky white opal.

open-hearth process An obsoletemethod of making steel by heating pig ironin an open-hearth furnace by burning amixture of producer gas and air. Oxygen isinjected through pipes to oxidize impuri-ties and burn off carbon, and lime is usedto form a slag. The basic oxygen processhas replaced it.

optical activity The ability of certaincompounds to rotate the plane of polariza-tion of plane-polarized light when the light

is passed through them. Optical activitycan be observed in crystals, gases, liquids,and solutions. The amount of rotation de-pends on the concentration of the activecompound.

Optical activity is caused by the interac-tion of the varying electric field of the lightwith the electrons in the molecule. It occurswhen the molecules are asymmetric – i.e.they have no plane of symmetry. Such mol-ecules have a mirror image that cannot besuperimposed on the original molecule. Inorganic compounds this usually means thatthey contain a carbon atom attached tofour different groups, forming a chiral cen-ter. The two mirror-image forms of anasymmetric molecule are optical isomers.One isomer will rotate the polarized lightin one sense and the other by the sameamount in the opposite sense. Such isomersare described as dextrorotatory (symbol dor (+)) or levorotatory (symbol l or (–)), ac-cording to whether they rotate the plane tothe ‘right’ or ‘left’ respectively (rotation to

oolite

158

(S)-lactic acid(R)-lactic acid

COOH

COH

H

CH3

COOH

CH

OH

CH3

Optical activity: two forms of lactic acid

N

N

N

XM

N

X

N

X

N

NM

N

X

Optical activity: two forms of an octahedral complex. The bidentate ligands are represented by acurved line

the left is clockwise to an observer viewingthe light coming toward the observer). Amixture of the two isomers in equalamounts does not show optical activity.Such a mixture is sometimes called the (±)or dl-form, a racemate, or a racemic mix-ture

Optical isomers have identical physicalproperties (apart from optical activity) andcannot be separated by fractional crystal-lization or distillation. Their general chem-ical behavior is also the same, althoughthey do differ in reactions involving otheroptical isomers. Many naturally occurringsubstances are optically active (only oneoptical isomer exists naturally) and bio-chemical reactions occur only with the nat-ural isomer. For instance, the natural formof glucose is d-glucose and living organ-isms cannot metabolize the l-form.

The terms ‘dextrorotatory’ and ‘levoro-tatory’ refer to the effect on polarized light.A more common method of distinguishingtwo optical isomers is by their D-form(dextro-form) or L-form (levo-form). Thisconvention refers to the absolute structureof the isomer according to specific rules.Sugars are related to a particular configu-ration of glyceraldehyde (2,3-dihydrox-ypropanal). For alpha amino acids the‘corn rule’ is used: the structure of the acidRC(NH2)(COOH)H is drawn with H atthe top; viewed from the top the groupsspell CORN in a clockwise direction for allD-amino acids (i.e. the clockwise order is–COOH,R,NH2). The opposite is true forL-amino acids. Note that this conventiondoes not refer to optical activity: D-alanineis dextrorotatory but D-cystine is levorota-tory.

An alternative is the R/S system forshowing configuration. There is an orderof priority of attached groups based on theproton number of the attached atom:I, Br, Cl, SO3H, OCOCH3, OCH3, OH,NO2, NH2, COOCH3, CONH2, COCH3,CHO, CH2OH, C6H5, C2H5, CH3, H

Hydrogen has the lowest priority. Thechiral carbon is viewed such that the groupof lowest priority is hidden behind it. If theother three groups are in descending prior-ity in a clockwise direction, the compound

is R-. If descending priority is anticlockwiseit is S-.

The existence of a carbon atom boundto four different groups is not the strictcondition for optical activity. The essentialpoint is that the molecule should be asym-metric. Inorganic octahedral complexes,for example, can show optical isomerism(see illustration). It is also possible for amolecule to contain asymmetric carbonatoms and still have a plane of symmetry.One structure of tartaric acid has two partsof the molecule that are mirror images,thus having a plane of symmetry. This(called the meso-form) is not optically ac-tive. See also resolution.

optical isomers See isomerism; opticalactivity.

optical rotary dispersion (ORD) Thephenomenon in which the amount of rota-tion of plane-polarized light by an opticallyactive substance depends on the wave-length of the light. Plots of rotation againstwavelength can be used to give informa-tion about the molecular structure of opti-cally active compounds.

optical rotation Rotation of the planeof polarization of plane-polarized light byan optically active substance.

orbital A region around an atomic nu-cleus in which there is a high probability offinding an electron. The modern picture ofthe atom, according to quantum mechan-ics, does not have electrons moving in fixedelliptical orbits. Instead, there is a finiteprobability that the electron will be foundin any small region at all possible distancesfrom the nucleus. In the hydrogen atom theprobability is low near the nucleus, in-creases to a maximum, and falls off to in-finity. It is useful to think of a region inspace around the nucleus – in the case ofhydrogen the region within a sphere –within which there is a high chance of find-ing the electron. Each of these regions,called an atomic orbital, corresponds to asubshell and can ‘contain’ either a singleelectron or two electrons with oppositespins. Another way of visualizing an or-

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orbital

160

s orbitaldxy orbital

px orbital py orbital pz orbital

z

y

x

z

y

x

z

y

x

z

y

x

z

y

x

z

y

x

dz2 orbital

Orbital: atomic orbitals

one s

one s

one s

+

+

+

ATOMIC ORBITALS HYBRID ORBITALS SHAPE

one p two sp3 linear

two p three sp2 planar

three p tetrahedralfour sp3

Orbital: hybrid orbitals

bital is as a cloud of electron charge (thecloud being the average distribution of thecharge with time).

Similarly, in molecules the electronsmove in the combined field of the nucleiand can be assigned to molecular orbitals.In considering bonding between atoms it isuseful to treat molecular orbitals as formedby overlap of atomic orbitals.

It is possible to calculate the shapes andenergies of atomic and molecular orbitalsby QUANTUM THEORY. The shapes of atomicorbitals depend on their orbital angularmomentum. For each shell there is, at amaximum, one s orbital, three p orbitals,five d orbitals, and seven f orbitals. The sorbitals are spherical, the p orbitals eachhave two lobes, the d orbitals have morecomplex shapes, typically with four lobes,and the f orbitals are more complex yet,often with eight lobes.

Molecular orbitals are formed by over-lap of atomic orbitals, and again there aredifferent types. If the orbital is completely

symmetrical about an axis between the nu-clei, it is a sigma orbital. This can occur,for instance, by overlap of two s orbitals,as in the hydrogen atom, or two p orbitalswith their lobes along the axis. However,two p orbitals overlapping at right anglesto the axis form a different type of molecu-lar orbital – a pi orbital – with regionsabove and below the axis. Pi orbitals arealso formed by overlap of d orbitals. Eachmolecular orbital can contain a pair ofelectrons, forming a sigma bond or pibond. A double bond, for example thebond in ethene, is a combination of a sigmabond and a pi bond.

Hybrid orbitals are atomic orbitalsformed by combinations of s, p, and datomic orbitals, and are useful in describ-ing the bonding in compounds. There arevarious types. In carbon, for instance, theelectron configuration is 1s22s22p2. Car-bon, in its outer (valence) shell, has onefilled s orbital, two filled p orbitals, andone ‘empty’ p orbital. These four orbitals

161

orbital

z

A

B

C

x x

x x

x x

s orbitals sigma

px orbitals

pz orbitals

sigma

pi

z

Orbital: molecular orbitals

may hybridize (sp3 hybridization) to act asfour equal orbitals arranged tetrahedrally,each with one electron. In methane, eachhybrid orbital overlaps with a hydrogen sorbital to form a sigma bond. Alterna-tively, the s and two of the p orbitals mayhybridize (sp2 hybridization) and act asthree orbitals in a plane at 120°. The re-maining p orbital is at right angles to theplane, and can form pi bonds. Finally, sphybridization may occur, giving two or-bitals in a line. More complex types of hy-bridization, involving d orbitals, explainthe geometries of inorganic complexes. Forexample, square-planar complexes can beformed by sp2d hybridization; octahedralcomplexes can involve sp3d2 hybridization.

The combination of two atomic orbitalsin fact produces two molecular orbitals.One – the bonding orbital – has a concen-tration of electron density between the nu-clei, and thus tends to hold the atomstogether. The other – the antibonding or-bital – has slightly higher energy and tendsto repel the atoms. If both atomic orbitalsare filled, the two molecular orbitals arealso filled and cancel each other out – thereis no net bonding effect. If each atomic or-bital has one electron, the pair occupies thelower energy bonding orbital, producing anet attraction.

ORD See optical rotary dispersion.

order The sum of the indices of the con-centration terms in the expression that de-termines the rate of a chemical reaction.For example, in the expression

rate = k[A]x[B]y

x is called the order with respect to A, y theorder with respect to B, and (x + y) theorder overall. The values of x and y are notnecessarily equal to the coefficients of Aand B in the molecular equation. Order isan experimentally determined quantity de-rived without reference to any equation ormechanism. Fractional orders do occur.For example, in the reaction

CH3CHO → CH4 + COthe rate is proportional to [CH3CHO]1.5

i.e. it is of order 1.5.

ore A mineral source of a chemical ele-ment.

ore dressing See beneficiation.

organic chemistry The chemistry ofcompounds of carbon. Originally the termorganic chemical referred to chemical com-pounds present in living matter, but now itcovers any carbon compound with the ex-ception of certain simple ones, such as thecarbon oxides, carbonates, cyanides, andcyanates. These are generally studied in in-organic chemistry. The vast numbers ofsynthetic and natural organic compoundsexist because of the ability of carbon toform chains of atoms (catenation). Otherelements are involved in organic com-pounds; principally hydrogen and oxygenbut also nitrogen, halogens, sulfur, andphosphorus.

organometallic compound An or-ganic compound containing a carbon–metal bond. Tetraethyl lead, (C2H5)4Pb, isa well-known example.

ortho- 1. Designating the form of a di-atomic molecule in which both nuclei havethe same spin direction; e.g. orthohydro-gen, orthodeuterium.2. Certain acids, regarded as formed froman anhydride and water, were named orthoacids to distinguish them from the less hy-drated meta acids. For example, H4SiO4

(from SiO2 + 2H2O) is orthosilicic acid;H2SiO3 (SiO2 + H2O) is metasilicic acid.See also meta-; para-.

orthoboric acid See boric acid.

orthohydrogen See hydrogen.

orthophosphates See phosphoric(V)acid.

orthophosphoric acid See phosphoric(V)acid.

orthophosphorous acid See phospho-nic acid.

ORD

162

orthorhombic crystal See crystal sys-tem.

osmiridium A very hard naturally oc-curring alloy consisting chiefly of osmiumand iridium, used to make pen nibs.

osmium A hard bluish white transitionelement, the third element of group 8 (for-merly part of subgroup VIIIB) of the peri-odic table. It is found associated withplatinum. Osmium is the densest of all met-als. It has a characteristic chlorinelike smellresulting from the production of os-mium(VIII) oxide (osmium tetroxide,OsO4) when heated in air. The metal isused in catalysts and in alloys for pen nibs,pivots, and electrical contacts.

Symbol: Os; m.p. 3045°C; b.p. 5027°C;r.d. 22.59 (20°C); p.n. 76; most commonisotope 192Os; r.a.m. 190.23.

osmium(IV) oxide (osmium tetroxide;OsO4) A volatile yellow crystalline solidwith chlorinelike penetrating poisonousodor, used as an oxidizing agent and, inaqueous solution, as a catalyst for organicreactions.

osmosis Systems in which a solvent isseparated from a solution by a semiperme-able membrane approach equilibrium bysolvent molecules on the solvent side of themembrane migrating through it to the so-lution side; this process is called osmosisand always leads to dilution of the solu-tion. The phenomenon is quantified bymeasurement of the osmotic pressure. Theprocess of osmosis is of fundamental im-portance in transport and control mecha-nisms in biological systems; for example,plant growth and general cell function.

Osmosis is a colligative property and itstheoretical treatment is similar to that forthe lowering of vapor pressure. The mem-brane can be regarded as equivalent to theliquid-vapor interface, i.e. one that permitsfree movement of solvent molecules but re-stricts the movement of solute molecules.The solute molecules occupy a certain areaat the interface and therefore inhibit sol-vent egress from the solution. Just as thedevelopment of a vapor pressure in a

closed system is necessary for liquid–vaporequilibrium, the development of an OS-MOTIC PRESSURE on the solution side is nec-essary for equilibrium at the membrane.

osmotic pressure Symbol: π The pres-sure that must be exerted on a solution toprevent the passage of solvent moleculesinto it when the solvent and solution areseparated by a semipermeable membrane.The osmotic pressure is therefore the pres-sure required to maintain equilibrium be-tween the passage of solvent moleculesthrough the membrane in either directionand thus prevent the process of osmosisfrom proceeding. The osmotic pressure canbe measured by placing the solution, con-tained in a small perforated thimble cov-ered by a semipermeable membrane andfitted with a length of glass tubing, in abeaker of the pure solvent. Solvent mol-ecules pass through the membrane, dilut-ing the solution and thereby increasing thevolume on the solution side and forcing thesolution to rise up the glass tubing. Theprocess continues until the pressure ex-erted by the solvent molecules on the mem-brane is balanced by the hydrostaticpressure of the solution in the tubing. Asample of the solution is then removed andits concentration determined. Osmosis is acolligative property; therefore the methodcan be applied to the determination of rel-ative molecular masses, particularly forlarge molecules, such as proteins, but it isrestricted by the difficulty of preparinggood semipermeable membranes.

Because the osmotic pressure is a col-ligative property it is directly proportionalto the molar concentration of the solute ifthe temperature remains constant; thus π isproportional to the concentration n/V,where n is the number of moles of soluteand V the solvent volume. The osmoticpressure is also proportional to the ab-solute temperature. Combining these twoproportionalities gives πV = nCT, whichhas the same form as the gas equation, PV= nRT, and experimental values of C aresimilar to those for R, the universal gasconstant. This gives considerable supportto the kinetic theory of colligative proper-ties.

163

osmotic pressure

Ostwald, Friedrich Wilhelm (1854–1932) Russian-born German physicalchemist. Ostwald was one of the main fig-ures involved in establishing physicalchemistry as a distinct subject. In 1888 heput forward the OSTWALD DILUTION LAW forthe degree of dissociation of an electrolyte.His own research was mainly concernedwith catalysts. This led to the discovery ofthe OSTWALD PROCESS for converting am-monia to nitric acid by passing air and am-monia over a platinum catalyst. Ostwaldwon the 1909 Nobel Prize for chemistryfor his work on catalysis. He also wrote aninfluential two-volume textbook entitledTextbook of General Chemistry (1885,1887).

Ostwald’s dilution law See dissocia-tion.

ox See oxalic acid.

oxalato See oxalic acid.

oxalic acid A dibasic organic carboxylicacid, CH2(COOH)2. It is of interest in in-organic chemistry because the carboxylateion functions as a bidentate ligand (the ox-alato ligand) in forming certain complexes.In the names of complexes it is given thesymbol ox.

oxidant See oxidation.

oxidation An atom, an ion, or a mol-ecule is said to undergo oxidation or to beoxidized when it loses electrons. Theprocess may be effected chemically, i.e. byreaction with an oxidizing agent (oxidant),or electrolytically, in which case oxidationoccurs at the anode. For example,

2Na + Cl2 → 2Na+ + 2Cl–

where chlorine is the oxidizing agent andsodium is oxidized, and

4CN– + 2Cu2+ → C2N2 + 2CuCNwhere Cu2+ is the oxidizing agent and CN–

is oxidized.The oxidation state of an atom is indi-

cated by the number of electrons lost or ef-fectively lost by the neutral atom, i.e. itsoxidation number. The oxidation numberof a negative ion is negative. The process ofoxidation is the converse of reduction. Seealso redox.

oxidation number See oxidation.

oxidation–reduction See redox.

oxidation state See oxidation.

oxide A compound of OXYGEN and an-other element. Oxides can be made by di-rect combination of the elements or byheating a hydroxide, carbonate, or othersalt. See also acidic oxide; basic oxide; neu-tral oxide; peroxide.

oxidizing acid An acid that acts as anoxidizing agent, e.g. sulfuric acid or nitricacid. Because it is oxidizing, nitric acid candissolve metals below hydrogen in the elec-trochemical series:

2HNO3 + M → MO + 2NO2 + H2OMO + 2HNO3 → H2O + M(NO3)2

oxidizing agent See oxidation.

oxyacid An ACID in which the replace-able hydrogen atom is part of a hydroxylgroup, including inorganic acids such asphosphoric(V) acid and sulfuric acid.

oxygen A colorless odorless diatomicgas, the first member of group 16 (formerlyVI) of the periodic table. It has the elec-tronic configuration [He]2s22p4 and itschemistry involves the acquisition of elec-trons to form either the di-negative ion O2–

or two covalent bonds. In each case theoxygen atom attains the configuration ofthe rare gas neon.

Oxygen is the most plentiful element inthe Earth’s crust, accounting for over 49%by mass. It accounts for 23% of the volumeof the atmosphere and is a constituent ofthe majority of minerals and rocks (e.g. sil-

Ostwald, Friedrich Wilhelm

164

CC

O O

O__

O

Oxalic acid: the oxalato ligand

icates, SiO2, carbonates, CaCO3, alumi-nosilicates, Al2SiO5) as well as being themajor constituent of the sea. Oxygen is anessential element for almost all livingthings. It is also used in solid propellantsfor rockets, in steel making and welding, inmedical environments and breathing appa-ratus, and in the manufacture of variouschemicals. Elemental oxygen has two al-lotropes: the diatomic molecule O2 and theless stable molecule OZONE (trioxygen O3),which is formed by passing an electric dis-charge through oxygen gas. There are sev-eral convenient laboratory routes by whichto produce oxygen:1. Heat on unstable oxides.

2HgO → 2Hg + O22. Decomposition of peroxides.

2H2O2 → 2H2O + O23. Heat on ‘-ate’ compounds, such as chlo-

rates or manganate(VII).2KClO3 → KCl + 3O2

2KMnO4 → K2MnO4 + MnO2 + O2Industrially oxygen is obtained by the

fractional distillation of liquid air.Oxygen reacts with elements from all

groups of the periodic table, includingxenon in group 18 (e.g. XeO3, XeO4,XeOF2, XeO2

2–).The nature of the oxides has long been

a convenient guide to metallic and non-metallic character. Thus, metals typicallycombine with oxygen to form basic oxides,which if soluble react with water to formbases via the OH– ion. The more elec-tropositive the metal, the more vigorousthe reaction. Most of the group 1 andgroup 2 metals react vigorously whenforming oxides, O2–, (e.g., Na2O, CaO);peroxides, O2

2–, (e.g., Na2O2, BaO2); andsuperoxides, O2

–, (e.g., RbO2, CsO2). Ox-ides of metallic elements are solids and x-ray studies show that discrete O2– ions doexist in the solid state even though they donot exist in solution:

O2– + H2O → 2OH–

The nonmetals typically form acidic ox-ides such as SO2, ClO2, and NO2. Whensoluble in water they generally dissolve toform acids. The less electronegative ele-ments among the nonmetals generally formweaker acids, e.g. B(OH)3 and H2CO3.The more electronegative elements form

stronger acids. The higher the oxidationstate of the central atom the stronger theacid (e.g. HOCl is weaker than HClO2,which is weaker than HClO4). Nonmetaloxides may be gaseous (SO2, CO2), liquid(N2O4), or solid (P2O5). The weakly non-metallic elements such as silicon may re-quire bases for dissolution and in somecases the oxides have extended polymericstructures, e.g., SiO2. In between the acidicand basic oxides are those that are ampho-teric, i.e. they behave acidically to strongbases and basically toward strong acids.This behavior is associated with elementsthat are physically metallic but are chemi-cally only weakly cationic, e.g., ZnO givesZn2– with acids and zincates, Zn(OH)4

2–,with bases. There is also a small class ofmixed oxides that can be regarded as saltsbetween a ‘basic’ and an ‘acidic’ oxide ofthe same metal: for example,

FeO + Fe2O3 → Fe(FeO2)2 = Fe3O42PbO + PbO2 → Pb3O4

The neutral oxides are essentially insol-uble nonmetal oxides such as N2O, NO,CO, and F2O, although extreme conditionscan lead to some degree of acidic behaviorin the case of CO, e.g.,

CO + OH– → HCOO–

Oxygen is very limited in its catenationbut an interesting range of O–O species isformed by the following: peroxide, O2

2–;superoxide, O2

–; oxygen gas, O2; oxygenylcation, O2

+. The O–O bond gets both pro-gressively shorter and stronger in the se-ries. The peroxides are formed with Na,Ca, Sr, and Ba and are formally related tohydrogen peroxide (prepared by the actionof acids on ionic peroxides). The superox-ides of K, Rb, and Cs are prepared by burn-ing the metal in air (the less electropositivemetals Na, Mg, and Zn form less stable su-peroxides). Oxygenyl cation, O2

+, isformed by the reaction of PtF6 (which hasa high electron affinity) with oxygen atroom temperature. Other oxygenyl cationspecies can be made with group 15 fluo-rides, e.g. O2

+AsF6–.

Oxygen occurs in three natural isotopicforms, 16O (99.76%), 17O (0.0374%), and18O (0.2039%); the rarer isotopes are usedin detailed studies of the behavior of oxy-

165

oxygen

gen-containing groups during reactions(tracer studies).

Symbol: O; m.p. –218.4°C; b.p.–182.962°C; d. 1.429 kg m–3 (0°C); p.n. 8;r.a.m. 15.9994.

ozone (trioxygen; O3) A poisonous bluediamagnetic allotrope of oxygen made nat-urally by high-energy ultraviolet radiationin the stratosphere and to a lesser extent byphotochemical reactions involving pollu-tants such as NOx. Ozone can also be madeartificially by passing oxygen through asilent electric discharge. Ozone is unstableand decomposes to oxygen on warming. In

the stratosphere it forms a layer ofozonized air that acts to screen the Earthfrom harmful short-wave ultraviolet radia-tion. In recent decades there has been con-siderable evidence to suggest that this layeris being depleted by fluorocarbons andother compounds produced by industry.Consequently most industrialized nationshave agreed to severely restrict their pro-duction and use of these chemicals.

Ozone is a strong oxidizing agent. Itsconcentration can be determined by react-ing it with iodide ions and titrating theiodine formed with standard sodium thio-sulfate(VI).

ozone

166

paint A suspension of powdered pig-ment in a liquid vehicle, used for decorativeand protective surface coatings. In oil-based paints, the vehicle contains solventsand a drying oil or synthetic resin. Inwater-based or emulsion paints, the vehicleis mostly water with an emulsified resin.

palladium A soft silvery white ductiletransition metal, the second element ofgroup 10 (formerly part of subgroupVIIIB) of the periodic table. It occurs nativein association with platinum and also innickel, copper, gold, and silver ores, fromwhich it is obtained as a by-product. Al-though it will dissolve in acid, it will reactwith oxygen only at high temperatures,making it resistant to corrosion in most en-vironments. Consequently it finds use indentistry, surgical instruments, electricalrelays, and jewelry (white gold is an alloyof gold and palladium). It is also used as acatalyst in hydrogenation processes. Hy-drogen is purified by diffusing it through ahot palladium barrier.

Symbol: Pd; m.p. 1552°C; b.p. 3140°C;r.d. 12.02 (20°C); p.n. 46; most commonisotope 106Pd; r.a.m. 106.42.

paper chromatography A techniquewidely used for the analysis of mixtures.Paper CHROMATOGRAPHY usually employsa specially produced paper as the station-ary phase. A base line is marked in pencilnear the bottom of the paper and a smallsample of the mixture is spotted onto itusing a capillary tube. The paper is thenplaced vertically in a suitable solvent,which rises up to the base line and beyondby capillary action. The componentswithin the sample mixture dissolve in thismobile phase and are carried up the paper.However, the paper holds a quantity of

moisture and some components will have agreater tendency than others to dissolve inthis moisture than in the mobile phase. Inaddition, some components will cling tothe surface of the paper. Therefore, as thesolvent moves through the paper, certaincomponents will be left behind and sepa-rated from each other.

When the solvent has almost reachedthe top of the paper, the paper is removedand quickly dried. The paper is developedto locate the positions of colorless fractionsby spraying with a suitable chemical, e.g.ninhydrin, or by exposure to ultraviolet ra-diation. The components are identified bycomparing the distance they have traveledup the paper with standard solutions thathave been run simultaneously, or by com-puting an RF value. A simplified version ofpaper chromatography uses a piece of filterpaper. The sample is spotted at the centerof the paper and solvent passed through it.Separation of the components of the mix-ture again takes place as the mobile phasespreads out on the paper.

Paper chromatography is an applica-tion of the partition law.

167

P

developingchamber

paper strip

mobile phase

spotted samples

Paper chromatography

para- Designating the form of a di-atomic molecule in which both nuclei haveopposite spin directions; e.g. parahydro-gen, paradeuterium.

See also meta-; ortho-.

parahydrogen See hydrogen.

paramagnetism See magnetism.

partial ionic character The electronsof a covalent bond between atoms orgroups with different electronegativitieswill be polarized toward the more elec-tronegative constituent; the magnitude ofthis effect can be measured by the ioniccharacter of the bond. When the effect issmall the bond is referred to simply as apolar bond and is adequately treated usingdipole moments; as the effect growsstronger the theoretical treatment requiresother contributions to ionic character. Ex-amples of partial ionic character (from nu-clear quadrupole resonance) are H–I, 21%;H–Cl, 40%; Li–Br, 90%.

partial pressure See Dalton’s law.

partition coefficient If a solute dis-solves in two nonmiscible liquids, the par-tition coefficient is the ratio of theconcentration in one liquid to the concen-tration in the other liquid.

pascal Symbol: Pa The SI derived unit ofpressure, equal to a pressure of one newtonper square meter (1 Pa = 1 N m–2).

Paschen–Back effect See Zeeman ef-fect.

Paschen series A series of lines in the in-frared spectrum emitted by excited hydro-gen atoms. The lines correspond to theatomic electrons falling into the third low-est energy level and emitting energy as ra-diation. The wavelength (λ) of theradiation in the Paschen series is given by1/λ = R(1/32 – 1/n2) where n is an integerand R is the Rydberg constant. The series isnamed for German physicist Louis Paschen(1865–1947), who discovered it in 1908.See also spectral series.

passive Describing a metal that is unre-active because of the formation of a thinprotective layer. Iron, for example, doesnot dissolve in concentrated nitric(V) acidbecause of the formation of a thin oxidelayer.

Pauli, Wolfgang (1900–58) Austrian-born Swiss theoretical physicist. Pauli isbest known for his enunciation of the Pauliexclusion principle in 1925. This enabledthe electronic structure of atoms to be un-derstood, particularly how the shell struc-ture of atoms, and hence the periodic tableof the elements, comes about. Pauli wonthe 1945 Nobel Prize for physics for thiswork. Pauli made many other importantcontributions to physics including his pre-diction of the neutrino in beta decay, theincorporation of spin into quantum me-chanics, and the explanation of paramag-netism in metals. He also wrote severalclassic books and reviews on quantum me-chanics.

Pauli exclusion principle See atom.

Pauling, Linus Carl (1901–94) Ameri-can chemist. Pauling was one of the great-est scientists of the 20th century. His earlywork was on determining the structure ofcomplex minerals by x-ray diffraction. In1928–29 this led to Pauling’s rules govern-ing the structure of complex minerals.Pauling was also a major pioneer in the ap-plication of quantum mechanics to chemi-cal bonding. In 1931 he published a classicpaper entitled The Nature of the ChemicalBond that explained how a chemical bondis formed from a pair of electrons. Paulingalso introduced the concept of hybridiza-tion to explain the chemical bonding of thecarbon atom. He also considered partiallyionic bonds and put together his ideasabout chemical bonding in his great bookThe Nature of the Chemical Bond, the firstedition of which was published in 1939. Inthe mid-1930s Pauling turned his attentionto molecules of biological interest and wasone of the founders of molecular biology.Together with Robert Corey, he showedthat many proteins have helical shapes. Healso worked on sickle-cell anemia. With E.

para-

168

Bright Wilson he co-authored the book In-troduction to Quantum Mechanics (1935)and wrote the influential books GeneralChemistry (1948) and College Chemistry(1950). Pauling was awarded the 1954Nobel Prize for chemistry. In the 1950s hebecame concerned with nuclear weapons,particularly their testing in the atmosphere.This led to him being awarded the 1962Nobel Prize for peace.

p-block elements The elements of themain groups 13 (B to Tl), 14 (C to Pb), 15(N to Bi), 16 (O to Po), 17 (F to At), and 18(He to Rn). They are so called because theyhave outer electronic configurations thathave occupied p levels.

pearl A lustrous spherical accretion ofnacre, comprised chiefly of calcium car-bonate (CaCO3), that builds up on a parti-cle of foreign matter inside the shells ofoysters and some other mollusks.

pearl spar See dolomite.

peat A brown or black material formedby the partial decomposition of plant re-mains in marshy ground, the first stage inthe formation of coal. It is used to makecharcoal and compost. When dried, it canalso be burned as a fuel.

PEC See photoelectrochemical cell.

pentahydrate A crystalline solid con-taining five molecules of water of crystal-lization per molecule of compound.

pentavalent (quinquevalent) Having avalence of five.

percentage composition A way of ex-pressing the composition of a chemicalcompound in terms of the percentage (bymass) of each of the elements that make itup. It is calculated by dividing the mass ofeach element (taking into account the num-ber of atoms present) by the relative mole-cular mass of the whole molecule. Forexample, methane (CH4) has a relativemolecular mass of 16 and its percentage

composition is 12/16 = 75% carbon and (4× 1)/16 = 25% hydrogen.

perchloric acid See chloric(VII) acid.

perfect gas (ideal gas) See gas laws; ki-netic theory.

peridot See olivine.

period One of the horizontal rows in theconventional periodic table. Each periodrepresents the elements arising from pro-gressive filling of the outer shell (i.e. the ad-dition of one extra electron for each newelement), the elements being arranged inorder of ascending proton number. In astrict sense hydrogen and helium representone period but convention refers to the el-ements lithium to neon (8 elements) as thefirst short period (n = 2), and the elementssodium to argon (8 elements) as the secondshort period (n = 3). With entry to the n =4 level there is filling of the 4s orbital, thenback-filling of the 3d orbitals, before the4p are filled. Thus this set is based on thefilling of a total of 18 electrons (potassiumto krypton) and is called a long period. Thenext set, rubidium to xenon, is similarly along period. See also Aufbau principle.

periodic acid See iodic(VII) acid.

periodic law The law upon which themodern periodic table is based. Enunciatedin 1869 by Mendeléev, this law stated thatthe properties of the elements are a peri-odic function of their atomic masses: ifarranged in order of increasing atomicmass then elements having similar proper-ties occur at fixed intervals. Certain excep-tions or gaps in the table lead to the viewthat the nuclear charge is a more charac-teristic function, thus the modern state-ment of the periodic law is that the physicaland chemical properties of elements are aperiodic function of their proton number.

periodic table A table of the elementsarranged in order of increasing protonnumber to show similarities in chemicalbehavior between elements. Horizontalrows of elements are called periods. Across

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periodic table

a period there is a general trend frommetallic to nonmetallic behavior. Verticalcolumns of related elements are calledgroups. Down a group there is an increasein atomic size and in electropositive (metal-lic) behavior.

Originally the periodic table wasarranged in eight groups with the alkalimetals as group I, the halogens as groupVII, and the rare gases as group 0. Thetransition elements were placed in a blockin the middle of the table. Groups weresplit into two subgroups. For example,group I contained the main-group ele-ments, Li, Na, K, Rb, Cs, in subgroup IAand the subgroup IB elements, Cu, Ag, Au.The system was confusing because therewas a difference in usage for subgroupsand the current form of the table has 18groups (see Appendix).

See also group; period.

permanent gas A gas that cannot be liq-uefied by pressure alone – that is, a gasabove its critical temperature. See criticaltemperature.

permanent hardness A type of waterhardness caused by the presence of dis-solved calcium, iron, and magnesium sul-fates or chlorides. This form of hardnesscannot be removed by boiling. Comparetemporary hardness. See also water soften-ing.

permanganate See manganate(VII).

Permutit (Trademark) A substance usedto remove unwanted chemicals that havedissolved in water. It is a zeolite consistingof a complex chemical compound, sodiumaluminum silicate. When hard water ispassed over this material, calcium andmagnesium ions exchange with sodiumions in the Permutit. This is a good exam-ple of ION EXCHANGE. Once all the availablesodium ions have been used up, the Permu-tit can be regenerated by washing it with asaturated solution of sodium chloride. Theexcess sodium ions then exchange with thecalcium and magnesium ions in the Permu-tit.

peroxide 1. An oxide containing the–O–O– ion.2. A compound containing the –O–O–group.

peroxosulfuric(VI) acid (Caro’s acid;H2SO5) A white crystalline solid formedby reacting hydrogen peroxide with sulfu-ric acid. It is a powerful oxidizing agent.

perturbation theory An approxima-tion technique in which a system is dividedinto two parts, with one part that can besolved exactly and a second part thatmakes the system too complicated to besolved exactly. For perturbation theory tobe applicable it is necessary that the secondpart is a small correction to the first part.The effects of the small corrections are ex-pressed as an infinite series, called a per-turbation series, which modifies the resultsthat could be calculated exactly. A classicexample of perturbation theory is the mod-ification of the orbits of planets around theSun due to their gravitational interactionswith other planets. In quantum mechanicsan application of perturbation theory is thecalculation of the effects of small electric ormagnetic fields on atomic energy levels.

peta- Symbol: P A prefix used with SIunits, denoting 1015.

petrochemicals Organic chemicals ob-tained from crude oil or natural gas.

petrol See petroleum.

petroleum (crude oil) A mixture of hy-drocarbons formed originally in shallowbasins from decayed, chiefly marine ani-mals and plants, found today beneath theground trapped between layers of sedimen-tary rock. It is obtained by drilling. Differ-ent oilfields produce petroleum withdiffering compositions. The mixture is sep-arated into fractions by fractional distilla-tion in a vertical column. The mainfractions are:Diesel oil (gas oil) in the boiling range220–350°C, consisting mainly of C13–C25hydrocarbons. It is used in diesel engines.

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170

Kerosene (paraffin) in the range160–250°C, consisting mainly of C11 andC12 hydrocarbons. It is a fuel both for do-mestic heating and jet engines.Gasoline (petrol) in the range 40–180°C,consisting mainly of C5–C10 hydrocarbons.It is used as motor fuel for gasoline enginesand as a raw material for making otherchemicals.Refinery gas, consisting of C1–C4 gaseoushydrocarbons such as propane.

In addition, lubricating oils and paraf-fin wax are obtained from the residue. Theblack material left is composed of bitumenand asphalt.

pewter A tarnish-resistant alloy of tinand lead, today usually containing80–90% tin. Modern pewter is hardenedby antimony and copper, which in the bestgrades completely replaces the lead con-tent. It is less dense than old pewter, andhaving a bright or a soft satiny finish, isoften used for decorative and ceremonialpurposes.

pH The logarithm to base 10 of the rec-iprocal of the hydrogen-ion concentrationof a solution. In pure water at 25°C theconcentration of hydrogen ions is 1.00 ×10–7 mol l–1, thus the pH equals 7 at neu-trality. An increase in acidity increases thevalue of [H+], decreasing the value of thepH below 7. An increase in the concentra-tion of hydroxide ion [OH–] proportion-ately decreases [H+], therefore increasingthe value of the pH above 7 in basic solu-tions. Potential of hydrogen or pH valuescan be obtained approximately by using in-dicators. More precise measurements useelectrode systems.

phase One of the physically separableparts of a chemical system. For example, amixture of ice (solid phase) and water (liq-uid phase) consists of two phases. A systemconsisting of only one phase is said to behomogeneous. A system consisting of morethan one phase is said to be heterogeneous.

phase diagram A graphical representa-tion of the state in which a substance willoccur at a given pressure and temperature.

The lines show the conditions under whichmore than one phase can coexist at equi-librium. For one-component systems (e.g.water) the point at which all three phasescan coexist at equilibrium is called thetriple point and is the point on the graph atwhich the pressure–temperature curves in-tersect.

phase equilibrium A state in which theproportions of the various phases in achemical system are fixed. When two ormore phases are present at fixed tempera-ture and pressure a dynamic condition willbe established in which individual particleswill leave one phase and enter another; atequilibrium this will be balanced by anequal number of particles making the re-verse change, leaving the total compositionunchanged.

phase rule In a system at equilibriumthe number of phases (P), number of com-ponents (C), and number of degrees of free-dom (F) are related by the formula:

P + F = C + 2

phenolphthalein An acid–base indica-tor that is colorless in acid solutions andbecomes red if the pH rises above the tran-sition range of 8–9.6. It is used as the indi-cator in titrations for which the end pointlies clearly on the basic side (pH > 7), e.g.oxalic acid or potassium hydrogentartrateagainst caustic soda.

philosopher’s stone See alchemy.

phlogiston theory A former theory ofcombustion (disproved in the 18th centuryby Lavoisier). Flammable compounds weresupposed to contain a substance (phlogis-ton), which was presumed to be releasedwhen they burned, leaving ash.

phosgene See carbonyl chloride.

phosphates See phosphoric(V) acid.

phosphide A compound of phosphoruswith a more electropositive element.

phosphine (phosphorus(III) hydride;

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phosphine

PH3) A very poisonous colorless gas thatis slightly soluble in water. It has a charac-teristic fishy smell. It can be made by react-ing water and calcium phosphide or by theaction of yellow phosphorus on a concen-trated alkali. Phosphine usually ignitesspontaneously in air because of contamina-tion with diphosphane. It decomposes intoits elements if heated to 450°C in the ab-sence of oxygen and it burns in oxygen orair to yield phosphorus oxides. It reactswith solutions of metal salts to precipitatephosphides. Like its nitrogen analog am-monia it forms salts, called phosphoniumsalts. It also forms complex addition com-pounds with metal ions.

phosphinic acid (hypophosphorous acid;H3PO2) A white crystalline solid. It is amonobasic acid forming the anion H2PO2

–

in water. The sodium salt, and hence theacid, can be prepared by heating white (oryellow) phosphorus with sodium hydrox-ide solution. The free acid and its salts arepowerful reducing agents.

phosphonic acid (phosphorous acid; or-thophosphorous acid; H3PO3) A color-less deliquescent solid that can be preparedby the action of water on phosphorus(III)oxide or phosphorus(III) chloride. It is adibasic acid producing the anions H2PO3

–

and HPO32– in water. The acid and its salts

are weak reducing agents. On warming,phosphonic acid decomposes to phosphineand phosphoric(V) acid.

phosphonium ion The ion PH4+, de-

rived from phosphine.

phosphonium salts See phosphine.

phosphor A substance that shows lumi-nescence or phosphorescence.

phosphorescence 1. The absorption ofenergy by atoms followed by emission ofelectromagnetic radiation. Phosphores-cence is a type of luminescence, and is dis-tinguished from fluorescence by the factthat the emitted radiation continues forsome time after the source of excitation hasbeen removed. In phosphorescence the ex-

cited atoms have relatively long lifetimesbefore they make transitions to lower en-ergy states. However, there is no definedtime distinguishing phosphorescence fromfluorescence.2. In general usage the term is applied tothe emission of ‘cold light’ – light producedwithout a high temperature. The namecomes from the fact that white phosphorusglows in the dark as a result of a chemicalreaction with oxygen. The light comesfrom excited atoms produced directly inthe reaction – not from the heat produced.It is thus an example of chemilumines-cence. There are also a number ofbiochemical examples termed biolumines-cence; for example, phosphorescence issometimes seen in the sea emanating frommarine organisms, or on rotting wood em-anating from certain fungi (known as ‘foxfire’).

phosphoric(V) acid (orthophosphoricacid; H3PO4) A white solid that can bemade by reacting phosphorus(V) oxidewith water or by heating yellow phos-phorus with nitric acid. The naturallyoccurring phosphates (orthophosphates,M3PO4) are salts of phosphoric(V) acid.Aqueous solutions of phosphoric(V) acidhave a sharp taste and have been used inthe manufacture of soft drinks. At about220°C phosphoric(V) acid is converted topyrophosphoric acid (H4P2O7). Pure py-rophosphoric acid in the form of colorlesscrystals is made by warming solid phos-phoric(V) acid with phosphorus(III) chlor-ide oxide. Metaphosphoric acid is obtainedas a polymer, (HPO3)n, by heating phos-phoric(V) acid to 320°C. Both meta- andpyrophosphoric acids change to phos-phoric(V) acid in aqueous solution; thechange takes place faster at higher temper-atures. Phosphoric(V) acid is tribasic,forming three series of salts with the anionsH2PO4

–, HPO42–, and PO4

3–. These seriesof salts are acidic, neutral, and alkaline re-spectively. With the exception of lithiumphophate the alkali metal phosphates aresoluble in water, but other metal phos-phates are insoluble. Phosphates are usefulin water softening and as fertilizers.

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172

phosphorous acid See phosphonic acid.

phosphorus A reactive solid nonmetal-lic element, the second element in group 15(formerly VA) of the periodic table. It hasthe electronic configuration [Ne]3s23p3

and is therefore formally similar to nitro-gen. It is, however, very much more reac-tive than nitrogen and is never found innature in the uncombined state. Phospho-rus is widespread throughout the world;economic sources are rock phosphate (con-sisting largely of calcium phosphate,Ca3(PO4)2) and the apatites, variously occurring as both fluoroapatite (3Ca3-(PO4)2.CaF2) and as chloroapatite (3Ca3-(PO4)2CaCl2). Guano formed from theskeletal phosphate of fish in sea-bird drop-pings is also an important source ofphosphorus. The largest amounts of phos-phorus compounds produced are used infertilizers and detergents, with smalleramounts being employed in safety matches,pesticides, special alloys and glasses, foodand drinks, and fireworks. Phosphorus isan essential constituent of living tissue,bones, and nucleic acids, and plays a veryimportant part in metabolic processes andmuscle action. Phosphorus is also requiredfor the development of vigorous root sys-tems in green plants.

The element is obtained industrially bythe reduction of rock phosphate using sand(SiO2) and coke (C) in an electric furnace.The phosphorus is distilled off as P4 mol-ecules and collected under water as whitephosphorus (defined below):

2Ca3(PO4)2 + 6SiO2 + 10C → P4 +6CaSiO3 + 10CO

There are three common allotropes ofphosphorus and several other modifica-tions of these, some of which have indefi-nite structures. The common forms andtheir methods of preparation are:1. white (or yellow) phosphorus recovered

by distillation of phosphorus as shownabove; soluble in some organic solvents

2. red phosphorus obtained by the actionof heat on white phosphorus; insolublein organic solvents

3. black phosphorus obtained by the ac-tion of heat on white phosphorus underhigh pressures using an Hg catalyst.

Phosphorus has a sufficiently high ion-ization potential for the chemistry of itscompounds to be almost entirely covalent.Phosphorus compounds are formally P(III)and P(V) compounds. The element forms ahydride, PH3 (phosphine), which is analo-gous to ammonia, but it is not formed bydirect combination. Phosphorus(V) hy-drides are not known.

When phosphorus is burned in air theproduct is the highly hygroscopic whitesolid P4O10 (commonly called phosphoruspentoxide, P2O5); this is the P(V) oxide. Ina limited supply of air the P(III) oxide,P4O6 (called phosphorus(III) oxide) is ob-tained, contaminated with unreacted phos-phorus. The two oxides both react withwater to form acids:P4O6 + 6H2O →

4H3PO3 phosphonic acidP4O10 + 2H2O →

4HPO3 metaphosphoric acidP4O10 + 6H2O →

4H3PO4 phosphoric acid.There are also many polyphosphoric

acids and salts with different ring sizes anddifferent chain lengths. Pyrophosphoricacid is obtained by melting phosphoricacid; further dehydration above 200°Cleads to metaphosphoric acids:

2H3PO4 → H4P2O7 + H2OH4P2O7 → 2HPO3 + H2O

Polyphosphates with molecular massesfrom 1000 to 10 000 are industrially im-portant in detergent formulations, whilepolyphosphates with lower mass numbersare used to complex metal ions (sequestra-tion).

Phosphorus forms the P(III) halides,PX3, with all the halogens, and P(V)halides, PX5, with X = F, Cl, Br. The P(III)halides are formed by direct combination,but because of hazards in the reaction ofphosphorus and fluorine, PF3 is preparedfrom PCl3.

The P(III) halide molecules are all pyra-midal (as is PH3) and are readily hy-drolyzed. The P(V) halides are also readilyhydrolyzed by water in two stages givingfirst the oxyhalide, then phosphoric acid:

PX5 + H2O → POX3 + 2HXPOX3 + 3H2O → H3PO4 + 3HX

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phosphorus

In the gas phase the trihalides are pyra-midal and the pentahalides are trigonalbipyramids. However, in tetrachloro-methane solution phosphorus pentachlo-ride is a dimer with two chlorine bridges,exhibiting the ready expansion of the coor-dination number to six. In the solid state,(PCl5) is [PCl4]+[PCl6]– (the first tetrahe-dral, the second octahedral) obtained bytransfer of a chloride ion.

Phosphorus interacts directly with sev-eral metals and metalloids to give:1. Ionic phosphides such as Na3P, Ca3P2,

Sr3P2.2. Molecular phosphides such as P4S3 and

P4S5.3. Metallic phosphides such as Fe2P.

Symbol: P; m.p. 44.1°C (white) 410°C(red under pressure); b.p. 280.5°C; r.d.1.82 (white) 2.2 (red) 2.69 (black) (all at20°C); p.n. 15; most common isotope 31P;r.a.m. 30.973762.

phosphorus(III) bromide (phosphorustribromide; PBr3) A colorless liquidmade by reacting phosphorus with bro-mine. It is readily hydrolyzed by water tophosphonic acid and hydrogen bromide.Phosphorus(III) bromide is important inorganic chemistry, being used to replace ahydroxyl group with a bromine atom.

phosphorus(V) bromide (phosphoruspentabromide; PBr5) A yellow crystallinesolid that sublimes easily. It can be madeby the reaction of bromine and phospho-rus(III) bromide. Phosphorus(V) bromideis readily hydrolyzed by water to phos-phoric(V) acid and hydrogen bromide. Itsmain use is in organic chemistry to replacea hydroxyl group with a bromine atom.

phosphorus(III) chloride (phosphorustrichloride; PCl3) A colorless liquidformed from the reaction of phosphoruswith chlorine. It is rapidly hydrolyzed bywater to phosphonic acid and hydrogenchloride. Phosphorus(III) chloride is usedin organic chemistry to replace a hydroxylgroup with a chlorine atom.

phosphorus(V) chloride (phosphoruspentachloride; PCl5) A white easily sub-

limed solid formed by the action of chlo-rine on phosphorus(III) chloride. It is hy-drolyzed by water to phosphoric(V) acidand hydrogen chloride. Its main use is as achlorinating agent in organic chemistry toreplace a hydroxyl group with a chlorineatom.

phosphorus(III) chloride oxide (phos-phorus trichloride oxide; phosphorus oxy-chloride; phosphoryl chloride, POCl3) Acolorless liquid that can be obtained by re-acting phosphorus(III) chloride with oxy-gen or by distilling phosphorus(III)chloride with potassium chlorate. The re-actions of phosphorus(III) chloride oxideare similar to those of phosphorus(III)chloride. Water hydrolysis yields phos-phoric(V) acid. Phosphorus(III) chlorideoxide forms complexes with many metalions.

phosphorus(III) hydride See phos-phine.

phosphorus(III) oxide (phosphorus tri-oxide; P2O3) A white waxy solid with acharacteristic smell of garlic; it usually ex-ists as tetrahedral P4O6 molecules in whichthe phosphorus atoms are linked to eachother through oxygen bridges. It is solublein organic solvents, and is made by burningphosphorus in a limited supply of air.Phosphorus(III) oxide oxidizes in air tophosphorus(V) oxide at room temperatureand inflames above 70°C. It dissolvesslowly in cold water or dilute alkalis to givephosphonic acid or one of its salts. Phos-phorus(III) oxide reacts violently with hotwater to form phosphine and phos-phoric(V) acid.

phosphorus(V) oxide (phosphorus pen-toxide; P2O5) A highly deliquescentwhite powder that is soluble in organic sol-vents. It usually exists as P4O10 molecules.Phosphorus(V) oxide can be prepared byburning phosphorus in an abundant supplyof oxygen. It readily combines with waterto form phosphoric(V) acid and is there-fore used as a drying agent for gases. It is auseful dehydrating agent because it is ableto remove the elements of water from cer-

phosphorus(III) bromide

174

tain oxyacids and other compounds con-taining oxygen and hydrogen; for example,it reacts with nitric acid (HNO3) to give ni-trogen pentoxide (N2O5) on heating.

phosphorus oxychloride See phos-phorus(III) chloride oxide.

phosphorus pentabromide See phos-phorus(V) bromide.

phosphorus pentachloride See phos-phorus(V) chloride.

phosphorus pentoxide See phospho-rus(V) oxide.

phosphorus tribromide See phospho-rus(III) bromide.

phosphorus trichloride See phospho-rus(III) chloride.

phosphorus trichloride oxide Seephosphorus(III) chloride oxide.

phosphorus trioxide See phosphorus(III)oxide.

phosphoryl chloride See phosphorus(III)chloride oxide.

phot A unit of illumination in the c.g.s.system defined as an illumination of onelumen per square centimeter. In SI units itis equal to 104 lux.

photochemical reaction A reactionbrought about by light; examples includethe bleaching of colored material, the re-duction of silver halides, and the photosyn-thesis of carbohydrates. Chemical changesoccur only when the reacting atoms ormolecules absorb photons of the appropri-ate energy. The amount of substance thatreacts is proportional to the quantity of en-ergy absorbed. For example, in the reac-tion between hydrogen and chlorine, it isnot the concentrations of hydrogen orchlorine that dictate the rate of reactionbut the intensity of the radiation.

photochemistry The branch of chem-

istry dealing with reactions induced bylight or ultraviolet radiation.

photoelectric effect The emission ofelectrons from a solid (or liquid) surfacewhen it is irradiated with electromagneticradiation. For most materials the photo-electric effect occurs with ultraviolet radia-tion or radiation of shorter wavelength;some show the effect with visible radiation.

In the photoelectric effect, the numberof electrons emitted depends on the inten-sity of the radiation and not on its fre-quency. The kinetic energy of the electronsthat are ejected depends on the frequencyof the radiation. This was explained, byEinstein, by the idea that electromagneticradiation consists of streams of photons.The photon energy is hv, where h is thePlanck constant and v the frequency of theradiation. To remove an electron from thesolid a certain minimum energy must besupplied, known as the work function, φ.Thus, there is a certain minimum thresholdfrequency v0 for radiation to eject elec-trons: hv0 = θ. If the frequency is higherthan this threshold the electrons areejected. The maximum kinetic energy (W)of the electrons is given by Einstein’s equa-tion:

W = hv – φThe photoelectric effect also occurs

with gases. See photoionization.

photoelectrochemical cell (PEC) Atype of voltaic cell in which one of the elec-trodes is a light-sensitive semiconductor(such as gallium arsenide). A current is gen-erated by light falling on the semiconduc-tor, causing electrons to pass between theelectrode and the electrolyte and producechemical reactions in the cell.

photoelectron An electron ejected froma solid, liquid, or gas by the PHOTOELECTRIC

EFFECT or by PHOTOIONIZATION.

photoelectron spectroscopy See pho-toionization.

photoemission The emission of photo-electrons by the photoelectric effect or byphotoionization.

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photoemission

photoionization The ionization ofatoms or molecules by electromagnetic ra-diation. Photons absorbed by an atom mayhave sufficient photon energy to free anelectron from its attraction by the nucleus.The process is

M + hv → M+ + e–

where h is the Planck constant and v is thefrequency of the radiation.

As in the photoelectric effect, the radia-tion must have a certain minimum thresh-old frequency. The energy of thephotoelectrons ejected is given by W = hv –I, where W is the kinetic energy and I is theionization potential of the atom or mol-ecule. Analysis of the energies of the emit-ted electrons gives information on theionization potentials of the substance – atechnique known as photoelectron spec-troscopy.

photolysis A chemical reaction that isproduced by electromagnetic radiation(light or ultraviolet radiation). Many pho-tolytic reactions involve the formation offree radicals.

photon energy See photoelectric effect.

physical change A change to a sub-stance that does not alter its chemicalproperties. Physical changes (e.g. melting,boiling, and dissolving) are comparativelyeasy to reverse.

physical chemistry The branch ofchemistry concerned with the physicalproperties of compounds and how thesedepend on the chemical bonds involved.

physisorption See adsorption.

piano-stool compound See sandwichcompound.

pi bond See orbital.

pico- Symbol: p A prefix used with SIunits, denoting 10–12. For example, 1 pico-farad (pF) = 10–12 farad (F).

pi complex A complex in which the

metal ion is bound to a pi-electron system,as in FERROCENE.

pig iron The crude iron produced froma blast furnace, containing carbon, silicon,and other impurities. The molten iron isrun out of the furnace into channels called‘sows’, which branch out into a number ofoffshoots called ‘pigs’, in which the metal isallowed to cool.

pigment An insoluble, particulate color-ing material as used in PAINT.

pi orbital See orbital.

pipette A device used to transfer aknown volume of solution from one con-tainer to another; in general, several sam-ples of equal volume are transferred forindividual analysis from one stock solu-tion. Pipettes are of two types, bulbpipettes, which transfer a known and fixedvolume, and graduated pipettes, which cantransfer variable volumes.

pitchblende (uraninite) A highly ra-dioactive black mineral consisting mostlyof uranium(VI) oxide, UO3 along with ura-nium radioactive decay series elementssuch as thorium and radium. It is the chiefore of uranium and radium.

pK The logarithm to the base 10 of thereciprocal of an acid’s dissociation con-stant (Ka):

log10(1/Ka)

planar chromatography See chro-matography; thin-layer chromatography.

Planck constant Symbol: h A funda-mental constant; the ratio of the energy(W) carried by a photon to its frequency(v). A basic relationship in the quantumtheory of radiation is W = hv. The value ofh is 6.626 196 × 10–34 J s. The Planck con-stant appears in many relationships inwhich some observable measurement isquantized (i.e. can take only specific dis-crete values rather than any of a range ofvalues). It is named for German physicist

photoionization

176

Max Karl Planck (1858–1947), who esti-mated its value in 1900.

plane polarization A type of polariza-tion of electromagnetic radiation in whichthe vibrations take place entirely in oneplane.

plane-polarized See polarization.

plasma A very hot mixture of ions andelectrons, as in an electrical discharge.Sometimes, a plasma is described as afourth state of matter.

plaster of Paris See calcium sulfate.

platinum A silvery-white malleable duc-tile transition metal the third element ofgroup 10 (formerly part of subgroupVIIIB) of the periodic table. It occurs nativeor in association with other platinum met-als in Australia, Canada, Russia, and SouthAfrica. It is also found in certain copperand nickel ores, from which it is recoveredduring refining. It is resistant to oxidationand is not attacked by acids (except aquaregia) or alkalis. Platinum is used as a cata-lyst for ammonia oxidation (to make nitricacid) and in catalytic converters. It is alsoused in thermocouples and surgical instru-ments, and to make jewelry.

Symbol: Pt; m.p. 1772°C; b.p. 3830 ±100°C; r.d. 21.45 (20°C); p.n. 78; mostcommon isotope 195Pt; r.a.m. 195.08.

platinum black A finely divided blackform of platinum produced, as a coating,by evaporating platinum onto a surface inan inert atmosphere. Platinum-black coat-ings are used as pure absorbent electrodecoatings in experiments on electric cells,and as catalysts. They are also used, likecarbon-black coatings, to improve the abil-ity of a surface to absorb radiation.

platinum(II) chloride (PtCl2) A gray-brown powder prepared by the partial de-composition of platinum(IV) chloride. Itmay also be prepared by passing chlorineover heated platinum. Platinum(II) chlor-ide is insoluble in water but dissolves in

concentrated hydrochloric acid to form acomplex acid.

platinum(IV) chloride (PtCl4) A red-dish-brown hygroscopic solid prepared bythe action of heat on chloroplatinic acid.Crystals of the pentahydrate, PtCl4.5H2O,are formed; anhydrous platinum(IV) chlor-ide is prepared by treating the hydratedcrystals with concentrated sulfuric acid.The chloride dissolves in water to producea strongly acidic solution. Platinum(IV)chloride forms a series of hydrates having1, 2, 4, and 5 molecules of water; thetetrahydrate is the most stable. Thestrongly acidic solution produced on dis-solving it in water probably containsH2[PtCl4(OH)2].

platinum–iridium An alloy of platinumcontaining up to 30% iridium. Hardnessand resistance to chemical attack increaseas the iridium content is increased. It isused in jewelry, electrical contacts, and hy-podermic needles. The international proto-type of the kilogram is a platinum–iridiumalloy.

platinum metals The second and thirdtransition series metals ruthenium (Ru),osmium (Os), rhodium (Rh), iridium (Ir),palladium (Pd), and platinum (Pt), some-times classed together due to their relatedproperties and similar compounds. All the metals, for example, are hard andcorrosion-resistant.

plumbane (lead(IV) hydride; PbH4) Acolorless unstable gas that can be obtainedby the action of acids on a mixture of mag-nesium and lead pellets.

plumbic Designating a lead(IV) com-pound.

plumbous Designating a lead(II) com-pound.

plutonium A radioactive silvery ele-ment of the actinoid series of metals. It is atransuranic element found on Earth only inminute quantities in uranium ores, but isreadily obtained, as 239Pu, by neutron

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bombardment of natural uranium. Thereadily fissionable 239Pu is a major nuclearfuel and nuclear explosive. Plutoniumemits high levels of alpha particles and isthus highly toxic; the particles accumulatein bone where they can cause leukemia andradiation poisoning.

Symbol: Pu; m.p. 641°C; b.p. 3232°C;r.d. 19.84 (25°C); p.n. 94; most stable iso-tope 244Pu (half-life 8.2 × 107 years).

point defect See defect.

point group See molecular symmetry.

poison 1. A substance that destroyscatalyst activity.2. Any substance that endangers biologicalactivity, whether by physical or chemicalmeans.

polar Describing a compound with mol-ecules that have a permanent dipole mo-ment. Hydrogen chloride and water areexamples of polar compounds.

polar bond A covalent bond in whichthe bonding electrons are not sharedequally between the two atoms. A bond be-tween two atoms of different electronega-tivity is said to be polarized in the directionof the more electronegative atom, i.e. theelectrons are drawn preferentially towardthe atom. This leads to a small separationof charge and the development of a bondDIPOLE MOMENT as in, for example, hydro-gen fluoride, represented as H→F or asHδ+–Fδ– (F is more electronegative).

The charge separation is much smallerthan in ionic compounds; molecules inwhich bonds are strongly polar are said todisplay partial ionic character. The effectof the electronegative element can be trans-mitted beyond adjacent atoms. See also in-termolecular forces.

polarimeter (polariscope) An instru-ment for measuring optical activity. Seeoptical activity.

polariscope See polarimeter.

polarizability The ease with which an

electron cloud is deformed, i.e. polarized.In ions, an increase in size or negativecharge leads to an increase in polarizabil-ity. The concept is of particular use in thetreatment of covalent contributions to pre-dominantly ionic bonds embodied in Fa-jans’ Rules. Thus ions, such as I–, Se2–,Te2–, are especially prone to covalent char-acter, particularly in combination withions of small size and high positive charge(i.e. high ionic potential).

polarization 1. The restriction of the vi-brations in a transverse wave so that the vi-bration occurs in a single plane.Electromagnetic radiation, for instance, isa transverse wave motion. It can bethought of as an oscillating electric fieldand an oscillating magnetic field, both atright angles to the direction of propagationand at right angles to each other. Usually,the electric vector is considered since it isthe electric field that interacts with chargedparticles of matter and causes the effects. In‘normal’ unpolarized radiation, the electricfield oscillates in all possible directions per-pendicular to the wave direction. On re-flection or on transmission through certainsubstances (e.g. Polaroid) the field is con-fined to a single plane. The radiation isthen said to be plane-polarized. If the tip ofthe electric vector describes a circular helixas the wave propagates, the light is said tobe circularly polarized.2. See polarizability.3. The reduction of current in a voltaic cell,caused by the build-up of products of thechemical reaction. Commonly, the cause isthe build-up of a layer of bubbles (e.g. ofhydrogen). This reduces the effective areaof the electrode, causing an increase in thecell’s internal resistance. It can also pro-duce a back e.m.f. Often a substance suchas manganese(IV) oxide (a depolarizer) isadded to prevent hydrogen build-up.

polar molecule A molecule in which theindividual polar bonds are not perfectlysymmetrically arranged and are thereforenot ‘in balance’. Thus the charge separa-tion in the bonds gives rise to an overallcharge separation in the molecule as, for

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example, in water. Such molecules possessa dipole moment.

polarogram See polarography.

polarography An analytical method inwhich current is measured as a function ofpotential. A special type of cell is used inwhich there is a small, easily polarizablecathode (the dropping-mercury electrode,consisting of a thin tube through whichmercury is slowly dripped into the solu-tion) and a large nonpolarizable anode(reference cell). The analytical reactiontakes place at the cathode and is essentiallya reduction of the cations, which are dis-charged according to the order of theirelectrode potential values. The data is ex-pressed in the form of a polarogram, whichis a plot of current against applied voltage.As the applied potential is increased a pointis reached at which the ion is discharged.There is a step-wise increase in current,which levels off because of polarization ef-fects. The potential at half the step height(called the half-wave potential) is used toidentify the ion. Most elements can be de-termined by polarography. The optimumconcentrations are in the range10–2–10–4M; modified techniques allowdeterminations in the parts per millionrange.

polar solvent See solvent.

pollution Any damaging or unpleasantchange in the environment that resultsfrom the physical, chemical, or biologicalside-effects of human industrial or socialactivities. Pollution can affect the atmos-phere, rivers, lakes, seas, and soil.

Air pollution is caused by the domesticand industrial burning of carbonaceousfuels, by industrial processes, and by gasesin car exhausts. Among recent problemsare industrial emissions of sulfur(IV)oxide, a cause of acid rain, and emissionsof chlorofluorocarbons (CFCs), previouslywidely used in refrigeration, aerosols, etc.,and linked to the depletion of ozone in thestratosphere. Carbon dioxide, produced byburning fossil fuels, is slowly building up inthe atmosphere, which could result in an

overall increase in the temperature of theEarth due to its greenhouse effect. Car ex-hausts also emit carbon monoxide and, ifleaded gasoline (petrol) is used, lead. At-mospheric CO has not yet reached danger-ous levels, but vegetation near main roadshas in the past been found to contain a highproportion of lead, with levels sufficientlyhigh in urban areas to cause concern aboutthe effects on children. Leaded gasoline hasthus been banned in many countries. Pho-tochemical smog, caused by the action ofsunlight on volatile hydrocarbons and ni-trogen oxides from car exhausts, is a prob-lem in several countries. Catalyticconverters have helped reduce harmfulemissions from vehicle exhausts.

Water pollutants include those that arebiodegradable, such as sewage effluent,which cause no permanent harm if ade-quately treated and dispersed, as well asthose that are nonbiodegradable, such ascertain chlorinated hydrocarbon pesticides(e.g. DDT) and heavy metals, such as lead,copper, and zinc in some industrial efflu-ents, which cause heavy-metal pollution.When these pollutants accumulate in theenvironment they can become very concen-trated in food chains. The pesticides DDT,aldrin, and dieldrin are now banned inmany countries. Water supplies can be-come polluted by leaching of nitrates, pes-ticides, or animal wastes from agriculturalland. The discharge of waste heat can causethermal pollution of the environment,which can be reduced by the use of coolingtowers. In lakes, rivers, and the sea,spillage from tankers and the discharge ofinadequately treated sewage effluent arethe main problems.

Other forms of pollution are noise fromaircraft, traffic, and industry and radioac-tivity from improper disposal of radioac-tive waste.

polonium A strongly radioactive raremetallic or metalloid element belonging togroup 16 (formerly VIA) of the periodictable. It occurs in very minute quantities inuranium ores. Over 30 radioisotopes areknown, nearly all alpha-particle emitters.Polonium is a volatile metal and evapo-rates with time. It is also extremely haz-

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ardous; a quantity of polonium quicklyreaches a temperature of a few hundred de-grees Celsius because of alpha emission.For this reason it has been used as a light-weight heat source in space satellites.

Symbol: Po; m.p. 254°C; b.p. 962°C;r.d. 9.32 (20°C); p.n. 84; stablest isotope209Po (half-life 102 years).

polyatomic Describing a molecule (orion or radical) that consists of severalatoms (three or more). Examples are cal-cium carbonate (CaCO3) and methane(CH4).

polybasic Describing an acid that hastwo or more replaceable hydrogen atoms.For example, phosphorus(V) acid, H3PO4,is tribasic.

polychromic acids See chromium(IV)oxide.

polycrystalline Describing a substancecomposed of very many minute interlock-ing crystals that have solidified together.

polycyclic Describing a compound thathas two or more rings in its molecules.

polydioxoboric(III) acid See boric acid.

polymer A compound in which thereare very large molecules made up of re-peating molecular units called MONOMERS.Polymers do not usually have a definite rel-ative molecular mass, because there arevariations in the lengths of different chains.They may be natural substances (e.g. cellu-lose, starch, silicates, or proteins) or syn-thetic materials (e.g. nylon or silicone). Thetwo major classes of synthetic polymers arethermosetting and thermoplastic. The for-mer are infusible, and heat may only makethem harder, whereas the latter soften onheating.

polymorphism The ability of certainchemical substances to exist in more thanone physical form. For elements this iscalled ALLOTROPY. The existence of twoforms is called dimorphism. Crystallinestructures of compounds can vary with

temperature as a result of different packingarrangements of the particles. There is atransition temperature between forms andusually a marked change in density.

polyvalent Describing an atom orgroup that has a valence of more than one.

porcelain A ceramic traditionally madefrom feldspar, kaolin (China clay), marble,and quartz.

Porter, George, Baron (1920–2001)British physical chemist. In collaborationwith Ronald NORRISH, Porter developed thetechnique of flash photolysis for investigat-ing fast chemical reactions, starting in thelate 1940s. He shared the 1967 Nobel Prizefor chemistry with Norrish and ManfredEIGEN.

positive catalyst See catalyst.

potash See potassium carbonate; potas-sium hydroxide.

potash alum See alum; aluminumpotassium sulfate.

potassamide See potassium monoxide.

potassium A light soft silvery reactivealkali metal; the third element of group 1(formerly IA) of the periodic table. It hasthe argon electronic configuration of argonplus an outer 4s1 electron. The elementgives a distinct lilac color to flames which,however, is easily masked by the intenseyellow coloration resulting from any traceimpurity of sodium. Consequently theflames must be viewed through blue cobaltglass. The ionization potential of potas-sium is low and its chemistry is largely thechemistry of the K+ ion. Potassium ac-counts for 2.4% of the lithosphere and oc-curs in large salt deposits such as carnallite(KCl.MgCl2.6H2O) and schönite (K2SO4),and sylvite (KC1). Large amounts alsooccur in mineral forms that are not ofmuch use for recovery of potassium, e.g.,orthoclase, K2Al2Si6O10. The commercialusage of potassium is much less than thatof sodium; industrially the element is ob-

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tained by electrolysis of the fused hydrox-ide. Potassium hydroxide is in turn ob-tained by electrolysis of carnallite solutions(the magnesium is initially precipitated asMg(OH)2), and both hydrogen and chlo-rine are recovered as by-products.

The chemistry of potassium and itscompounds is very similar to that of theother group 1 elements. Particular ways inwhich the chemistry of potassium differsfrom that of sodium can be summarized asfollows:1. Burning in air produces the superoxide

KO2.2. Because of the larger size of the K+ ion

and hence lower lattice energies, potas-sium salts are often more soluble thancorresponding sodium salts.

3. Potassium salts are usually less heavilyhydrated than corresponding sodiumsalts.The most abundant isotope of potas-

sium is 39K (93.1%), with 41K also a stableisotope (6.8%). The naturally occurringradioactive isotope 40K (0.11%) has a half-life of 1.28 × 109 years.

Symbol: K; m.p. 63.65°C; b.p. 774°C;r.d. 0.862 (20°C); p.n. 19; r.a.m. 39.0983.

potassium–argon dating A techniquefor dating rocks. It depends on the ra-dioactive decay of the radioisotope 40K to40Ar (half-life 1.27 × 1010 years) and isbased on the assumption that argon hasbeen trapped in the rock since the time thatthe rock cooled (i.e. since it formed). If theratio 40Ar/40K is measured it is possible toestimate the age of the rock. See also ra-dioactive dating.

potassium bicarbonate See potassiumhydrogencarbonate.

potassium bromide (KBr) A white orcolorless cubic crystalline solid that is ex-tremely soluble in water. It is formed by theaction of bromine on hot potassium hy-droxide solution or by the neutralization ofthe carbonate with hydrobromic acid. It isused extensively in the manufacture ofphotographic plates, films, and papers andwas once used as a sedative in medicine.

potassium carbonate (pearl ash; potash;K2CO3) A white deliquescent solid pre-pared by thermal decomposition of po-tassium hydrogencarbonate (KHCO3).Potassium carbonate is very soluble inwater, its solutions being strongly alkalinedue to salt hydrolysis. It is used in the lab-oratory as a drying agent and industriallyin the manufacture of soft soap, hard glass,and in dyeing and wool finishing proce-dures. Potassium carbonate crystallizes outbetween 10–25°C as K2CO3.3H2O; it de-hydrates at 100°C to K2CO3.H2O and at130°C to K2CO3.

potassium chlorate (KClO3) A whitesoluble crystalline solid prepared by theelectrolysis of a concentrated solution ofpotassium chloride. Industrially, it is pre-pared by the fractional crystallization of asolution containing sodium chlorate (V)and potassium chloride. When heated justabove its melting point of 356°C, potas-sium perchlorate is formed or it can bemade by subjecting a hot solution of potas-sium chloride to electrolysis. When heatedfurther, potassium chlorate decomposes toyield oxygen and potassium chloride. It is apowerful oxidizing agent and is used in ex-plosives, matches, weedkillers, and fire-works, and as a disinfectant. It oxidizesiodide ions to iodine when in an acidicmedium.

potassium chloride (KCl) A white orcolorless ionic solid prepared by neutraliz-ing hydrochloric acid with potassium hy-droxide solution. Potassium chlorideoccurs naturally as the minerals sylvite andcarnallite, which both occur in deposits leftby evaporated shallow seas. It is more sol-uble than sodium chloride in hot water butless soluble in cold water. On evaporationof an aqueous solution, colorless cubiccrystals similar to those of sodium chlorideare produced. Potassium chloride is used asa fertilizer, in the manufacture of potas-sium hydroxide, and in photography.

potassium chromate (K2CrO4) Abright yellow toxic solid prepared byadding potassium hydroxide solution to asolution of potassium dichromate. It is ex-

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tremely soluble in water. Addition of anacid to an aqueous solution of potassiumchromate converts the chromate ions intodichromate ions. The salt is used as an in-dicator in silver nitrate titrations and inpigments and enamels. The crystals are iso-morphous with potassium sulfate.

potassium cyanide (KCN) A whiteionic solid that is very soluble in water andextremely poisonous. It smells of bitter al-monds. It is made industrially by combin-ing potassium hydroxide with hydrogencyanide. Potassium cyanide is used to re-cover gold and silver, in electroplating, andin fumigants. It is also used as a reducingagent and in chemical analysis. Aqueoussolutions of potassium cyanide are stronglyhydrolyzed, the solutions being alkaline.On standing, these solutions slowly evolvehydrocyanic acid.

potassium dichromate (K2Cr2O7) Anorange-red crystalline toxic solid preparedby adding potassium chloride solution to aconcentrated solution of sodium dichro-mate and crystallizing out or by acidifyinga solution of potassium chromate andevaporating the same. It is less soluble thansodium dichromate in cold water but moresoluble in hot water. Potassium dichro-mate is used as an oxidizing agent in volu-metric analysis and organic chemistry, andalso in the manufacture of dyes, glass,glues, and ceramics. It also finds use in fire-works, photography, and lithography. Thecrystals are triclinic, anhydrous, nondeli-quescent, and also a fire risk due to theirstrong oxidative capacity. Addition of analkali to a solution of the dichromate yieldsthe chromate.

potassium hydride (KH) A white orlight gray ionic solid prepared by passinghydrogen over heated potassium, the metalbeing suspended in an inert medium. It isan excellent reducing agent but also a firehazard because it reacts with water to re-lease hydrogen.

potassium hydrogencarbonate (potas-sium bicarbonate; KHCO3) A whitecrystalline solid prepared by passing car-

bon dioxide through a saturated solutionof potassium carbonate. Potassium hydro-gencarbonate is more soluble in water thansodium hydrogencarbonate. When heated,it undergoes thermal decomposition to givethe carbonate, water, and carbon dioxide.The salt forms monoclinic crystals. Its so-lutions are strongly alkaline as a result ofsalt hydrolysis.

potassium hydrogentartrate (cream oftartar; HOOC(CHOH)2COO–K+) A whitecrystalline solid, an acid salt of tartaric acidthat occurs in grape juice. It is used as theacid ingredient of BAKING POWDER.

potassium hydroxide (caustic potash;lye; KOH) A highly corrosive white solidmanufactured by the electrolysis of potas-sium chloride solution in a mercury cell. Itcan also be prepared by heating eitherpotassium carbonate or potassium sulfatewith slaked lime. In the laboratory, it canbe made by reacting potassium, potassiummonoxide, or potassium superoxide withwater. Potassium hydroxide resemblessodium hydroxide but is more soluble inwater and alcohol. It is preferred oversodium hydroxide for the absorption ofcarbon dioxide and sulfur(IV) oxide, dueto its greater solubility. Potassium hydrox-ide is used as an electrolyte in Ni–Cd, alka-line (Zn–Mn), mercury (Zn–Hg), andsilver (Zn–Ag) storage batteries and in theproduction of soft soaps. It forms crys-talline hydrates with 1, 1.5, and 2 mol-ecules of water.

potassium iodate (KIO3) A white solidformed either by adding iodine to a hotconcentrated solution of potassium hy-droxide or by the electrolysis of potassiumiodide solution. No hydrates are known. Itis a source of iodide and iodic acid. Whentreated with a dilute acid and a reducingagent, the iodate ions are reduced to io-dine. It is used in chemical analysis, and asa food additive.

potassium iodide (KI) A white ionicsolid readily soluble in water. It is preparedby dissolving iodine in hot concentratedpotassium hydroxide solution. Both the io-

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dide and iodate are formed but the latter isremoved by fractional crystallization.Potassium iodide has a sodium chloridecubic lattice. In solution with iodine itforms potassium triiodide PI3. Potassiumiodide is used in medicine, particularly inthe treatment of goiter resulting from io-dine deficiency. It is also added to table saltto help prevent goiter, and is used in pho-tography. Dilute acidified solutions ofpotassium iodide can act as reducingagents, manganate(VII) ions being reducedto manganese(II) ions, copper(II) ions tocopper(I) ions, and iodate ions to iodine.

potassium manganate(VII) (potassiumpermanganate; KMnO4) A purple solidsoluble in water. It is prepared by oxidizingpotassium manganate(VI) with chlorine.Potassium permanganate is used in volu-metric analysis as an oxidizing agent, as abactericide, and as a disinfectant. In aque-ous solution its behavior as an oxidizingagent depends on the pH of the solution.

potassium monoxide (K2O) An ionicsolid that is white when cold and yellowwhen hot. It is prepared by heating potas-sium with potassium nitrate. Potassiummonoxide dissolves violently in water toform potassium hydroxide solution. Thehydrate K2O.3H2O is known. Potassiummonoxide dissolves in liquid ammoniawith the formation of potassium hydroxideand potassamide (KNH2).

potassium nitrate (saltpeter; niter ornitre; KNO3) A white solid, soluble inwater, formed by fractional crystallizationof sodium nitrate and potassium chloridesolutions. It occurs naturally as niter ornitre (saltpeter) in rocks in Hungary,Spain, India, South Africa, and Brazil.When heated it decomposes to give the ni-trite and oxygen. Unlike sodium nitrate it isnondeliquescent. Potassium nitrate is astrong oxidizing agent and is used in gun-powder, fireworks, fertilizers, and in thelaboratory preparation of nitric acid.

potassium nitrite (KNO2) A creamydeliquescent solid that is readily soluble inwater. It reacts with cold dilute mineral

acids to produce solutions of nitrous acid.Potassium nitrite is used in organic chem-istry.

potassium permanganate See potas-sium manganate(VII).

potassium sulfate (K2SO4) A whitesolid prepared by the neutralization of ei-ther potassium hydroxide or potassiumcarbonate with dilute sulfuric acid. It oc-curs naturally as schönite in deposits atStassfurt, Germany. Potassium sulfate issoluble in water, forming a neutral solu-tion. It is used as a fertilizer and in thechemical industry in the preparation ofalums. It is also used in cement and inglass-making. The anhydrous salt crystal-lizes in the rhombic form.

potassium sulfide (K2S) A yellowish-brown deliquescent solid prepared by satu-rating an aqueous solution of potassiumhydroxide with hydrogen sulfide and thenadding an equal volume of potassium hy-droxide. Industrially it is produced byheating potassium sulfate and carbon athigh temperature. The sulfide crystallizesout from aqueous solution as K2S.5H2O.Its aqueous solutions undergo hydrolysis,the solution being strongly alkaline. Potas-sium sulfide dust is a fire risk.

potassium superoxide (KO2) A yellowparamagnetic solid prepared by burningpotassium in excess oxygen. When treatedwith cold water or dilute mineral acids, hy-drogen peroxide is produced. If heatedstrongly, it yields oxygen and potassiummonoxide. Potassium superoxide is a pow-erful oxidizing agent.

potassium thiocyanate (KSCN) A col-orless hygroscopic solid. Its solution isused in a test for iron(III) compounds, withwhich it turns a blood-red color.

potential energy See energy.

potential energy curve A curve of thepotential energy of electrons in a diatomicmolecule in which the potential energy ofan electronic state is plotted vertically and

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the interatomic distance is plotted horizon-tally, with the minimum of the curve beingthe average internuclear distance.

potentiometric titration A titration inwhich an electrode is used in the reactionmixture. The end point can be found bymonitoring the electric potential during thetitration.

praseodymium A soft ductile malleablesilvery-yellow element of the lanthanoidseries of metals. It occurs in associationwith other lanthanoids in minerals such asbastnaesite and monazite. Praseodymiumis used in several alloys, as a catalyst, incarbon arc lamp electrodes, in misch metal,in enamel, and in yellow glass for eye pro-tection.

Symbol: Pr; m.p. 931°C; b.p. 3512°C;r.d. 6.773 (20°C); p.n. 59; most commonisotope 141Pr; r.a.m. 140.91.

precipitate A suspension of small solidparticles formed in a liquid as a result of achemical reaction.

precursor A substance from which an-other substance is formed in a chemical re-action.

pressure Symbol: p The pressure on asurface due to forces from another surfaceor from a fluid acting at 90° to unit area ofthe surface:

pressure = force/areaThe SI derived unit by which pressure ismeasured is the PASCAL (Pa).

Priestley, Joseph (1733–1804) Englishchemist and minister. Priestley was one ofthe most eminent chemists of the 18th cen-tury. In 1774 he discovered oxygen, but be-cause he believed in the phlogiston theoryof combustion he called it ‘dephlogisti-cated air’. He also discovered and isolateda number of other compounds. Priestleywas also interested in electricity and optics.

Prigogine, Ilya (1917– ) Russian-born Belgian chemist. Prigogine is bestknown for extending thermodynamics andstatistical mechanics to systems that are far

from equilibrium. He called such systems‘dissipative structures’. Prigogine has beenparticularly interested in applying his ideasto biological and social systems. He wonthe 1977 Nobel Prize for chemistry for hisinvestigations of non-equilibrium systems.

primary cell A voltaic cell in which thechemical reaction that produces the e.m.f.is not reversible. Compare accumulator.

primary standard A substance that canbe used directly for the preparation of stan-dard solutions without reference to someother concentration standard. Primarystandards should be easy to purify, dry, ca-pable of preservation in a pure state, unaf-fected by air or CO2, of a high molecularmass (to reduce the significance of mass de-termination errors), stoichiometric, andreadily soluble. Any likely impuritiesshould be easily identifiable.

principal quantum number See atom.

producer gas (air gas) A combustiblemixture of carbon monoxide (25–30%),nitrogen (50–55%), and hydrogen (10–15%), prepared by passing air with a littlesteam in it through a thick layer of white-hot coke in a furnace or ‘producer’. Pro-ducer gas is used as a fuel while still hot toprevent heat loss for such purposes as in-dustrial heating, the firing of retorts, and inglass furnaces. Compare water gas.

product See chemical reaction.

promethium A soft silvery radioactiveelement of the lanthanoid series of metals.It occurs naturally in trace amounts in themineral pitchblende as the result of ra-dioactive decay of such elements as ura-nium, thorium, and plutonium. It can alsobe produced artificially by the fission ofuranium. Promethium-147 is used in someminiature nuclear-powered batteries foruse on spacecraft.

Symbol: Pm; m.p. 1168°C; b.p. 2730°C(approx.); r.d. 7.22 (20°C); p.n. 61; moststable isotope 145Pm (half-life 18 years).

promoter (activator) A substance that

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improves the efficiency of a CATALYST. Itdoes not itself catalyze the reaction but as-sists the catalytic activity. For example,alumina or molybdenum promotes thecatalytic activity of finely divided iron inthe Haber process. The manner in which apromoter functions is not fully understood;no one theory explains all the examples.

protactinium A toxic radioactive ele-ment of the actinoid series of metals. It oc-curs in minute quantities in uranium oressuch as autunite, tobernite, and pitch-blende as a radioactive decay product ofactinium. It is also synthesized by bom-barding thorium nuclei with neutrons.

Symbol: Pa; m.p. 1840°C; b.p. 4000°C(approx.); r.d. 15.4 (calc.); p.n. 91; moststable isotope 231Pa (half-life 32 500years).

protic acid See acid.

proton An elementary particle with apositive charge (+1.602 192 × 10–19 C) andrest mass 1.672 614 × 10–27 kg. Protonsare nucleons, found in all nuclides.

proton number (atomic number) Symbol:Z The number of protons in the nucleusof an atom. In a neutral atom it is alsoequal to the number of electrons orbitingthe nucleus. The proton number deter-mines the chemical properties of an ele-ment because the element’s electronstructure, which determines chemicalbonding, depends on electrostatic attrac-tion to the positively charged nucleus. Anelement’s proton number determines itsposition in the periodic table.

Prussian blue See cyanoferrate.

prussic acid See hydrocyanic acid.

pseudo-first order Describing a reac-tion that appears to exhibit first-order ki-netics under special conditions, eventhough the ‘true’ order is greater than one.For example, in the hydrolysis of a sub-

stance in the presence of a large volume ofwater, the concentration of water remainsapproximately constant. The rate of reac-tion is thus found experimentally to be pro-portional to the concentration of thesubstance, even though it also depends onthe amount of water present. Such a reac-tion is described as ‘bimolecular of the firstorder’.

pseudohalogens A small group of sim-ple inorganic compounds with symmetricalmolecules that resemble the halogens insome reactions and compounds. For exam-ple, cyanogen (C2N2) has compounds anal-ogous to halogen compounds (e.g. HCN,KCN, CH3CN, etc.). Thiocyanogen(S2C2N2), is another example.

p-type semiconductor See semicon-ductor.

Pyrex (Trademark) A particularlystrong, heat resistant, chemically inertborosilicate glass commonly used for labo-ratory glassware.

pyrites A mineral sulfide of a metal; e.g.iron pyrites, FeS2.

pyrolusite See manganese(IV) oxide.

pyrolysis The decomposition of chemi-cal compounds by subjecting them to veryhigh temperature.

pyrometer An instrument used in thechemical industry to measure high temper-ature, e.g. in reactor vessels.

pyrophoric 1. Describing a compoundthat ignites spontaneously in air.2. Describing a metal or alloy (mischmetal) that sparks when struck. Lighterflints are made of pyrophoric alloy.

pyrophosphoric acid See phos-phoric(V) acid.

pyrosulfuric acid See oleum.

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pyrosulfuric acid

quadrivalent (tetravalent) Having a va-lence of four.

qualitative analysis Analysis carriedout with the purpose of identifying thecomponents of a sample. Classical meth-ods involved simple preliminary tests fol-lowed by a carefully devised scheme ofsystematic tests and procedures. Modernmethods include the use of such techniquesas infrared spectroscopy and emissionspectrography. Compare quantitativeanalysis.

quantitative analysis Analysis carriedout with the purpose of determining theconcentration of one or more of the knowncomponents of a sample. Classical wetmethods include volumetric and gravimet-ric analysis. A wide range of more moderninstrumental techniques are also used, in-cluding polarography and various types ofchromatography and spectroscopy. Com-pare qualitative analysis.

quantized Describing a physical quan-tity that can take only certain discrete val-ues, and not a continuous range of values.

quantum (plural quanta) A definiteamount of energy released or absorbed in aprocess. Energy often behaves as if it were‘quantized’ in this way. The quantum ofelectromagnetic radiation is the photon.

quantum electrodynamics The use ofquantum mechanics to describe how parti-cles and electromagnetic radiation interact.

quantum mechanics See quantum the-ory.

quantum number An integer or half in-

teger that specifies the value of a quantizedphysical quantity (e.g. energy, angular mo-mentum, etc.). See atom; Bohr theory; spin.

quantum states States of an atom, elec-tron, particle, etc., specified by a unique setof quantum numbers.

quantum theory A mathematical the-ory originally introduced by Max Planck(1900) to explain the radiation emittedfrom hot bodies. Quantum theory is basedon the idea that energy (or certain otherphysical quantities) can be changed only incertain discrete amounts for a given sys-tem. Other early applications were the ex-planations of the photoelectric effect andthe Bohr theory of the atom.

Quantum mechanics is a system of me-chanics that developed from quantum the-ory and is used to explain the behavior ofatoms, molecules, etc. In one form it isbased on de Broglie’s idea that particles canhave wavelike properties – this branch ofquantum mechanics is called wave me-chanics. See orbital.

quartz See silicon(IV) oxide.

quasicrystal A type of crystal structurein which there is long-range order but inwhich the symmetry of the repeating unit isnot one allowed in normal crystals. Thereare examples of solids, such as the com-pound AlMn, that have icosahedral sym-metry and a long-range repeating order inthe stucture. These are known as qua-sicrystals.

quicklime See calcium oxide.

quinquevalent See pentavalent.

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racemate See optical activity.

racemic mixture See optical activity.

racemization The conversion of an op-tical isomer into a racemic mixture, whichis not optically active.

rad An m.k.s. unit formerly used to mea-sure absorbed dose of ionizing radiation,defined as being equivalent to an absorp-tion of 10–2 joule of energy in one kilogramof material. In SI units it is equal to 10–2

gray.

radian Symbol: rad The SI derived unitof plane angle, equal to that subtended atthe center of a circle by an arc whose lengthis equal to the circle’s radius. One completerevolution (360°) is 2π radian.

radiation In general, the emission of en-ergy from a source, either as waves (light,sound, etc.) or as moving particles (betarays or alpha rays).

radical A group of atoms in a molecule.See also free radical.

radioactive Describing an element ornuclide that exhibits natural radioactivity.

radioactive dating (radiometric dating)A technique for dating archaeological spec-imens, rocks, etc., by measuring the extentto which some radionuclide has decayed togive a product. See carbon; potassium–argon dating; rubidium–strontium dating;uranium–lead dating.

radioactive decay series The series ofdefinite isotopes into which a given ra-

dioactive element is transformed as it de-cays.

radioactive isotope See radioisotope.

radioactivity The disintegration ordecay of certain unstable nuclides withemission of radiation. The emission ofALPHA PARTICLES, BETA PARTICLES, andGAMMA RAYS are the three most importantforms of radiation that occur during decay.

radiocarbon dating See carbon.

radiochemistry The chemistry of ra-dioactive isotopes of elements. Radio-chemistry involves such topics as thepreparation of radioactive compounds, theseparation of isotopes by chemical reac-tions, the use of radioactive labels in stud-ies of mechanisms, and experiments on thechemical reactions and compounds of ra-dioactive elements.

radioisotope (radioactive isotope) A ra-dioactive isotope of an element. Tritium,for instance, is a radioisotope of hydrogen.Radioisotopes are extensively used in re-search as souces of radiation and as tracersin studies of chemical reactions. Thus, if anatom in a compound is replaced by a ra-dioactive nuclide of the element (a label) itis possible to follow the course of thechemical reaction. Radioisotopes are alsoused in medicine for diagnosis and treat-ment.

radiolysis Chemical decomposition pro-duced by high-energy radiation, i.e. x-rays,gamma rays, or particles.

radiometric dating See radioactive dat-ing.

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radio waves A form of electromagneticradiation with wavelengths ranging fromapproximately 10–1 to 103 meters. See alsoelectromagnetic radiation.

radium A soft white radioactive lumi-nescent metallic element of the alkaline-earth group, the sixth element of group 2(formerly IIA) of the periodic table. Ra-dium is found in uranium ores, especiallypitchblende and carnotite, from which it isultimately obtained via electrolysis using amercury electrode. It was formerly used inluminous paints and radiotherapy, andcontinues to be used as a radiation sourcein research laboratories. It has severalshort-lived radioisotopes and one long-lived isotope, radium-226 (half-life 1602years).

Symbol: Ra; m.p. 700°C; b.p. 1140°C;r.d. 5 (approx. 20°C); p.n. 88; r.a.m.226.0254.

radon A colorless odorless monatomicradioactive gas, the sixth member of therare gases; i.e. group 18 (formerly VIIIA) ofthe periodic table. It occurs naturally as aproduct of the uranium radioactive decayseries. It has 19 short-lived radioisotopes;the most stable, radon-222, is a decayproduct of radium-226 and itself disinte-grates into an isotope of polonium with ahalf-life of 3.82 days. 222Rn is sometimesused in radiotherapy. Radon accumulatesin uranium mines and can also seep intothe basements of buildings constructedover rocks or sands containing uranium orradium ores, where it may pose a healthrisk.

Symbol: Rn; m.p. –71°C; b.p. –61.8°C;mass density 9.73 kg m–3 (0°C); p.n. 86.

raffinate The liquid remaining after thesolvent extraction of a dissolved substance.See solvent extraction.

r.a.m. See relative atomic mass.

Raman effect A change in the fre-quency of electromagnetic radiation, suchas light, that occurs when a photon of ra-diation undergoes an inelastic collisionwith a molecule. This type of scattering is

called Raman scattering, in contrast tonormal (Rayleigh) scattering. The Ramaneffect was first observed by the Indianphysicists Sir Chandrasekhara VenkataRaman (1888–1970) and his colleague Sir Kariamanikkam Srinivasa Krishnan in1928. The effect has been used extensivelyin Raman spectroscopy for the deter-mination of molecular structure, particu-larly since the advent of the laser. In some cases it is possible to draw conclu-sions about molecular structure by observ-ing whether certain lines are present orabsent.

Ramsay, Sir William (1852–1916)British chemist, famous for his discovery ofthe rare gases. After a talk by LordRayleigh in 1892 on some discrepancies inthe density of nitrogen, Ramsay predictedthat these discrepancies were due to a hith-erto undiscovered gas. After careful exper-iments he was able to isolate a new elementwhich Rayleigh and Ramsay named argon.Ramsay was also able to identify helium, incollaboration with the spectroscopic obser-vations of Sir William Crookes. (Heliumhad previously been discovered in the Sun.)These discoveries led Ramsay to speculatein his book The Gases of the Atmosphere(1896) that there is a whole group of inertelements in the periodic table. Ramsay andhis colleagues found the missing elementsbetween 1894 and 1901.

Raney nickel A catalytic form of nickelproduced by treating a nickel–aluminumalloy with sodium hydroxide. The alu-minum dissolves (as aluminate) in thesodium hydroxide, leaving the Raneynickel as a spongy mass, which is py-rophoric when dry. It is used especially forcatalyzing hydrogenation reactions.

Raoult’s law A relationship betweenthe pressure exerted by the vapor of a solu-tion and the presence of a solute. It statesthat the partial vapor pressure of a solventabove a solution (p) is proportional to themole fraction of the solvent in the solution(X) and that the proportionality constant isthe vapor pressure of pure solvent, (p0), atthe given temperature: i.e. p = p0X. It was

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discovered by François Raoult (1830–1901), a French chemist. Solutions thatobey Raoult’s law are said to be ideal solu-tions. Because of solvation forces this be-havior is rare and in general Raoult’s lawholds only for dilute solutions.

rare earths See lanthanoids.

rare gases (noble gases; inert gases; group0 elements; group 18 elements) A groupof monatomic low-boiling gases belongingto group 18 of the periodic table and occu-pying a position between the highly elec-tronegative group 17 elements and thehighly electropositive group 1 elements.They are helium (He), neon (Ne), argon(Ar), krypton (Kr), xenon (Xe), and radon(Rn). Their completely filled s and p levelsgives a closed-shell structure that is associ-ated with a general lack of chemical reac-tivity and very high ionization energies. Itis the acquisition of a stable rare-gas con-figuration that is associated with determin-ing the valence of many covalent and ioniccompounds.

Prior to 1962 the rare gases were fre-quently called inert gases because chemicalcompounds with them were not known(there were a few clathrates and supposed‘hydrates’), but the realization that the ion-ization potential of xenon was sufficientlylow to be accessible to chemical reactionled to the preparation of several fluorides,oxides, oxyfluorides, and a hexafluoro-platinate of xenon. Several unstable kryp-ton and radon compounds have also beensynthesized.

Argon forms about 1% of the atmos-phere but the other gases are present in airin only minute traces. They are obtained byfractional distillation of liquid air or of nat-ural gas. Considerable amounts of argonare produced as a by-product from ammo-nia-plant tail gas. Applications for the raregases depend heavily on their inertness; forexample, welding (Ar), filling light bulbs(Ar), gas-stirring in high temperature met-allurgy (Ar), powder technology (Ar), dis-charge tubes (Ne), and chemical researchas inert carriers (He, Ar). Neon has beenproposed as an oxygen diluent for deep-seadiving because of its lower solubility in

blood. Liquid helium is extensively used inlow-temperature work, and radon is usedtherapeutically as a source of alpha-parti-cles.

The gases helium and argon are formedby radioactive decay in various minerals,for example argon by radioactivity decayof 40K. This mechanism can be applied toage-determination of minerals, a techniqueknown as potassium-argon dating. Radonis generally obtained as 222Rn from thedecay of radium. Its half-life is only about3.8 days.

rate constant (velocity constant; specificreaction rate) Symbol: k The constant ofproportionality in the rate expression for achemical reaction. For example, in a reac-tion A + B → C, the rate may be propor-tional to the concentration of A multipliedby that of B; i.e.

rate = k[A][B]where k is the rate constant for this partic-ular reaction. The constant is independentof the concentrations of the reactants butdepends on temperature; consequently thetemperature at which k is recorded must bestated. The units of k vary depending onthe number of terms in the rate expression,but are easily determined rememberingthat rate has the unit s–1.

rate-determining step (limiting step)The slowest step in a multistep reaction.Many chemical reactions are made up of anumber of steps in which the one with thelowest rate is the one that determines therate of the overall process.

The overall rate of a reaction cannot ex-ceed the rate of the slowest step. For exam-ple, the first step in the reaction betweenacidified potassium iodide solution and hy-drogen peroxide is the rate-determiningstep:

H2O2 + I– → H2O + OI– (slow)H+ + OI– → HOI (fast)

HOI + H+ + I– → I2 + H2O (fast)

rate of reaction A measure of theamount of reactant consumed in a chemi-cal reaction in unit time. It is thus a mea-sure of the number of effective collisionsbetween reactant molecules. The rate at

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which a reaction proceeds can be measuredby the rate the reactants disappear or bythe rate at which the products are formed.The principal factors affecting the rate ofreaction are temperature, pressure, concen-tration of reactants, light, and the action ofa catalyst. The units usually used to mea-sure the rate of a reaction are mol dm–3 s–1.See also mass action.

rationalized units A system of units inwhich the equations defining the systemhave a logical form related to the shape ofthe system. SI units form a rationalized sys-tem of units. For example, SI formulae con-cerned with circular symmetry contain afactor of 2π; those concerned with radialsymmetry contain a factor of 4π.

raw material A substance from whichother substances are made. In the chemicalindustry it may be simple (such as nitrogenfrom air, used to make ammonia) or com-plex (such as coal and petroleum, used tomake a wide range of products).

reactant See chemical reaction.

reaction See chemical reaction.

reaction coordinate See energy profile.

reagent A compound that reacts withanother. The term is usually used for com-mon laboratory chemicals, e.g. sodium hy-droxide, hydrochloric acid, etc. used forexperiment and analysis.

realgar The mineral form of arsenic(II)sulfide As4S4, one of the chief ores of ars-enic. It is bright red in color and was for-merly used as a pigment, in tanning, and infireworks. Its use for these purposes hasbeen largely discontinued due to its toxic-ity.

real gas See gas laws.

rearrangement A reaction in which thegroups of a compound rearrange them-selves to form a different compound.

reciprocal proportions, law of Seeequivalent proportions.

recrystallization The repeated crystal-lization of a compound in order to ensurepurity of the sample or crystals of betterform.

red lead See dilead(II) lead(IV) oxide.

redox (oxidation–reduction) Relating tothe process of oxidation and reduction,which are intimately connected in that dur-ing oxidation by chemical agents the oxi-dizing agent itself becomes reduced, andvice versa. Thus an oxidation process is al-ways accompanied by a reduction process.In electrochemical processes this is equallytrue, oxidation taking place at the anodeand reduction at the cathode. These sys-tems are often called redox systems, partic-ularly when the interest centers on bothcompounds.

Oxidizing and reducing power is indi-cated quantitatively by the redox potentialor standard electrode potential, EŠ.Redox potentials are normally expressedas reduction potentials. They are obtainedby electrochemical measurements and thevalues are referred to the H+/H2 couple forwhich EŠ is set equal to zero. Thus in-creasingly negative potentials indicate in-creasing ease of oxidation or difficulty ofreduction. Thus in a redox reaction the halfreaction with the most positive value ofEŠ is the reduction half and the half reac-tion with the least value of EŠ (or mosthighly negative) becomes the oxidationhalf. See also electrode potential; oxida-tion; reduction.

redox potential See redox.

redox systems See redox.

red phosphorus See phosphorus.

reducing agent See reduction.

reduction The gain of electrons by suchspecies as atoms, molecules, or ions. Itoften involves the loss of oxygen from acompound, or addition of hydrogen. Re-

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duction can be effected chemically, i.e. bythe use of reducing agents (electrondonors), or electrically, in which case thereduction process occurs at the cathode.For example,

2Fe3+ + Cu → 2Fe2+ + Cu2+

where Cu is the reducing agent and Fe3+ isreduced, and

2H2O + SO2 + 2Cu2+ →4H+ + SO4

2– + 2Cu+

where SO2 is the reducing agent and Cu2+

is reduced.Tin(II) chloride, sulfur(IV) oxide, and

the hydrogensulfite ion are common reduc-ing agents. See also redox.

reduction potential See electrode po-tential.

refining The process of removing impu-rities from a substance or of extracting asubstance from a mixture.

refractory Describing a compound (e.g.an inorganic oxide) that has a very highmelting point. Refractory materials areused as furnace linings.

relative atomic mass (r.a.m.; averageatomic mass) Symbol: Ar The ratio of theaverage mass per atom of the naturally oc-curring isotopes or, if given in parentheses,the most stable isotope of an element to1/12 of the mass of an atom of nuclide 12C.It was formerly called atomic weight.

relative density The ratio of the densityof a given substance to the density of somereference substance. The relative densitiesof liquids are usually measured with refer-ence to the density of water at 4°C. Rela-tive densities are also specified for gases;usually with respect to air at STP. The tem-perature of the substance is either stated oris understood to be 20°C. Relative densitywas formerly called specific gravity.

relative molecular mass Symbol:Mr The ratio of the average mass permolecule of the naturally occurring form ofan element or compound to 1/12 of themass of an atom of nuclide 12C. This ratiowas formerly called molecular weight. It

does not have to be used only for com-pounds that have discrete molecules; forionic compounds (e.g. NaCl) and giant-molecular structures (e.g. BN) the formulaunit is used.

relaxation The process by which an ex-cited species loses energy and falls to alower energy level (such as the groundstate).

rem An m.k.s. unit formerly used formeasuring equivalent ionizing radiationdoses absorbed by living tissue. One rem isequivalent to a dosage of one roentgen ofx-rays or gamma rays absorbed by an av-erage adult male. In SI units it is equal to10–2 sievert.

resin A yellowish insoluble organic com-pound exuded by trees as a viscous liquidthat gradually hardens on exposure to airto form a brittle amorphous solid. Syn-thetic resins are artificial polymers used inmaking adhesives, insulators, and paints.

resolution (of racemates) The separa-tion of a racemate into the two optical iso-mers. This cannot be done by normalmethods, such as crystallization or distilla-tion, because the isomers have identicalphysical properties. The main methods in-volve mechanical, chemical, or biochemi-cal techniques.

resonance (mesomerism) The behaviorof many molecules or ions cannot be ade-quately explained by a single structureusing simple single and double bonds. Thebonding electrons of the molecule or ionhave instead a different distribution, one inwhich the actual bonds can be regarded asa hybrid of two or more conventionalforms called resonance forms or canonicalforms. The result is a resonance hybrid. Forexample, three equivalent LEWIS STRUC-TURES can be written for sulfur(VI) oxide,depending on which of the three oxygenatoms is double bonded to the sulfur atom.However, experiments reveal that all thebonds in sulfur(VI) oxide are of the samelength and strength. This means that thebonds to the oxygen atoms must be neither

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double nor single, but a hybrid of the pos-sible Lewis structures.

resonance forms See resonance.

resonance ionization spectroscopy(RIS) A type of spectroscopy that detectsspecific types of atoms using lasers. Thelaser ionizes the atoms of interest. The fre-quency of the laser is chosen so that onlythe atoms of interest in a sample are ex-cited by the laser. This method is very se-lective because ionization occurs only forthose atoms whose energy levels fit in withthe frequency of the laser light. This selec-tivity has led to many practical uses for thistechnique.

retort A piece of laboratory apparatusconsisting of a glass bulb with a long nar-row neck. In industrial chemistry, variousmetallic vessels in which distillations or re-actions take place are called retorts.

reverbatory furnace A furnace forsmelting metals. It has a curved roof so thatheat is reflected downward, the fuel beingin one part of the furnace and the ore in theother.

reverse osmosis A process by whichwater containing a salt is separated via asemipermeable membrane into pure waterand salt water streams by subjecting the in-coming water to pressures significantlygreater than its OSMOTIC PRESSURE. This ac-tion causes pure water to be forced throughthe membrane, leaving the salt behind. Seeosmosis.

reversible change A change in the pres-sure, volume, or other properties of a sys-tem, in which the system remains atequilibrium throughout the change. Suchprocesses could be reversed; i.e. returned tothe original starting position through thesame series of stages. They are never real-ized in practice. An isothermal reversiblecompression of a gas, for example, wouldhave to be carried out infinitely slowly andinvolve no friction, etc. Ideal energy trans-fer would have to take place between the

gas and the surroundings to maintain aconstant temperature.

In practice, all real processes are irre-versible changes in which there is not anequilibrium throughout the change. In anirreversible change, the system can still bereturned to its original state, but notthrough the same series of stages. For aclosed system, there is always an entropyincrease involved in an irreversible change.

reversible reaction A chemical reactionthat can proceed in either direction. For ex-ample, the reaction:

N2 + 3H2 ˆ 2NH3

is reversible. In general, there will be anequilibrium mixture of reactants and prod-ucts.

rhenium A dense silvery rare transitionmetal, the third element of group 7 (for-merly VIIB) of the periodic table. It usuallyoccurs naturally with molybdenum andplatinum ores, and is usually extractedfrom flue dust in molybdenum smelters.The metal is chemically similar to man-ganese. It has the second highest meltingpoint of all the elements, making it espe-cially useful for alloys for electrical con-tacts, thermocouples, and otherhigh-temperature environments. It is alsoused as a catalyst.

Symbol: Re; m.p. 3180°C; b.p. 5630°C;r.d. 21.02 (20°C); p.n. 75; most commonisotope 187Re; r.a.m. 186.207.

rheology The study of the ways inwhich matter can flow. This topic is of par-ticular interest in the study of polymers.

rhodium A rare silvery hard transitionmetal, the second element of group 9 (for-merly part of subgroup VIIIB) of the peri-odic table. It is difficult to work and highlyresistant to corrosion. Rhodium occurs na-tive in the natural alloy osmiridium but ismost often obtained from copper andnickel ore refinery residues. It is used inprotective finishes, high-temperature al-loys, and as a catalyst. Rhodium com-pounds are characteristically pink.

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Symbol: Rh; m.p. 1966°C; b.p. 3730°C;r.d. 12.41 (20°C); p.n. 45; most commonisotope 103Rh; r.a.m. 102.90550.

rhombic crystal See crystal system.

ring A closed loop of atoms in a mol-ecule, as in the crown-shaped allotropes ofsulfur (S8) and selenium (Se8). A fused ringis one joined to another ring in such a waythat they share two atoms.

RIS See resonance ionization spectros-copy.

rock A definable part of the Earth’scrust made up of a mixture of minerals,some of which usually predominate inabundance over other, accessory minerals.Rock may be hard (for example, granite),soft (chalk), consolidated (clay), or loose(sand). See also mineral.

rock salt (halite) A transparent natu-rally occurring mineral form of sodiumchloride.

roentgen Symbol: R A c.g.s. unit of ra-diation exposure, formerly used for x-raysand gamma rays, defined in terms of theionizing effect of one electrostatic unit ofelectricity on a cubic centimeter of air. In SIunits one roentgen is equal to 2.58 × 10–4

coulomb per kilogram of dry air.

Rose’s metal A fusible alloy containing50% bismuth, 25–28% lead, and the bal-ance in tin. Its low melting point (about100°C) leads to its use in fire-protectiondevices.

rotary dryers Devices commonly usedin the chemical industry for the drying,mixing, and sintering of solids. They con-sist essentially of a heated rotating inclinedcylinder, which is longer in length than indiameter. Gases flow through the cylinderin either a countercurrent or co-current di-rection to regulate the flow of solids, whichare fed into the end of the cylinder. Rotarydryers can be applied to both batch andcontinuous processes.

rotational spectroscopy The spectro-scopic study of the rotational motion ofmolecules. Rotational spectroscopy givesinformation about interatomic distances.The transitions between different rota-tional energy levels in molecules corre-spond to the microwave and far infraredregions of the electromagnetic spectrum. Itis possible for there to be transitions be-tween rotational energy levels in pure rota-tional spectra only if the molecule has apermanent dipole moment. In the near in-frared region rotational transitions are su-perimposed on vibrational transitions,resulting in a vibrational–rotational spec-trum. This type of spectrum is considerablymore complicated than a purely rotationalspectrum.

rubidium A soft light silvery highly re-active alkali metal; the fourth element ofgroup 1 (formerly IA) of the periodic table.It is found in small amounts in several com-plex silicate minerals, including lepidolite,and in some brines. Naturally occurringrubidium comprises two isotopes, one ofwhich, 87Rb, is radioactive (half-life 5 ×1010 years). Rubidium is used in vacuumtubes, photocells, and in making specialglass. Rubidium–strontium dating relies onthe decay rate of 87Rb.

Symbol: Rb; m.p. 39.05°C; b.p. 688°C;r.d. 1.532 (20°C); p.n. 37; most commonisotope 85Rb; r.a.m. 85.4678.

rubidium–strontium dating A tech-nique for dating rocks. It depends on theradioactive decay of the radioisotope 87Rbto 87Sr (half-life 4.7 × 1010 years). If theratio 87Sr/87Rb is measured and comparedto the implied original quantity of 87Rb, itis possible to estimate the age of the rock.See also radioactive dating.

ruby A rare and highly valued gemstonevariety of the mineral corundum (alu-minum oxide, Al2O3) colored red bychromium impurities. Rubies can also bemade synthetically. They are used in jew-elry, as bearings in watches, and in somelasers.

Russell–Saunders coupling The type

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of coupling of electronic angular momen-tum vectors that describes light atoms.Russell–Saunders coupling is applicable ifthe energies of electrostatic Coulomb re-pulsion are considerably greater than theenergies of SPIN–ORBIT COUPLING. The con-cept of Russell–Saunders coupling was pio-neered by the American astronomer HenryNorris Russell (1877–1957) and Americanphysicist Frederick Albert Saunders(1875–1963) in 1925.

rusting The corrosion of iron in air toform an oxide. Both oxygen and moisturemust be present for rusting to occur. Theprocess is electrolytic, involving electro-chemical reactions in which different areasof the wet iron serve as anode and cathode:

Fe(s) → Fe2+(aq) + 2e–

2H2Ol + O2(aq) + 4e– → 4OH–(aq)Iron(II) hydroxide precipitates and is

oxidized to the red hydrated iron(III) oxideFe2O3.H2O. Rusting is greatly speeded upby impurities in the iron and electrolytes inthe water.

ruthenium A hard brittle silver-whitetransition metal, the second element ofgroup 8 (formerly part of subgroup VIIIB)of the periodic table. It occurs naturallywith platinum in the natural alloy osmirid-ium and in some sulfide ores. Its relativelyhigh melting point and hardness makes ituseful in hard heat-resistant alloys, such asthose used for electrical contacts. It is alsoused as a catalyst, and as a pigment in dec-orative glasses and enamels. Ruthenium isalso used in jewelry alloyed with palla-dium.

Symbol: Ru; m.p. 2310°C; b.p.3900°C; r.d. 12.37 (20°C); p.n. 44; mostcommon isotope 102Ru; r.a.m. 101.07.

Rutherford, Ernest, Lord Rutherford(1871–1937) New Zealand-born physicist.Ernest Rutherford is generally regarded asone of the greatest experimental physicistsever. He conducted fundamental researchon radioactivity and nuclear physics. In1899–1900 he found that there are threetypes of radioactivity. He called the differ-

ent types of radiation alpha rays, beta rays,and gamma rays. He showed that radioac-tivity is characterized by a half-life. Fol-lowing a suggestion of Rutherford, HansGeiger and Ernest Marsden performed anexperiment involving the scattering ofalpha particles which Rutherford inter-preted in 1911 as showing that an atomconsists of a nucleus at the center sur-rounded by electrons. In 1919 he showedthat artificial disintegration of the nucleuscould be induced by bombarding it withalpha particles. Rutherford also pioneeredthe use of radioactivity in dating rocks. Hewon the 1908 Nobel Prize for chemistry.

rutherfordium A radioactive synthetictransactinide metallic element, the firsttransactinide. Atoms of rutherfordium areproduced by bombarding 249Cf with 12C orby bombarding 248Cm with 18O. Only avery few atoms have ever been created.

Symbol: Rf; m.p. 2100°C (est.); b.p.5200°C (est.); r.d. 23 (est.); p.n. 104; moststable isotope 261Rf (half-life 65 s).

rutile A naturally occurring reddish-brown to black form of titanium(IV)oxide, TiO2. It is an ore of titanium and isalso used in making ceramics and to coatwelding rods. Clear varieties are used as asemiprecious gemstone.

Rydberg constant A constant that oc-curs in formula for the frequencies of spec-tral lines in atomic spectra. For thehydrogen atom it has the value 1.0968 ×107m–1. The value of the Rydberg constantcan be calculated from the BOHR THEORY ofthe hydrogen atom and from quantum me-chanics. These calculations showed that:

R = Μ02me4c3/8h3

where Μ0 is the magnetic constant, m ande are the mass and charge respectively of anelectron, h is the Planck constant and c isthe speed of light in a vacuum. The Ryd-berg constant is named for the Swedishphysicist Johannes Robert Rydberg (1854–1919) for his work on atomic spectra. Seealso hydrogen atom spectrum.

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sacrificial protection A method of pro-tection against electrolytic corrosion (espe-cially rusting). In protecting steel pipelines,for instance, zinc or magnesium rods areburied in the ground at points along theline and connected to the pipeline. Rustingof iron is an electrochemical process inwhich Fe2+ ions dissolve in the water incontact with the surface. A more elec-tropositive element, such as zinc, protectsagainst this because Zn2+ ions dissolve inpreference. In other words, the zinc rod is‘sacrificed’ for the steel pipeline. The sameeffect occurs with galvanized iron. If thezinc coating is scratched, the exposed irondoes not rust until all the zinc has dissolvedaway. (Note that a tin coating, being lesselectropositive than iron, has the oppositeeffect.)

safety lamp (Davy lamp) A type of oillamp for use in coal mines designed so thatit will not ignite any methane (firedamp)present and cause an explosion. The flameis surrounded by a fine metal gauze whichdissipates the heat from the flame but neverreaches the ignition temperature ofmethane. If methane is present it burns in-side the gauze; the lamp can be used as atest for pockets of methane.

sal ammoniac See ammonium chloride.

saline Containing a salt, especially an al-kali-metal halide such as sodium chloride.

salt A compound with an acidic and abasic radical, or a compound formed bytotal or partial replacement of the hydro-gen in an acid by a metal; the product ofneutralizing an acid with a base. In generalterms a salt is a material that has identifi-able cationic and anionic components.

salt bridge An electrical contact be-tween two half cells, used to prevent mix-ing. A glass U-tube filled with potassiumchloride in agar is often used.

saltcake An impure form of SODIUM SUL-FATE, formerly produced industrially as aby-product of the LEBLANC PROCESS.

salt hydrate A metallic salt in which themetal ions are surrounded by a fixed num-ber of water molecules. The water mol-ecules are bound to the metal ions byion–dipole interactions. In the solid statethe water molecules form part of the crys-tal structure. It is thus a clathrate.

Salt hydrates that contain several mol-ecules of water for each metal ion can losethem progressively (effloresce) if the pres-sure of water vapor is kept below the dis-sociation pressure of the system; eventuallyan anhydrous salt is formed. Alternatively,if the vapor pressure is continuously raisedthe system will add molecules of water(deliquesce) until a saturated solutionforms.

salting out The addition of an ionic saltto a solution in order to precipitate a solute(or cause it to be evolved as a gas). The dis-solved substance is more soluble in purewater than in the ionic solution. Certaincolloids may also be precipitated in thisway.

saltpeter See potassium nitrate.

sal volatile See ammonium carbonate.

samarium A hard brittle silver-gray ele-ment of the lanthanoid series of metals. Itoccurs in association with other lan-thanoids in such minerals as allanite, bast-

195

S

naesite, gadolinite, monazeite, andsamarskite. Samarium is alloyed withcobalt to produce powerful permanentmagnets. It is also used in alloys for nuclearreactor parts, as a catalyst in organic reac-tions, and as an ingredient in optical glass.

Symbol: Sm; m.p. 1077°C; b.p.1791°C; r.d. 7.52 (20°C); p.n. 62; mostcommon isotope 152Sm; r.a.m. 150.36.

sand A mineral form of silicon(IV) oxide(silica, SiO2) consisting chiefly of separategrains of quartz. It results from erosion ofquartz-containing rocks; the grains may berounded by the tumbling action of water orwind. Sand is used in concrete and mortarand in making glass and other ceramics.

sandstone A sedimentary rock consist-ing of consolidated sand grains cementedtogether with calcium carbonate, clay, oroxides of iron. It is used mainly as a build-ing stone.

sandwich compound A type oforganometallic compound formed betweentransition metal ions and aromatic com-pounds, in which the metal ion is ‘sand-wiched’ between two rings. Four, five, six,seven, and eight-membered rings areknown to form complexes with a numberof elements including V, Cr, Mn, Co, Ni,and Fe. The bonding in such compounds isnot between the metal ion and individualatoms on the ring; rather it involves bond-ing of d electrons of the ion with the pi-electron system of the ring as a whole.FERROCENE is the original example, havingtwo parallel cyclopentadienyl rings sand-wiching the iron ion. There are a numberof related types of compound. Thus, halfsandwich compounds (sometimes knownas piano-stool compounds) have only onering bound to the central ion. Bent sand-wich compounds have two rings that arenot parallel, and the ion is attached toother ligands. Multidecker sandwich com-pounds have more than two parallel rings.See illustration.

sapphire A valuable gemstone variety ofthe mineral corundum (aluminum oxide,Al2O3) colored blue, green, pink, violet, or

other colors by impurities. Sapphires canalso be made synthetically. They are usedin jewelry and certain kinds of lasers.

saturated solution A solution that con-tains the maximum equilibrium amount ofsolute at a given temperature. A solution issaturated if it is in equilibrium with itssolute. If a saturated solution of a solid iscooled slowly, the solid may stay tem-porarily in solution; i.e. the solution maycontain more than the equilibrium amountof solute. Such solutions are said to be su-persaturated solutions.

saturated vapor A vapor that is in equi-librium with its solid or liquid phase. A sat-urated vapor is at the maximum pressure(the saturated vapor pressure) at a giventemperature. If the temperature of a satu-rated vapor is lowered, the vapor con-denses. Under certain circumstances, thesubstance may stay temporarily in thevapor phase; i.e. the vapor contains morethan the equilibrium concentration of thesubstance. The vapor is then said to be asupersaturated vapor.

s-block elements The elements of thefirst two groups of the periodic table; i.e.group 1 (H, Li, Na, K, Rb, Cs, Fr) andgroup 2 (Be, Mg, Ca, Sr, Ba, Ra). They areso called because their outer shells have theelectronic configurations ns1 or ns2. The s-block excludes those with inner (n – 1)dlevels occupied (i.e. it excludes transitionelements, which also have s2 and occasion-ally s1 configurations).

scandium A light soft reactive silverytransition metal, the first member of group3 (formerly IIIB) of the periodic table. It isfound in minute amounts in over 800 min-erals, especially thortveitite ((Sc,Y)2Si2O7)and minerals often associated with lan-thanoids. Scandium is used in high-inten-sity lights, in electronic devices, and intitanium carbide alloys. Scandium has sev-eral radioactive isotopes, some of whichare used as tracers in medicine.

Symbol: Sc; m.p. 1541°C; b.p. 2831°C;r.d. 2.989 (0°C); p.n. 21; most commonisotope 45Sc; r.a.m. 44.955910.

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scanning tunneling microscope (STM)A type of microscope that makes use ofquantum-mechanical tunneling to investi-gate the atomic structure of a sample ofmaterial. In this technique a sharp con-ducting tip is brought near a surface. Thiscauses electrons to tunnel from the surfaceto the tip, with the probability of this oc-

curring depending on both the distance be-tween the surface and the tip and the elec-tron density at the surface. The tunnelingproduces an electric current, which is keptconstant by moving the tip up or down asit is moved over the surface; a computerconverts the movements into an image.Single atoms can be observed using STM.

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scanning tunneling microscope

Ni

Ni

FeMo

CO

COCO

parallel sandwich half sandwich (piano stool)

multidecker sandwichbent sandwich

Ti

H

H

Sandwich compound

Scheele, Karl Wilhelm (1742–86)Swedish chemist. Scheele was one of thegreatest chemists of the 18th century. Hediscovered a large number of elements andcompounds including oxygen (indepen-dently of Joseph PRIESTLEY), nitrogen, chlo-rine (although he did not recognize it as anelement), citric acid, glycerol, hydrogencyanide, hydrogen fluoride, hydrogen sul-fide, lactic acid, and uric acid. He also dis-covered that light affects certain silversalts. He wrote a book entitled A ChemicalTreatise on Air and Fire (1777) in which hedescribed his experiments.

schönite See potassium sulfate.

Schottky defect See defect.

Schrödinger, Erwin (1887–1961) Aus-trian physicist. Schrödinger developed thebranch of quantum mechanics known aswave mechanics. In this work Schrödingerformulated what is known as the Schrö-dinger equation. He solved his equation forsimple systems such as the hydrogen atom.He also showed how wave mechanics andthe matrix mechanics of Werner HEISEN-BERG are equivalent. Schrödinger sharedthe 1933 Nobel Prize for physics with PaulDIRAC for his work on quantum mechanics.Schrödinger wrote a book entitled What isLife? (1944) which was influential in stim-ulating interest in molecular biology.

Schrödinger equation The fundamen-tal equation in the WAVE MECHANICS for-mulation of quantum mechanics. Forcalculating energy levels of electrons inatoms, molecules, and solids, the time-in-dependent Schrödinger equation is used.This equation can be written:

∇2ψ + 8π2m(E – U)ψ/h2 = 0where ∇2 is an operator, U the potentialenergy, h the Plank constant, and ψ thewavefunction. E is the energy of the sys-tem. The operator ∇2 is:

∂/∂x2 + ∂/∂y2 + ∂/∂z2

scrubber The part of a chemical plantthat removes impurities from a gas by pass-ing it through a liquid.

seaborgium A radioactive synthetictransactinide metallic element created bybombarding 249Cf with 18O nuclei using acyclotron. Six isotopes are known.

Symbol: Sg; m.p., b.p. and r.d. un-known; p.n. 106; most stable isotope 266Sg(half-life 27.3s).

second Symbol: s The base unit of timein all metric systems, including the SI. Inthe SI it is defined as the duration of9 192 631 770 periods of a particularwavelength of radiation corresponding to atransition between two hyperfine levels inthe ground state of the cesium-133 atom,an unvarying physical constant. Earliermetric systems defined the second as thefraction 1/86400 of a mean solar day. Ir-regularities in the Earth’s rotation, how-ever, make the mean solar day less thanconstant.

secondary cell See accumulator.

second-order reaction A reaction inwhich the rate of reaction is proportionalto the product of the concentrations of twoof the reactants or to the square of the con-centration of one of the reactants; i.e.

rate = k[A][B]or

rate =k[A]2

For example, the hydrolysis by dilutealkali of an ester is a second-order reaction:

rate = k[ester][alkali]The rate constant for a second-order re-

action has the units mol–1 dm3 s–1. Unlike afirst-order reaction, the time for a definitefraction of the reactants to be consumed isdependent on the original concentrations.

sedimentation The settling of a suspen-sion, either under gravity or in a centrifuge.The speed of sedimentation can be used toestimate the average size of the particles.This technique is used with an ultracen-trifuge to find the relative molecularmasses of macromolecules.

seed A small crystal added to a gas orliquid to assist solidification or precipita-tion of a substance from a solution. Theseed, usually a crystal of the substance to

Scheele, Karl Wilhelm

198

be formed, enables particles to pack intopredetermined positions so that a largercrystal can form.

selenium A metalloid or nonmetallic el-ement; the third member of group 16 (for-merly VIA) of the periodic table. It exists inseveral allotropic forms. It occurs inminute quantities in sulfide ores and is alsorecovered from anode sludges formed dur-ing electrolytic refining. The common graymetallic allotrope is both a semiconductorand very light-sensitive, and thus finds usein photocells, solar cells, and xerography.It is also used in some glasses and enamels,and in trace amounts is an essential dietarymineral. The red allotrope is unstable andreverts to the gray form under normal con-ditions.

Symbol: Se; m.p. 217°C (gray); b.p.684.9°C (gray); r.d. 4.79 (gray); p.n. 34;most common isotope 80Se; r.a.m. 78.96.

self-organization The order that canarise in a system that is far from thermody-namic equilibrium. The existence of self-organization does not contradict thesecond law of thermodynamics since self-organization occurs in systems that areopen and through which there is an energyflow. Self-organization can be analyzed interms of chaos theory and nonequilibriumstatistical mechanics and thermodynamics.

Semenov, Nikolay Nikolaevich (1896–1986) Russian physical chemist. Semenovstudied chemical chain reactions in the1920s. He showed how such reactionscould lead to combustion and violent ex-plosions when branching occurs in thechain. He gave an account of his work inthe influential book Chemical Kinetics andChain Reactions (1934), the English trans-lation of which was published in 1935. Heshared the 1956 Nobel Prize for chemistrywith Sir Cyril HINSHELWOOD.

semiconductor A crystalline materialthat conducts only under certain condi-tions. Its conductivity, unlike that of met-als, increases with temperature because thehighest occupied and lowest unoccupiedenergy levels are very close. A semiconduc-

tor can be formed by introducing smallamounts of an impurity to an ultrapurematerial. If these impurities can withdrawelectrons from the occupied energy level,leaving ‘holes’ that permit conduction, theresult is known as a p-type semiconductor.Alternatively, these impurities may donateelectrons that enter the unoccupied energylevel, thus forming an n-type semiconduc-tor. Semiconductors are used extensivelyby the electronics industry in such devicesas integrated circuits.

semipermeable membrane A mem-brane that, when separating a solutionfrom a pure solvent, permits the solventmolecules to pass through it but does notallow the transfer of solute molecules. Syn-thetic semipermeable membranes are gen-erally supported on a porous material,such as unglazed porcelain or fine wirescreens, and are commonly formed of cel-lulose or related materials. They are used inosmotic studies, gas separations, reverseosmosis water systems for homes and bev-erage industries, and in medical applica-tions.

Equilibrium is reached at a semiperme-able membrane if the chemical potentialson both sides become identical; migrationof solvent molecules towards the solutionis an attempt by the system to reach equi-librium. The pressure required to halt thismigration is the OSMOTIC PRESSURE.

semipolar bond See coordinate bond.

septivalent See heptavalent.

sequestration The formation of a com-plex with an ion in solution, so that the iondoes not have its normal activity. Seques-tering agents are often chelating agents.

sesqui- Prefix indicating a 2/3 ratio in acompound. A sesquioxide, for example,would have the formula M2O3. Asesquicarbonate is a mixed carbonate andhydrogencarbonate of the typeNa2CO3.NaHCO3.2H2O, which contains2 CO3 units and 3 sodium ions.

shell A group of electrons that share the

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same principal quantum number n. Earlywork on x-ray emission studies used theterms K, L, M, and these are still some-times used for the first three shells: n = 1,K-shell; n = 2, L-shell; n = 3, M-shell. Seealso atom.

sheradizing A technique used to obtaincorrosion-resistant articles of iron or steelby heating with zinc dust in a sealed rotat-ing drum for several hours at about 375°C.At this temperature the two metals com-bine, forming internal layers of zinc–ironand zinc–steel alloys and an external layerof pure zinc. Sheradizing is principally usedfor small parts, such as springs, washers,nuts, and bolts. It is named for its inventor,English metallurgist Sherard Cowper-Coles (1866–1936).

short period See period.

SI See SI units.

side reaction A chemical reaction thattakes place to a limited extent at the sametime as a main reaction. Thus the mainproduct of a reaction may contain smallamounts of other compounds.

Sidgwick, Nevil Vincent (1873–1952)British chemist. The first notable contribu-tion by Sidgwick to chemistry was his bookThe Organic Chemistry of Nitrogen(1910). He subsequently became interestedin the electronic structure of atoms, includ-ing work on coordination compounds andthe hydrogen bond. He summarized his re-search in an influential book The Elec-tronic Theory of Valency (1927). He spentthe rest of his career trying to understandcompounds in terms of valence theory.This culminated in his monumental two-volume book The Chemical Elements andtheir Compounds (1950).

siemens Symbol: S The SI derived unitof electrical conductance, equal to a con-ductance of one ampere per volt.1S = 1Av–1. In earlier metric systems the equivalentof the siemens was known as the mho. It isnamed for German electrical engineer andinventor Ernst von Siemens (1816–92).

sievert Symbol: Sv The SI derived unit ofabsorbed dose equivalent in living tissue,equal to that produced by ionizing radia-tion which, when multiplied by certain di-mensionless factors, gives a value of onejoule per kilogram (1 J kg–1). The dimen-sionless factors are used to modify the ab-sorbed dose to take account of the fact thatdifferent types of radiation cause differentbiological effects. The sievert is named forthe Swedish radiologist and physicist RolfSievert (1896–1966).

sigma bond See orbital.

sigma orbital See orbital.

silane Any of a small number of hy-drides of silicon: SiH4, Si2H6, Si3H8, etc.Silanes can be made by the action of acidson magnesium silicide (Mg2Si). They areunstable compounds and ignite sponta-neously in air. Only the first few membersof the series are known and there is not therange of ‘hydrosilicon’ compounds to com-pare with the hydrocarbons.

silica See silicon(IV) oxide.

silica gel A gel made by coagulatingsodium silicate sol. The gel is dried by heat-ing and used as a catalyst support and as adrying agent. The silica gel used in desicca-tors and in packaging to remove moistureis often colored with a cobalt salt to indi-cate whether it is still active (blue = dry;pink = moist).

silicates A large number of compoundscontaining metal ions and complex sili-con–oxygen combinations. The negativeions in silicates are of the form SiO4

4–,Si2O7

6–, etc., and many are polymeric, con-taining SiO4 units linked in long chains,sheets, or three-dimensional arrays. Alumi-nosilicates and borosilicates are similarmaterials containing aluminum or boronatoms in the structure. Silicates occur nat-urally in many rocks and minerals.

silicic acid A colloidal or jellylike hy-drated form of silicon(IV) oxide (SiO2),made by adding an acid (such as hy-

sheradizing

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201

Si

O

OOO

=

Si O4– as in Be2SiO

4 (phenacite)

4

Si2O 2– as in Sc

2Si

2O

7 (thortveitite)5

Si3O 6– as in BaTiSi

3O

9 (bentonite)

9

single chain : pyroxenes

double chain : amphiboles sheet : micas

Si6O 12– as in Be

3Al

2Si

6O

18 (beryl)

18

Silicate: examples of silicate structures

drochloric acid) to a soluble SILICATE.

silicide A compound of silicon with amore electropositive element.

silicon A hard brittle gray semiconduct-ing metalloid; the second element of group14 (formerly IVA) of the periodic table. Ithas the electronic configuration of neonwith four additional outer electrons; i.e.,[Ne]3s23p2.

Silicon accounts for 27.7% of the massof the Earth’s crust and occurs in a widevariety of silicates with metals, and inclays, micas, and sand, which is largely sil-icon(IV) oxide (SiO2). The element is ob-tained on a small scale by the reduction ofSiO2 by carbon or calcium carbide. It isemployed in certain alloys, but its majoruse by far is in semiconducting electroniccomponents and chips. For semiconductorapplications very pure silicon is producedby direct reaction of silicon with anHCl/Cl2 mixture to give silicon tetrachlo-ride (SiCl4), which can then be purified byfractional distillation. This compound isthen decomposed on a hot wire in an at-mosphere of hydrogen. For ultrapure sam-ples zone refining is used. Silicon has twoallotropes: the familiar gray one has a dia-mond lattice; the other is an amorphousbrownish powder.

Silicon does not react with hydrogenexcept under extreme conditions in thepresence of chlorine, in which case com-pounds such as trichlorosilane (HSiCl3) areobtained. The hydride SiH4 itself may beprepared in the laboratory by hydrolysis ofmagnesium silicide, in which case a num-ber of other hydrides up to Si6H14 are alsoobtained, or more conveniently by reduc-tion of silicon tetrachloride with lithiumtetrahydridoaluminate:

SiCl4 + LiAlH4 → SiH4 + LiCl + AlCl3The silanes are mildly explosive.

Silicon combines with oxygen whenheated in air to form silicon(IV) oxide(SiO2). The mineral silicates constitute avast collection of materials with a widerange of structures including chains, rings,lattices, twisted chains, sheets, and vary-ingly cross-linked structures all based ondifferent assemblies of [SiO4]2– tetrahedra.

Being a metalloid, silicon is somewhat am-photeric and consequently silica is weaklyacidic, dissolving in fused alkalis to formthe appropriate silicate.

Symbol: Si; m.p. 1410°C; b.p. 2355°C;r.d. 2.329 (20°C); p.n. 14; most commonisotope 28Si; r.a.m. 28.0855

silicon bronze See bronze.

silicon carbide (carborundum; SiC) Ablack very hard crystalline solid made byheating silicon(IV) oxide with carbon in anelectric furnace. Sand and coke may be em-ployed for lower grades. It is used as anabrasive and as a refractory material.

silicon(IV) chloride (silicon tetrachlo-ride; SiCl4) A colorless fuming liquidwhich rapidly hydrolyzes in moist air orwater. It is used to produce pure silica formaking special kinds of glass.

silicon dioxide See silicon(IV) oxide.

silicones Polymeric synthetic siliconcompounds containing chains of alternat-ing silicon and oxygen atoms, with organicgroups bound to the silicon atoms. Sili-cones are used as lubricants and water re-pellants and in waxes and varnishes.Silicone rubbers are superior to naturalrubbers in their resistance to both high andlow temperatures and to chemicals. Seealso siloxanes.

silicon(IV) oxide (silicon dioxide; silica;SiO2) A hard crystalline compound oc-curring naturally chiefly in three crystallineforms: quartz (hexagonal), tridymite(rhombic), and crystobalite (tetragonal orcubic). Sand is mostly fine particles ofquartz. Fused silica is a glassy substanceused in laboratory apparatus. Silicon(IV)oxide is used in the manufacture of glass, inrefractory materials, and in metallurgicalfurnaces.

silicon tetrachloride See silicon(IV)chloride.

siloxanes Compounds containing Si–O–Sigroups with organic groups bound to the

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silicon atoms. The silicones are polymersof siloxanes.

silver A soft ductile malleable brilliantwhite precious transition metal, the secondelement of group 11 (formerly IB) of theperiodic table. Silver occurs native and insuch minerals as argentine (silver sulfide,Ag2S) and chlorargyrite (AgCl). It is alsofound in gold, copper, and lead ores, fromwhich it is extracted as a by-product duringrefining. Silver is not as reactive as copper,but is more reactive than gold. It darkens inair due to the formation of silver sulfide. Itis used in jewelry, dental amalgams, elec-trical conductors, and batteries. Until re-cently it was also used in coinage alloys,tableware, and as a reflective surface formirrors. Silver compounds are extensivelyused in photography.

Symbol: Ag; m.p. 961.93°C; b.p.2212°C; r.d. 10.5 (20°C); p.n. 47; mostcommon isotope 107Ag; r.a.m. 107.8682.

silver(I) bromide (AgBr) A pale yellowprecipitate obtained by adding a solublebromide solution to a solution of silver(I)nitrate. The halide dissolves in concen-trated ammonia solution but not in diluteammonia solution. It is used extensively inphotography for making light-sensitiveemulsions. Silver(I) bromide crystallizes ina similar manner to sodium(I) chloride.Unlike silver(I) chloride and silver(I) io-dide, silver(I) bromide does not absorb am-monia gas.

silver(I) chloride (AgCl) A compoundprepared as a curdy white precipitate bythe addition of a soluble chloride solutionto a solution of silver(I) nitrate. It occurs innature as the mineral chlorargyrite. Sil-ver(I) chloride is insoluble in water but dis-solves in concentrated hydrochloric acidand in concentrated ammonia solution. It is rather unreactive but is affected bysunlight, and is thus used extensively inphotography to make light-sensitive emul-sions. Silver(I) chloride crystallizes in thesodium chloride lattice pattern.

silver(I) iodide (AgI) A pale yellow pre-cipitate prepared by the addition of a solu-

ble iodide solution to a solution of silver(I)nitrate. Silver(I) iodide occurs in nature asiodargyrite in the form of hexagonal crys-tals. It is virtually insoluble in ammonia so-lution, and is used in the photographicindustry. Silver(I) iodide will absorb am-monia gas. It is trimorphic, contractingwhen heated and expanding when cooled.

silver(I) nitrate (AgNO3) A white crys-talline poisonous solid prepared by dis-solving silver in dilute nitric acid andcrystallizing the solution. Silver(I) nitrate isdimorphous, crystallizing in the ortho-rhombic and hexagonal systems. It under-goes thermal decomposition to yield silver,dinitrogen oxide, and oxygen. Probablythe most important silver salt, it is used inthe medical industry as an antiseptic andfor the treatment of warts, and in the pho-tographic industry to make light-sensitiveemulsions. In the laboratory, it is used bothas an analytical and volumetric reagent.

silver(I) oxide (argentous oxide; Ag2O)A brown amorphous solid formed by theaddition of sodium or potassium hydrox-ide solution to a solution of silver(I) ni-trate. Silver(I) oxide turns moist red litmusblue and decomposes (at 160°C) to give sil-ver and oxygen. Silver(I) oxide dissolves inconcentrated ammonia solution, formingthe complex ion [Ag(NH3)2]+. It is used inorganic chemistry.

silver(II) oxide (argentic oxide; AgO) Ablack diamagnetic substance prepared bythe oxidation of either silver or silver(I)oxide using ozone. Alternatively it can beprepared as a precipitate by the addition ofa solution of potassium persulfate(K2S2O8) to one of silver(I) nitrate.

simple cubic crystal See cubic crystal.

single bond A covalent bond betweentwo elements that involves one pair of elec-trons only. It is represented by a single line,for example H–Br, and is usually a sigmabond, although it can be a pi bond. Com-pare multiple bond. See also orbital.

sintering A process in which certain

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sintering

powdered substances (e.g. metals, ceram-ics) coagulate into a single mass whenheated to a temperature below the sub-stance’s melting point. Sintered glass is aporous material used for laboratory filtra-tion.

SI units (SI; Système International d’Unités) The modern coherent rational-ized internationally adopted metric systemof units. It has seven BASE UNITS and two di-mensionless units, formerly called supple-mentary units. DERIVED UNITS are formedby multiplication and/or division of baseunits. Standard prefixes are used for deci-mal multiples and submultiples of SI units,along with standard symbols for both unitsand prefixes.

slag (basic slag) Glasslike compounds ofcomparatively low melting point formedduring the extraction of metals when theimpurities in an ore react with a flux. Thefact that the slag is a liquid of compara-tively low density means that it can be sep-arated from the liquid metal on which itfloats. In a blast furnace, the flux used islimestone (CaCO3) and the main impurityis silica (SiO2). The slag formed is mainlycalcium silicate:

CaCO3 + SiO2 → CaSiO3It is used as railroad ballast and in fertiliz-ers and concrete.

slaked lime See calcium hydroxide.

slaking See calcium hydroxide.

slip planes The planes of weakness in acrystal, e.g. the boundaries between the oc-tahedral layers in graphite.

slurry A thin paste of suspended solidparticles in a liquid.

Smalley, Richard Errett (1943– )American chemist. In 1981 Smalley de-vised a procedure to produce microclustersof a hundred or so atoms, by vaporizing ametal using a laser. The released atoms arecooled by a jet of helium and condense intovariously sized clusters. In 1985 a visitingBritish chemist, Harold KROTO, persuaded

Smalley to direct his laser beam at agraphite target, leading to the discovery ofa new alloprope of carbon, now known asBUCKMINSTERFULLERENE.

smelting An industrial process for ex-tracting metals from their ores at high tem-peratures. Generally the ore is reducedwith carbon (for zinc and tin) or carbonmonoxide (for iron). Copper and lead areobtained by reduction of the oxide with thesulfide; for example:

2Cu2O + Cu2S → 6Cu + SO2A flux is also used to combine with impu-rities and form a SLAG on top of the moltenmetal.

smithsonite See calamine.

soap See detergents.

soapstone A soft type of TALC which hasa greasy feel and which is easy to carve tomake ornaments. It was formerly knownas steatite.

soda See sodium carbonate; sodium hy-droxide.

soda ash See sodium carbonate.

soda glass See glass.

soda lime A gray solid produced byadding sodium hydroxide solution to cal-cium oxide, to give a mixture of Ca(OH)2and NaOH on evaporation. It is used in thelaboratory as a drying agent and as an ab-sorbent for carbon dioxide.

sodamide (NaNH2) A white ionic solidformed by passing dry ammonia oversodium at 300–400°C. The compound re-acts with water to give sodium hydroxideand ammonia. It reacts with red-hot car-bon to form sodium cyanide and with ni-trogen(I) oxide to form sodium azide.Sodamide is used in the manufacture ofsodium hydroxide and in the explosives in-dustry.

soda water A solution of carbon diox-ide dissolved in water under pressure, used

SI units

204

205

BASE AND DIMENSIONLESS SI UNITS

Physical quantity Name of SI unit Symbol for SI unitlength meter mmass kilogram(me) kgtime second selectric current ampere Athermodynamic temperature kelvin Kluminous intensity candela cdamount of substance mole mol*plane angle radian rad*solid angle steradian sr*supplementary units

DERIVED SI UNITS WITH SPECIAL NAMES

Physical quantity Name of SI unit Symbol for SI unitfrequency hertz Hzenergy joule Jforce newton Npower watt Wpressure pascal Paelectric charge coulomb Celectric potential difference volt Velectric resistance ohm Ωelectric conductance siemens Selectric capacitance farad Fmagnetic flux weber Wbinductance henry Hmagnetic flux density tesla Tluminous flux lumen lmilluminance (illumination) lux lxabsorbed dose gray Gyactivity becquerel Bqdose equivalent sievert Sv

DECIMAL MULTIPLES AND SUBMULTIPLES USED WITH SI UNITS

Submultiple Prefix Symbol Multiple Prefix Symbol10–1 deci- d 101 deca- da10–2 centi- c 102 hecto- h10–3 milli- m 103 kilo- k10–6 micro- µ 106 mega- M10–9 nano- n 109 giga- G10–12 pico- p 1012 tera- T10–15 femto- f 1015 peta- P10–18 atto- a 1018 exa- E10–21 zepto- z 1021 zetta- Z10–24 yocto- y 1024 yotta- Y

in fizzy drinks. When the pressure is re-leased, the liquid effervesces with bubblesof carbon dioxide gas. See also carbonicacid.

sodium A soft silver-white reactive al-kali metal; the second element of group 1(formerly IA) of the periodic table. It hasthe electronic configuration of neon plusone additional outer 3s electron. The ele-ment undergoes electronic excitation inflames and in sodium-vapor lamps to givea distinctive yellow color arising from in-tense emission at the so called ‘sodium-D’line pair. The element’s ionization poten-tial is rather low and the sodium atom thusreadily loses its electron (i.e. the metal isstrongly reducing). The chemistry ofsodium is largely the chemistry of themonovalent Na+ ion.

Sodium occurs widely as NaCl in sea-water and as deposits of halite in dried-uplakes etc. (2.6% of the Earth’s crust). Theelement is obtained commercially via theDowns process by electrolysis of NaClmelts in which the melting point is reducedby the addition of calcium chloride;sodium is produced at the steel cathode.The metal is extremely reactive, vigorouslyso with the halogens and also with water,in the latter case to give hydrogen andsodium hydroxide. It is used as a coolant infast-breeder nuclear reactors. The chem-istry of sodium is very similar to that of theother members of group 1.

Nearly all sodium compounds are solu-ble in water. Sodium hydroxide is pro-duced commercially by the electrolysis ofbrine using diaphragm cells or mercury-cathode cells; chlorine is a coproduct.

Solutions of sodium metal in liquid am-monia are blue and have high electricalconductivities; the main current carrier ofsuch solutions is the solvated electron.Such solutions are used in both organic andinorganic chemistry as efficient reducingagents.The metal itself has a body-centered crystalstructure.

Symbol: Na; m.p. 97.81°C; b.p. 883°C;r.d. 0.971 (20°C); p.n. 11; most commonisotope 23Na; r.a.m. 22.989768.

sodium acetate See sodium ethanoate.

sodium aluminate (NaAlO2) A whitesolid produced in the laboratory by addingexcess aluminum to a hot concentrated so-lution of sodium hydroxide. Once the reac-tion has been initiated, sufficient heat isliberated to keep the reaction going; hy-drogen is also produced. In solution thealuminate ions have the structure Al(OH)4

–

and NaAl(OH)4 is known as sodium(I)tetrahydroxoaluminate(III). Addition ofsodium hydroxide to an aluminum salt so-lution produces a white gelatinous precipi-tate of aluminum hydroxide, whichdissolves in excess alkali to give a solutionof sodium(I) tetrahydroxoaluminate(III).Sodium aluminate is used in cleaning prod-ucts, in the treatment of waste effluent, andin zeolite production. It is also used as amordant.

sodium azide (NaN3) A white solid pre-pared by passing nitrogen(I) oxide overheated sodamide. On heating, sodiumazide decomposes to give sodium and ni-trogen. It is used as a reagent in organicchemistry and in the manufacture of explo-sives (detonators).

sodium bicarbonate See sodium hy-drogencarbonate.

sodium bisulfate See sodium hydrogen-sulfate.

sodium bisulfite See sodium hydrogen-sulfite.

sodium borate See disodium tetrabo-rate decahydrate.

sodium bromide (NaBr) A white solidformed by the action of bromine on hotsodium hydroxide solution. Alternatively,it can be made by the action of dilute hy-drobromic acid on sodium carbonate orhydroxide. Sodium bromide is soluble inwater and forms crystals similar in shape tosodium chloride. It is used in medicine, an-alytical chemistry, and photography.Sodium bromide forms a hydrate,NaBr.2H2O.

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sodium carbonate (soda ash; Na2CO3)A white amorphous powder, which aggre-gates on exposure to air owing to the for-mation of hydrates. Industrially it isprepared by the SOLVAY PROCESS. On crys-tallizing from aqueous solution largetranslucent crystals of the decahydrate(Na2CO3.10H2O) (washing soda) areformed. These effloresce quickly in air,forming the monohydrate Na2CO3.H2O.Large quantities of sodium carbonate areused in the manufacture of sodium hy-droxide. Sodium carbonate is also used inphotography, as a food additive, and in thetextile trade. Washing soda is used as a do-mestic cleanser. The salt produces an alka-line solution in water by hydrolysis:

Na2CO3 + 2H2O → 2NaOH + H2CO3It is used in volumetric analysis to stan-dardize strong acids.

sodium chlorate(V) (NaClO3) A whitesolid formed by the action of chlorine onhot concentrated sodium hydroxide solu-tion, or by the electrolysis of a concen-trated sodium chloride solution. It issoluble in water. Sodium chlorate is a pow-erful oxidizing agent. If heated it yieldsoxygen and sodium chloride. The chlorateis used in explosives, in matches, as a weedkiller, and in the textile industry.

sodium chloride (common salt; salt;NaCl) A white solid prepared by the neu-tralization of hydrochloric acid with aque-ous sodium hydroxide. It occurs in seawater and natural brines. Natural solid de-posits of rock salt (halite) are also found incertain places. It is an essential dietary min-eral. The compound dissolves in waterwith the absorption of heat, its solubilitychanging very little with temperature. It isused to season and preserve food. Industri-ally, it is used as a raw material in the man-ufacture of sodium carbonate via theSOLVAY PROCESS, sodium hydroxide viaelectrolysis, and soap. Sodium chloride isalmost insoluble in alcohol.

sodium-chloride structure A form ofcrystal structure that consists of a face-cen-tered cubic arrangement of sodium ionswith chloride ions situated at the middle of

each edge and in the center of the cube.Electrostatic attraction holds the oppo-sitely charged ions together.

The lattice can also be thought of astwo interpenetrating face-centered cubes,one composed of sodium ions and theother of chloride ions. Other compounds(e.g. sodium bromide and potassium chlor-ide) having their ions arranged in the samepositions are also described as having thesodium-chloride structure.

sodium cyanide (NaCN) An extremelypoisonous white solid formed either by theaction of carbon on sodamide at high tem-perature or by passing ammonia oversodium at 300–400°C to form sodamideand then reacting it with carbon. The in-dustrial salt is prepared by absorbing hy-drogen cyanide into a sodium hydroxidesolution and purifying the solid by recrys-tallization from liquid ammonia. It is usedas a source of cyanide and hydrocyanicacid. In the cyanide process it is used in theextraction of silver and gold. Its aqueoussolutions are alkaline due to salt hydroly-sis.

sodium dichromate(VI) (Na2Cr2O7)An orange deliquescent solid that is verysoluble in water. It is made from finelypowdered chromite, which is heated withcalcium oxide and sodium carbonate in afurnace. If an alkali is added to a solutionof the dichromate, the chromate is pro-duced. At high temperatures, sodiumdichromate decomposes to give the chro-mate, chromic oxide, and oxygen. It is used

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chloride ionsodium ion

Sodium-chloride structure

as an oxidizing agent, particularly in or-ganic chemistry, and in volumetric analysisto estimate iron(II) ions and iodide ions. Itis also used as a mordant.

sodium dihydrogen orthophosphateSee sodium dihydrogen phosphate(V).

sodium dihydrogen phosphate(V)(sodium dihydrogen orthophosphate;NaH2PO4) A white solid prepared bytitrating phosphoric acid with sodium hy-droxide solution using methyl orange asthe indicator. On evaporation white crys-tals of the monohydrate are formed. It isused in some baking powders. The mono-hydrate crystallizes in the orthorhombicsystem.

sodium dioxide See sodium superox-ide.

sodium ethanoate (sodium acetate;CH3COONa) A white solid prepared bythe neutralization of ethanoic acid withsodium carbonate or sodium hydroxide.Sodium ethanoate reacts with sulfuric acidto form sodium hydrogensulfate andethanoic acid; with sodium hydroxide itgives rise to sodium carbonate andmethane. Sodium ethanoate is used in thedyeing industry.

sodium fluoride (NaF) A toxic whitesolid formed by the action of hydrofluoricacid on sodium carbonate or sodium hy-droxide (the reaction between metallicsodium and fluorine is too violent).Sodium fluoride is soluble in water and re-acts with concentrated sulfuric acid toyield hydrogen fluoride. It is used as a con-stituent of ceramic enamels, as an antisep-tic, as an agent to prevent fermentation,and in minute amounts to fluoridate watersupplies.

sodium hexafluoroaluminate (Na3-AlF6) A compound found in nature asthe mineral cryolite. It is usually white orcolorless, but may be reddish or brown be-cause of impurities. It is used to makeenamels, opaque glass, and glazes for ce-ramics. It crystallizes in the monoclinic sys-

tem but in forms that closely resemblecubes and isometric octahedrals. Sodiumhexafluoroaluminate is also used as a fluxin the manufacture of aluminum.

sodium hydride (NaH) A white crys-talline solid prepared by passing a streamof pure dry hydrogen over sodium at350°C; the sodium is usually suspended inan inert medium. Electrolysis of fusedsodium hydride yields hydrogen at theanode, suggesting the presence of the H–

ion. Sodium hydride reacts vigorously withwater to form sodium hydroxide solutionand hydrogen. It is used as a powerful re-ducing agent to convert water to hydrogen,concentrated sulfuric acid to hydrogen sul-fide, and iron(III) oxide to iron. Sodiumhydride bursts into flames spontaneouslyon contact with the halogens at room tem-perature. It dissolves in liquid ammonia togive sodamide, NaNH2.

sodium hydrogencarbonate (sodium bi-carbonate; baking soda; NaHCO3) Awhite solid formed either by passing an ex-cess of carbon dioxide through sodium car-bonate or hydroxide solution, or byprecipitation when cold concentrated solu-tions of sodium chloride and ammoniumhydrogencarbonate are mixed. Sodium hy-drogencarbonate decomposes on heatingto give sodium carbonate, carbon dioxide,and water. With dilute acids, it yields car-bon dioxide. It is used as an antacid and asa constituent of baking powder, and in ef-fervescent beverages and fire extinguishers.It is also used in the textile, ceramics, andpaper-making industries. Its aqueous solu-tions are alkaline as a result of salt hydrol-ysis. Sodium hydrogencarbonate formsmonoclinic crystals.

sodium hydrogensulfate (sodium bisul-fate; NaHSO4) A white solid formed ei-ther by the partial neutralization of sulfuricacid with sodium hydroxide solution, bythe action of concentrated sulfuric acid onsodium nitrate, or by heating equimolarquantities of sodium chloride and concen-trated sulfuric acid. In aqueous solutionsodium hydrogensulfate is strongly acidic.It crystallizes as the monohydrate

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(NaHSO4.H2O), which dehydrates onwarming. If heated strongly the pyrosulfate(Na2S2O7) is formed, which decomposes togive the sulfate and sulfur(VI) oxide.Sodium hydrogensulfate is used as a cheapsource of sulfuric acid and in the dyeing in-dustry.

sodium hydrogensulfite (sodium bisul-fite; NaHSO3) A white powder preparedby saturating a solution of sodium carbon-ate with sulfur(IV) oxide. It is isolated fromthe aqueous solution by precipitation withalcohol. If heated it undergoes thermal de-composition to give sodium sulfate, sul-fur(VI) oxide, and sulfur. Sodiumhydrogensulfite is used to sterilize equip-ment used in the brewing and winemakingindustries, and also in medicine as an anti-septic.

sodium hydroxide (caustic soda; NaOH)A white deliquescent slightly translucentsolid. In solution it is a strong alkali andelectrolyte. In the laboratory it can be pre-pared by reacting sodium, sodium monox-ide, or sodium peroxide with water.Industrially it can be prepared by the elec-trolysis of sodium chloride using a cell hav-ing a mercury cathode or a cell in which theanode and cathode are separated by a di-aphragm. Sodium hydroxide dissolvesreadily in water with the evolution of heat.Its solutions have a soapy feel and are verycorrosive. It is used to absorb acidic gases,such as carbon dioxide and sulfur(IV)oxide, and to remove heavy metals fromsewage and industrial effluents. Industri-ally it is also used in soap and paper man-ufacture and in the purification of bauxite.

sodium iodide (NaI) A white solidformed by reacting sodium carbonate orsodium hydroxide with hydroiodic acid;the solution is then evaporated. Sodium io-dide forms colorless crystals having a cubicshape. It is used as a source of iodine andin medicine and photography. Acidified so-lutions of sodium iodide exhibit reducingproperties owing to the formation of hy-droiodic acid.

sodium metasilicate See sodium sili-cate.

sodium monoxide (Na2O) A whitishdeliquescent solid formed by burningsodium in a deficiency of oxygen or alter-natively by reducing sodium peroxide orsodium hydroxide with the requisiteamount of sodium. It reacts violently withwater to form sodium hydroxide and withacids to form solutions of their salts.Sodium monoxide forms cubic crystals. Itdissolves in liquid ammonia to give a mix-ture of sodamide and sodium hydroxide.

sodium nitrate (Chile saltpeter; NaNO3)A white solid formed by the neutralizationof nitric acid with either sodium carbonateor sodium hydroxide. It occurs naturally inlarge quantities in South America in im-pure mineral deposits called caliche.Sodium nitrate is very soluble in water andon crystallization forms colorless deliques-cent crystals. On heating it decomposes togive sodium nitrite and oxygen. Whenheated with concentrated sulfuric acid, ni-tric acid is produced. Sodium nitrate isused as a fertilizer and as a source of ni-trates and nitric acid. The salt crystallizesin the rhombohedral system and is isomor-phous with the iodate.

sodium nitrite (NaNO2) A yellowish-white solid formed by the thermal decom-position of sodium nitrate. It is readilysoluble in water. The anhydrous salt formsorthorhombic crystals. When treated withcold dilute hydrochloric acid, sodium ni-trite forms nitrous acid. It is used in or-ganic chemistry and industrially as acorrosion inhibitor.

sodium orthophosphate See trisodiumphosphate(V).

sodium peroxide (Na2O2) A yellowish-white ionic solid formed by the direct com-bination of burning sodium with excessoxygen. It reacts with water to formsodium hydroxide and hydrogen peroxide;the latter decomposes rapidly in the alka-line solution to give oxygen. Sodium per-oxide is used as a bleaching agent and an

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oxidizing agent for such materials as wooland wood pulp. Sodium peroxide can con-vert nitrogen(II) oxide to sodium nitrateand iodine to sodium iodate.

sodium silicate (sodium metasilicate;Na2SiO3.5H2O) A colorless crystallinesolid used in detergents and other cleaningcompounds, in refractories, and in cementsfor ceramics. A concentrated aqueous solu-tion is known as water glass and is used asa sizing agent and for making precipatedsilica and silica gel.

sodium sulfate (Na2SO4) A white solu-ble solid often formed in the laboratory byheating a mixture of sodium chloride andconcentrated sulfuric acid. Industrially it ismore frequently prepared by reacting mag-nesium sulfate with sodium chloride. Itforms two hydrates, the decahydrate(Na2SO4.10H2O) and the heptahydrate(Na2SO4.7H2O). Sodium decahydrate,found in nature as the mineral mirabilite,also known as Glauber’s salt, is used in themanufacture of glass and as a purgative inmedicine. It effloresces on exposure to airto form the anhydrous salt, which is usedas a drying agent in organic chemistry. Atroom temperature sodium sulfate crystal-lizes in the orthorhombic system; around250°C there is a transition to the hexago-nal system. The decahydrate crystallizes inthe monoclinic form.

sodium sulfide (Na2S) A yellow-redsolid formed by the reduction of sodiumsulfate using carbon (or carbon monoxideor hydrogen) at a high temperature. It is acorrosive material and deliquesces to re-lease hydrogen sulfide. Sodium sulfideforms hydrates containing 4½, 5, and 9H2O. In aqueous solution it is readily hy-drolyzed, the solution being strongly alka-line. Its reducing properties make it usefulin metallurgy and in the production ofdyestuffs and wood pulp.

sodium sulfite (Na2SO3) A white solidformed by reacting the exact amount ofsulfur(IV) oxide with either sodium car-bonate or sodium hydroxide. It is readilysoluble in water and crystallizes as color-

less crystals of the heptahydrate (Na2-SO3.7H2O). When treated with dilute min-eral acids, sulfur(IV) oxide is evolved. Athigh temperatures, sodium sulfite under-goes thermal decomposition to givesodium sulfate and sodium sulfide. It isused as a bleaching agent for textiles andpaper, and as an antioxidant in somecanned goods.

sodium superoxide (sodium dioxide;NaO2) A pale yellow solid obtained byheating sodium peroxide in oxygen at490°C. It reacts with water to give hydro-gen peroxide, sodium hydroxide solution,and oxygen. Commercial sodium peroxidecontains 10% sodium superoxide.

sodium thiosulfate(IV) (hypo; Na2S2O3)A white solid prepared either by boilingsodium sulfite with flowers of sulfur or bypassing sulfur(IV) oxide into a suspensionof sulfur in boiling sodium hydroxide.Sodium thiosulfate is readily soluble inwater and crystallizes as large colorlesscrystals of the pentahydrate(Na2S2O3.5H2O). It reacts with diluteacids to give sulfur and sulfur(IV) oxide. Itis used in photography as the fixative‘hypo’ (it was formerly known as sodiumhyposulfate) and industrially as an an-tichlor. In volumetric analysis, solutions ofsodium thiosulfate are usually preparedfrom the pentahydrate. On heating it dis-proportionates to give sodium sulfate andsodium sulfide.

soft iron Iron that has a low carboncontent and is unable to form permanentmagnets.

soft solder See solder.

soft water See hardness.

sol A COLLOID consisting of solid parti-cles distributed in a liquid medium. A widevariety of sols are known; the colors oftendepend markedly on the particle size. Theterm aerosol is used for solid or liquidphases dispersed in a gaseous medium.

solder An alloy used in joining metals.

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The molten solder wets the surfaces to bejoined, without melting them, and solidi-fies on cooling to form a hard joint. Thesurfaces must be clean and free of oxide, aresult usually achieved with the aid of aflux. Soft solders consist of lead with up to60% tin and melt in the range 183–250°C.Soft-soldering is used, for example, inplumbing and making electrical joints.Brazing solders are copper–zinc alloys thathave higher melting points and producestronger joints than soft solders; silver canbe added to produce silver solders.

solid The state of matter in which theparticles occupy fixed positions, giving thesubstance a definite shape. The particlesare held in these positions by bonds. Threekinds of attraction fix the positions of theparticles: electrovalent, covalent, and inter-molecular. Since these bonds act over shortdistances the particles in solids are packedclosely together. The strengths of thesethree types of bonds are different and so,therefore, are the mechanical properties ofdifferent solids.

solid solution A solid composed of twoor more substances mixed together at themolecular level. Atoms, ions, or moleculesof one component in the crystal are at lat-tice positions normally occupied by theother component. Certain alloys are solidSOLUTIONS of one metal in another. Iso-morphic salts can also sometimes formsolid solutions, as in the case of crystallinealums.

solubility Symbol: S The amount of onesubstance that can dissolve in another toform a saturated solution under specifiedconditions of temperature and pressure.Solubilities are stated as moles of solute perkilogram of solvent (molality), or as kilo-grams of solute per cubic meter of solvent(density). The solubility of a solid in a liq-uid generally increases with temperature,whereas that of a gas in a liquid generallydecreases. An increase in pressure on a gasabove a liquid leads to a proportional in-crease in the solubility of the gas. See alsoconcentration.

solubility product Symbol: Ks If anionic solid is in contact with its saturatedsolution, there is a dynamic equilibriumbetween solid and solution:

AB(s) ˆ A+(aq) + B–(aq)The equilibrium constant for this is givenby

[A+][B–]/[AB]The concentration of undissolved solid[AB] is also constant, so

Ks = [A+][B–]Ks is the solubility product of the salt (at agiven temperature). For a salt A2B3, for in-stance:

Ks = [A+]2[B–]3, etc.Solubility products are meaningful only forsparingly soluble salts. If the product ofions exceeds the solubility product, precip-itation occurs.

solute A material that is dissolved in asolvent to form a solution.

solution A liquid system of two or morespecies that are intimately dispersed withineach other at a molecular level. The systemis therefore totally homogeneous. Themajor component is called the solvent(generally liquid in the pure state) and theminor component is called the solute (gas,liquid, or solid).

The process occurs because of a directintermolecular interaction of the solventwith the ions or molecules of the solute.This interaction is called solvation. Part ofthe energy of this interaction appears as achange in temperature on dissolution. Seealso solid solution; solubility.

solvation The attraction of a solutespecies (e.g. an ion) for molecules of sol-vent. In water, for example, a positive ionwill be surrounded by water molecules,which tend to associate around the ion be-cause of attraction between the positivecharge of the ion and the negative part ofthe polar water molecule. The energy ofthis solvation (hydration in the case ofwater) is the ‘force’ needed to overcome theattraction between positive and negativeions when an ionic solid dissolves. The at-traction of the dissolved ion for solventmolecules may extend for several layers. In

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the case of transition metal elements, ionsmay also form complexes by coordinationto the nearest layer of molecules.

Solvay process (ammonia–sodaprocess) An industrial process for mak-ing sodium carbonate. The raw materialsare calcium carbonate and sodium chloride(with ammonia). The calcium carbonate isheated:

CaCO3 → CaO + CO2Carbon dioxide is bubbled into a solutionof sodium chloride saturated with ammo-nia, precipitating sodium hydrogencarbon-ate and leaving ammonium chloride insolution. The sodium hydrogencarbonateis then heated:

2NaHCO3 → Na2CO3 + H2O + CO2The ammonia is regenerated by heating theammonium chloride with the calciumoxide:

2NH4Cl + CaO → CaCl2 + 2NH3 +H2O

solvent A liquid capable of dissolvingother materials (solids, liquids, or gases) toform a solution. The solvent is generallythe major component of the solution. Sol-vents can be divided into classes, the mostimportant being the following:Polar. A solvent in which the moleculespossess a moderate to high dipole momentand in which polar and ionic compoundsare easily soluble. Polar solvents are usu-ally poor solvents for nonpolar com-pounds. For example, water is a goodsolvent for many ionic species, such assodium chloride or potassium nitrate, andpolar molecules, such as the sugars, butdoes not dissolve paraffin wax.Nonpolar. A solvent in which the mol-ecules do not possess a permanent dipolemoment and consequently will solvatenonpolar species in preference to polarspecies. For example, some organic com-pounds are good solvents for iodine andparaffin wax, but do not dissolve sodiumchloride.Amphiprotic. A solvent that undergoesself-ionization and can act both as a protondonor and as an acceptor. Water is a goodexample and ionizes according to:

2H2O = H3O+ + OH–

Aprotic. A solvent that can neither acceptnor yield protons. An aprotic solvent istherefore the opposite to an amphiproticsolvent.

solvent extraction (liquid–liquid extrac-tion) A method of removing a substancefrom solution by shaking it with and dis-solving it in a better solvent that is imisci-ble with the original solvent.

solvolysis A reaction between a com-pound and the solvent in which it is dis-solved. See also hydrolysis.

sorption See absorption.

specific Denoting a physical quantityper unit mass. For example, volume (V) perunit mass (m) is called specific volume (v):

V/m = vIn certain physical quantities the term

does not have this meaning: for example,specific gravity is more properly called rel-ative density.

specific gravity See relative density.

specific heat capacity Symbol: c Theamount of heat needed to raise a unit massof a substance by one degree. It is measuredin joules per kilogram per Kelvin (J kg–1K–1) in SI units.

specific rotatory power Symbol: αmThe rotation of plane-polarized light in de-grees produced by a 10-centimeter lengthof solution containing one gram of a givensubstance per milliliter of stated solvent.The specific rotatory power is a measure ofthe optical activity of substances in solu-tion. It is measured at 20°C using the D-line of sodium.

specific volume See specific.

spectra See spectrum.

spectral line A particular wavelength oflight emitted or absorbed by an atom, ion,or molecule. See line spectrum.

spectral series A group of related lines

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in the absorption or emission spectrum ofa substance. The lines in a spectral seriesarise when the transitions all occur be-tween one particular energy level and a setof different levels. See also Bohr theory.

spectrograph An instrument for pro-ducing a photographic record of a spec-trum.

spectrographic analysis A method ofanalysis in which the sample is excited elec-trically (by an arc or spark) and emits radi-ation characteristic of its componentatoms. This radiation is passed through aslit, dispersed by a prism or a grating, andrecorded as a spectrum, either photograph-ically or photoelectrically. The photo-graphic method was widely used forqualitative and semiquantitative work butphotoelectric detection also allows widequantitative application.

spectrometer 1. An instrument for ex-amining the different wavelengths presentin electromagnetic radiation. Typically,spectrometers have a source of radiation,which is collimated by a system of lensesand/or slits. The radiation is dispersed by aprism or grating, and recorded photo-graphically or by a photocell. There aremany types for producing and investigat-ing spectra over the whole range of theelectromagnetic spectrum. Often spec-trometers are called spectroscopes. See alsospectrophotometer.2. Any of various other instruments for an-alyzing the energies, masses, etc., of parti-cles. See mass spectrometer.

spectrophotometer A form of spec-trometer able to measure the intensity ofradiation at different wavelengths in aspectrum, usually in the visible, infrared,or ultraviolet regions.

spectroscope See spectrometer.

spectroscopy 1. The production andanalysis of spectra. There are many spec-troscopic techniques designed for investi-gating the electromagnetic radiationemitted or absorbed by substances. Spec-

troscopy, in various forms, is used foranalysis of mixtures, for identifying anddetermining the structures of chemicalcompounds, and for investigating energylevels in atoms, ions, and molecules. In thevisible and longer wavelength ultraviolet,transitions correspond to electronic energylevels in atoms and molecules. The shorterwavelength ultraviolet corresponds totransitions in ions. In the x-ray region,transitions in the inner shells of atoms orions are involved. The infrared region cor-responds to vibrational changes in mol-ecules, with rotational changes at longerwavelengths.2. Any of various techniques for analyzingthe energy spectra of beams of particles orfor determining mass spectra.

spectrum (plural spectra) 1. A range ofelectromagnetic radiation emitted or ab-sorbed by a substance under particular cir-cumstances. In an emission spectrum, lightor other radiation emitted by the object isanalyzed to determine the particular wave-lengths produced. The emission of radia-tion may be induced by a variety ofmethods; for example, by high tempera-ture, bombardment by electrons, absorp-tion of higher-frequency radiation, etc. Inan absorption spectrum a continuous flowof radiation is passed through the sample.The radiation is then analyzed to deter-mine which wavelengths are absorbed. Seealso band spectrum; continuous spectrum;line spectrum.2. In general, any distribution of a prop-erty. For instance, a beam of particles mayhave a spectrum of energies. A beam ofions may have a mass spectrum (the distri-bution of masses of ions). See mass spec-trometer.

spelter Commercial zinc containingabout 3% impurities, mainly lead.

spin A property of certain elementaryparticles whereby the particle acts as if itwere spinning on an axis; i.e. it has an an-gular momentum. Such particles also havea magnetic moment. In a magnetic field thespins line up at an angle to the field direc-tion and precess around this direction. Cer-

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tain definite orientations to the field direc-tion occur such that msh/2π is the compo-nent of angular momentum along thisdirection. Here ms is the spin quantumnumber, which for an electron has values+1/2 and –1/2, and h is the Planck con-stant.

spin–orbit coupling The coupling be-tween the spin angular momentum and theorbital angular momentum of an electron.This is a FINE STRUCTURE feature of atomicspectra.

spin paired See electron pair.

spin quantum number See atom; spin.

square-planar See complex.

stabilization energy The difference inenergy between the delocalized structureand the conventional structure for a com-pound. The stabilization energy can be de-termined by comparing the experimentalvalue for the heat of formation with thatcalculated for a conventional structure.

stabilizer A substance added to preventchemical change (i.e. a negative catalyst).

stainless steel See steel.

stalactites and stalagmites Pillars ofcalcium carbonate found hanging from theceiling (stalactites) and standing on thefloor (stalagmites) in limestone caverns.They form when water containing dis-solved carbon dioxide forms weak car-bonic acid, which is able to react withlimestone rock (calcium carbonate) to givea solution of calcium hydrogencarbonate.This solution accumulates in drops on theroof of caves. As it evaporates, the reactionis reversed, causing calcium carbonate tobe deposited. Over a very long period,these deposits build up to create a stalactitehanging from the cavern roof. Water thatdrips onto the floor from the tip of this sta-lactite also evaporates to leave a deposit ofcalcium carbonate, and this deposit gradu-ally grows to form a stalagmite. Stalag-

mites and stalactites can eventually meet toform a single rock pillar.

standard cell A voltaic cell whose e.m.f.is used as a standard. See Clark cell; We-ston cadmium cell.

standard electrode A half cell used formeasuring electrode potentials. The hydro-gen electrode (standard hydrogen half cell)is the basic standard but, in practice,calomel electrodes are usually used.

standard hydrogen half cell See hy-drogen electrode.

standard pressure An internationallyagreed value of 101 325 Pa (approximately100 kPa), equal in non-SI units to a baro-metric height of 760 millimeters of mer-cury at 0°C or one ATMOSPHERE.

standard solution A solution that con-tains a known mass of reagent in a definitevolume of solution. A standard flask orvolumetric flask is used for this purpose.The solutions may be prepared by directdetermination of mass for primary stan-dards. If the reagent is not available in apure form or is deliquescent the solutionmust be standardized by titration againstanother known standard solution. See pri-mary standard.

standard state The standard conditionsused as a reference system in thermody-namics: pressure is 101 325 Pa; tempera-ture is 25°C (298.15 K); concentration isone mol. The substance under investiga-tion must also be pure and in its usualstate, given the above conditions.

standard temperature An internation-ally agreed value for which many measure-ments are quoted. It is the meltingtemperature of water, 0°C (273.15 K). Seealso STP.

stannane (tin(IV) hydride; SnH4) A col-orless poisonous volatile reactive gas pre-pared by the action of lithiumtetrahydroaluminate(III) on tin(II) chlor-ide. It is unstable, decomposing immedi-

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ately at 150°C. Stannane is used as a re-ducing agent.

stannate See tin.

stannic chloride See tin(IV) chloride.

stannic compounds Compounds oftin(IV).

stannite See tin.

stannous chloride See tin(II) chloride.

stannous compounds Compounds oftin(II).

Stark effect The splitting of atomicspectral lines due to the presence of an ex-ternal electric field. This phenomenon wasdiscovered by the German physicist Jo-hannes Stark (1874–1957) in 1913 and canbe explained using classical electron theoryand the BOHR THEORY of the atom.

states of matter The three physical con-ditions or phases in which substancesoccur: solid, liquid, and gas. The additionor removal of energy (usually in the formof heat) enables one state to be convertedinto another.

The major distinctions between thestates of matter depend on the kinetic ener-gies of their particles and the distances be-tween them. In solids, the particles havelow kinetic energy and are closely packed;in gases they have high kinetic energy andare very loosely packed; kinetic energy andseparation of particles in liquids are inter-mediate.

Solids, for instance, have fixed shapesand volumes, i.e. they do not flow like liq-uids and gases, and they are difficult tocompress. In solids the atoms or moleculesoccupy fixed positions in space. In mostcases there is a regular pattern of atoms,meaning that the solid is crystalline.

Liquids have fixed volumes (i.e. lowcompressibility) but flow to take up theshape of their container. Their atoms ormolecules move about at random, but theyare quite close to one another and theirmotion is hindered.

Gases have no fixed shape or volume.They expand spontaneously to fill the con-tainer and are easily compressed. The mol-ecules have almost free random motion.

A PLASMA is sometimes considered to bea fourth state of matter.

stationary phase See chromatography.

statistical mechanics The subject thatlinks the microscopic and macroscopic as-pects of a system by using statistical meth-ods to relate the properties of the very largenumber of microscopic particles, such asatoms and molecules, to the thermody-namics of the system. The derivation of theMAXWELL–BOLTZMANN DISTRIBUTION in thesecond half of the 19th century can be con-sidered to be the start of statistical me-chanics.

steam distillation A method of isolat-ing or purifying substances by exploitingDalton’s law of partial pressures to lowerthe boiling point of the mixture. When twoimmiscible liquids are distilled, the boilingpoint will be lower than that of the morevolatile component and consequently willbe below 100°C if one component is water.The method is particularly useful for re-covering materials from tarry mixtures.

steatite See soapstone.

steel An alloy of iron with smallamounts of carbon and, often, other met-als. Carbon steels contain 0.05–1.5% car-bon – the more carbon, the harder the steel.Alloy steels also contain small amounts ofother elements, such as chromium, man-ganese, and vanadium. Their properties de-pend on the composition. Stainless steels,for instance, are resistant to corrosion. Themain nonferrous constituent is chromium(10–25%) with up to 0.7% carbon.

step An elementary stage in a chemicalreaction, in which energy may be trans-ferred from one molecule to another,bonds may be broken or formed, or elec-trons may be transferred. For example, inthe reaction between hypochlorite ions and

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step

iodide ions in aqueous solution there arethree steps:Step 1

OCl–(aq) + H2O(l) → HOCl(aq) +OH–(aq)

Step 2I–(aq) + HOCl(aq) → HOI(aq) + Cl–(aq)

Step 3OH–(aq) + HOI(aq) → H2O(l) + OI–(aq)

steradian Symbol: sr The SI dimension-less unit of solid angle, equal to that sub-tended at the center of a sphere by the areaof a square on the sphere’s surface whosesides are equal in length to the radius of thesphere.

stereochemistry The branch of chem-istry concerned with the shapes of mol-ecules and the way these affect thechemical properties.

stereoisomerism See isomerism.

stereospecific Describing a chemical re-action involving an asymmetric atom (usu-ally carbon) that results in only onegeometric isomer. See isomerism; opticalactivity.

steric effect An effect in which theshape of a molecule influences its reac-tions. A particular example occurs in mol-ecules containing large groups, whichhinder the approach of a reactant (sterichindrance).

steric hindrance See steric effect.

still An apparatus for distillation.

Stock, Alfred (1876–1946) Germanchemist. Stock began studying boron hy-drides in 1909. Boron had previously beenlittle studied and was thought to react onlywith strongly electronegative elements,such as oxygen. However Stock found thatmagnesium boride and acid producedB4H10, the first of the several boron hy-drides that he discovered. It was clear fromthis and the other hydrides, B2H6 andB10H14, that the bond between boron andhydrogen could not be the familiar cova-

lent bond which occurs between carbonand hydrogen. Its actual form was notsolved until the work of William LIPSCOMB

in the 1950s.

stoichiometric coefficient See chemi-cal equation.

stoichiometric compound See stoi-chiometry.

stoichiometry The proportions inwhich elements form compounds. A stoi-chiometric compound is one in which theatoms have combined in small whole num-bers.

storage battery See accumulator.

STP (NTP) Standard temperature andpressure. Conditions used internationallywhen measuring quantities that vary withboth pressure and temperature (such as thedensity of a gas). The values are 101 325pascals (Pa) (approximately 100 kPa) and0°C (273.15 kelvin). See also standardpressure; standard temperature.

straight chain See chain.

strong acid An acid that is almost com-pletely dissociated into its component ionsin solution. Compare weak acid.

strong base A base that is completely oralmost completely dissociated into its com-ponent ions in solution.

strontia See strontium oxide.

strontium A soft reactive alkaline earthmetal; the fourth member of group 2 (for-merly IIA) of the periodic table. The elec-tronic configuration is that of krypton withtwo additional outer 5s electrons. Stron-tium is of low abundance in the Earth’scrust, strontium occurring in the mineralsstrontianite (SrCO3) and celestite (SrSO4).

The element is produced industrially byroasting the carbonate to give the oxideand then reducing the oxide with alu-minum:

3SrO + 2Al → Al2O3 + 2Sr

steradian

216

Strontium has a low ionization poten-tial, has large atoms, and is therefore veryelectropositive. Its chemistry is thereforecharacterized by high reactivity. The prop-erties of strontium fall into sequence withother alkaline earths. Thus it reacts directlywith oxygen (finely divided strontium willignite in air), nitrogen, sulfur, the halogens,and hydrogen to form respectively theoxide SrO, nitride Sr3N2, sulfide SrS,halides SrX2, and hydride SrH2, all ofwhich are largely ionic in character. Theoxide SrO and the metal react readily withwater to form the hydroxide Sr(OH)2,which is basic and between Ca(OH)2 andBa(OH)2 in terms of solubility. The car-bonate and sulfate are both insoluble.

As the metal is very electropositive thesalts are never much hydrolyzed in solutionand the ions are largely solvated as[Sr(H2O)6]2+.

Strontium is used in special alloys andto help create high vacuums. Its com-pounds flame bright red and thus find usein fireworks and flares. The fission of ura-nium in atomic bombs creates the long-lived radioactive isotopes 89Sr and 90Sr,which can accumulate in bone (in prefer-ence to calcium) and cause cancer.

Symbol: Sr; m.p. 769°C; b.p. 1384°C;r.d. 2.54 (20°C); p.n. 38; most commonisotope 88Sr; r.a.m. 87.62.

strontium bicarbonate See strontiumhydrogencarbonate.

strontium carbonate (SrCO3) A whiteinsoluble solid that occurs naturally as themineral strontianite. It can be prepared bypassing carbon dioxide over strontiumoxide or hydroxide or by passing the gasthrough a solution of a strontium salt.Strontium carbonate is then formed as aprecipitate. It is used as a slagging agent incertain metal furnaces and to produce a redcolor in fireworks, and also as a phosphorfor cathode-ray screens.

strontium chloride (SrCl2) A white del-iquescent solid obtained directly fromstrontium and chlorine or by passing chlo-rine over heated strontium oxide. It is usedin fireworks and flares to produce a red

color. The hexahydrate (SrCl2.6H2O) isprepared by the neutralization of hy-drochloric acid by strontium hydroxide orcarbonate.

strontium hydrogencarbonate (stron-tium bicarbonate; Sr(HCO3)2) A com-pound present in solutions, formed by theaction of carbon dioxide on a suspension incold water of strontium carbonate, towhich it reverts on heating:

SrCO3 + CO2 + H2O = Sr(HCO3)2

strontium hydroxide (Sr(OH)2) Asolid that normally occurs as the octahy-drate (Sr(OH)2.8H2O), which is preparedby crystallizing an aqueous solution ofstrontium oxide. Strontium hydroxide isreadily soluble in water and its aqueous so-lutions are strongly basic. It is used in thepurification of sugar.

strontium monoxide See strontiumoxide.

strontium nitrate (Sr(NO3)2) A color-less crystalline compound, often used inpyrotechnics to produce a bright red flame.

strontium oxide (strontia; strontiummonoxide SrO) A grayish-white powderprepared by the thermal decomposition ofstrontium carbonate, hydroxide, or nitrate.Strontium oxide is soluble in water, form-ing an alkaline solution because of the at-traction between the oxide ions andprotons from the water:

O2– + H2O → 2OH–

It is used as a drying agent and to makevarious strontium salts, as well as in pig-ments, soaps, and lubricants.

strontium sulfate (SrSO4) A whitesparingly soluble salt that occurs naturallyas the mineral celestite. It can be preparedby dissolving strontium oxide, hydroxide,or carbonate in sulfuric acid. It is used as asubstitute for barium sulfate in paints. It isalso used in fireworks to impart a red colorand in paints and glazes.

structural formula The formula of acompound showing the numbers and types

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structural formula

of atoms present, together with the way inwhich these are arranged in the molecule.Often this arrangement can be shown bygrouping the atoms, as in the structuralformula for hydrogen peroxide (HOOH),or by illustrating their orientation towardeach other in space. Compare empiricalformula; molecular formula.

structural isomerism See isomerism.

sublimate A solid formed by sublima-tion.

sublimation The conversion of a solidinto a vapor without the solid first melting.For instance, at standard pressure iodine,solid carbon dioxide, and ammoniumchloride sublime. At certain conditions ofexternal pressure and temperature an equi-librium can be established between thesolid phase and vapor phase.

subshell A subdivision of an electronshell. It is a division of the orbitals thatmake up a shell into sets of orbitals that aredegenerate (i.e. have the same energy) inthe free atom. For example, in the third orM-shell there are the 3s, 3p, and 3d sub-shells. The 3p subshell has three p orbitalsand the 3d sub shell has five d orbitals, in-dicating that they are 3- and 5-degenerate,respectively.

substitution reaction A reaction inwhich an atom or group of atoms in an or-ganic molecule is replaced by another atomor group.

substrate A material that is acted on bya catalyst.

sulfate A salt or ester of sulfuric acid.

sulfide A compound of sulfur with amore electropositive element. Simple sul-fides contain the ion S2–. Polysulfides canalso be formed containing ions with chainsof sulfur atoms (Sx

2–).

sulfinate See dithionite.

sulfinic acid See dithionous acid.

sulfite A salt or ester of sulfurous acid.

sulfonamide A type of organic com-pound with the general formulaR.SO2.NH2. Sulfonamides, which areamides of sulfonic acids, are active againstbacteria, and some are used in pharmaceu-ticals (‘sulfa drugs’).

sulfonate A salt or ester of sulfonic acid.See sulfonic acid.

sulfonation A reaction introducing the–SO2OH (sulfonic acid) group into an or-ganic compound. Sulfonation of aromaticcompounds is usually accomplished by re-fluxing with concentrated sulfuric acid forseveral hours. The attacking species is SO3(sulfur(VI) oxide) and the reaction is an ex-ample of electrophilic substitution.

sulfonic acid A type of organic com-pound containing the –SO2.OH group.The simplest example is benzenesulfonicacid C6H5SO2OH). Sulfonic acids arestrong acids. Electrophilic substitution canintroduce other groups onto the benzenering; the –SO2.OH group directs sub-stituents into the 3-position.

sulfonium compound An organiccompound of general formula R3SX,where R is an organic radical and X is anelectronegative radical or element; it con-tains the ion R3S+. An example is diethyl-methylsulfonium chloride, (C2H5)2.CH3.-S+Cl–, made by reacting diethyl sulfide withchloromethane.

sulfoxide An organic compound of gen-eral formula RSOR′, where R and R′ areorganic radicals. An example is dimethylsulfoxide, (CH3)2SO, commonly used as asolvent.

sulfur A nonmetallic solid element, yel-low in its common forms; the second mem-ber of group 16 (formerly VIA) of theperiodic table. It has the electronic config-uration [Ne]3s23p4.

Sulfur occurs in elemental form in Sicilyand some southern states of the USA, andin large quantities in combined forms such

structural isomerism

218

as sulfide ores (e.g. iron pyrite, FeS2) andsulfate minerals (e.g. anhydrite, (CaSO4)).It forms about 0.5% of the Earth’s crust.Elemental sulfur is extracted commerciallyby the Frasch process, in which super-heated water is forced down the outer ofthree concentric tubes leading into the de-posit. This causes the sulfur to melt. Air isthen blown down the central tube, causingthe molten sulfur to be forced out of thewell, where it is collected and cooled at thewell head. In contrast, the large amount ofsulfur obtained from the smelting of sulfideores or roasting of sulfates is not obtainedas elemental sulfur but is rather used di-rectly as SO2/SO3 for conversion to sulfuricacid via the contact process, which hasmany uses.

Sulfur exhibits allotropy and its struc-ture in all its phases is quite complex. Thecommon crystalline modification, rhombicsulfur, is in equilibrium with a triclinicmodification above 96°C. Both have struc-tures based on S8 rings, but the crystals arequite different. If molten sulfur is pouredinto water a dark red ‘plastic’ variety is ob-tained in a semielastic form. Its structureappears to be a helical chain of S atoms.Unlike selenium and tellurium, sulfur doesnot appear to have a gray ‘metal-like’ al-lotrope.

Sulfur reacts directly with hydrogen butthe position of the equilibrium at normaltemperatures precludes the use of this reac-tion as a preparative method for hydrogensulfide. The element burns readily in oxy-gen with a characteristic blue flame to formsulfur(IV) oxide (SO2) and traces of sul-fur(VI) oxide (SO3). Sulfur(IV) oxide isused as a food preservative, and to sterilizefruit juices and barrels used to age winesand spirits. Sulfur also forms a large num-ber of oxyacid species, some containingperoxide groups and some with two ormore S atoms. It forms four distinct fluo-rides, S2F2, SF4, SF6, S2F10; three chlorides,S2Cl2, SCl2, SCl4; a bromide S2Br2, but noiodide. Apart from SF6, which is surpris-ingly stable and inert, the halides are gen-erally susceptible to hydrolysis, commonlyto give SO2, sometimes H2S, and the hy-drogen halide. The halides SF4, S2Cl2, andSCl2 are frequently used as selective fluori-

nating agents and chlorinating agents inorganic chemistry. The chlorides are alsoindustrially important for hardening rub-bers. Sulfur hexafluoride may be preparedby direct reaction of the elements but SF4 isprepared by fluorinating SCl2 with NaF. Ina limited supply of chlorine, sulfur com-bines to give sulfur monochloride, S2Cl2; inexcess chlorine at higher temperatures thedichloride is formed,

2S + Cl2 → S2Cl2S2Cl2 + Cl2 → 2SCl2

In addition to these binary halides, sul-fur forms two groups of oxyhalides; thesulfur dihalide oxides (thionyl halides),SOX2 (F, Cl, and Br, but not I), and the sul-fur dihalide dioxides (sulfuryl halides),SO2X2 (F and Cl, but not Br and I).

Most metals react with sulfur to formsulfides, of which there are a large numberof structural types. With electropositive el-ements of groups 1 and 2 the sulfides arelargely ionic (S2–) in the solid phase. Hy-drolysis occurs in solution thus:

S2– + H2O → SH– + OH–

Most transition metals form sulfides,which although formally treated as FeS,CoS, NiS, etc., are largely covalent and fre-quently nonstoichiometric.

In addition to the complexities of thebinary compounds with metals, sulfurforms a range of binary compounds withelements such as Si, As, and P, which maybe polymeric and generally of rather com-plex structure, e.g. (SiS4)n, (SbS3)n, N4S4,As4S4, P4S3.

Sulfur is used in fungicides and in thevulcanizing process for rubber. It alsoforms a wide range of organic sulfur com-pounds, most of which have the typicallyrevolting smell of H2S.

Symbol: S; m.p. 112.8°C; b.p. 444.6°C;r.d. 2.07; p.n. 16; most common isotope32S; r.a.m. 32.066.

sulfur dichloride dioxide (sulfuryl chlor-ide; SO2Cl2) A colorless fuming liquidformed by the reaction of chlorine with sul-fur(IV) oxide in sunlight. It is used as achlorinating agent.

sulfur dioxide See sulfur(IV) oxide.

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sulfur dioxide

sulfuretted hydrogen See hydrogensulfide.

sulfuric(IV) acid See sulfurous acid.

sulfuric acid (oil of vitriol; H2SO4) Acolorless oily liquid manufactured by theCONTACT PROCESS. The concentrated acid isdiluted by adding it slowly to water, withcareful stirring. Concentrated sulfuric acidacts as an oxidizing agent, giving sulfur(IV)oxide as the main product, and also as adehydrating agent. The diluted acid acts asa strong dibasic acid, neutralizing basesand reacting with active metals and car-bonates to form sulfates.

Sulfuric acid is used in the laboratory todry gases (except ammonia) and to preparenitric acid. In industry, it is used to manu-facture fertilizers (e.g. ammonium sulfate),explosives, chemicals, rayon, pigments,and detergents. It is also used to clean met-als, and as the electrolyte in vehicle batter-ies.

sulfur monochloride See disulfurdichloride.

sulfurous acid (sulfuric(IV) acid; trioxo-sulfuric(IV) acid; H2SO3) A weak acidfound only in solution, made by passingsulfur(IV) oxide into water. The solution isunstable and smells of sulfur(IV) oxide. Itis used as a reducing agent, convertingiron(III) ions to iron(II) ions, chlorine tochloride ions, and orange dichromate(VI)ions to green chromium(III) ions.

sulfur(IV) oxide (sulfur dioxide;SO2) A colorless choking gas preparedby burning sulfur or by heating metal sul-fides in air, or by treating a sulfite with anacid. It is a powerful reducing agent, usedas a bleach. It is also used for sterilizingand as a food preservative. It dissolves inwater to form sulfurous acid (sulfuric(IV)acid) and combines with oxygen, in thepresence of a catalyst, to form sulfur(VI)oxide. This latter reaction is important inthe manufacture of sulfuric(VI) acid.

sulfur(VI) oxide (sulfur trioxide; SO3) Afuming volatile white solid prepared by

passing sulfur(IV) oxide and oxygen overhot vanadium(V) oxide, which acts as acatalyst, and then cooling the product inice. Sulfur(VI) oxide reacts vigorously withwater to form sulfuric acid. See also con-tact process.

sulfur trioxide See sulfur(VI) oxide.

sulfuryl Describing a compound con-taining the group =SO2.

sulfuryl chloride See sulfur dichloridedioxide.

superconductivity The property ofzero electrical resistance exhibited by cer-tain metals and metallic compounds at lowtemperatures. Metals superconduct at tem-peratures close to absolute zero but a num-ber of ceramic metal oxides are nowknown that superconduct at higher (94Kor better) temperatures. Synthetic organicsuperconductors have also been produced.

supercooling The cooling of a liquid ata given pressure to a temperature below itsmelting temperature at that pressure with-out solidifying it. The liquid particles loseenergy but do not spontaneously fall intothe regular geometrical pattern of the solid.A supercooled liquid is in a metastablestate and will usually solidify if a smallcrystal of the solid is introduced to act as a‘seed’ for the formation of crystals. As soonas this happens, the temperature returns tothe melting temperature until the substancehas completely solidified.

superfluidity A property of liquid he-lium at very low temperatures (2.186 K)when it makes a transition to a superfluidstate in which it has a high thermal con-ductivity and flows without friction. Seealso helium.

superheating The raising of a liquid’stemperature above its boiling temperature,accomplished by increasing the pressure.

supernatant Denoting a clear liquidthat lies above a sediment or a precipitate.

sulfuretted hydrogen

220

superoxide An inorganic compoundcontaining the O2

– ion.

superphosphate A mixture – mainlycalcium hydrogen phosphate and calciumsulfate – used as a fertilizer. It is made fromcalcium phosphate and either sulfuric acidor phosphoric(V) acid.

supersaturated solution See saturatedsolution.

supersaturated vapor See saturatedvapor.

supramolecular chemistry A branchof chemistry concerned with the synthesisand study of large structures consisting ofmolecules assembled together in a definitepattern. In a supramolecule the molecularunits (which are sometimes known as syn-thons) are joined by intermolecular bonds– i.e. by hydrogen bonds or by ionic attrac-tions. A particular interest in supramolecu-lar research is the idea of ‘self-assembly’ –i.e. that the molecules form well-definedstructures spontaneously as a result of theirgeometry and chemical properties. In thisway, supramolecular chemistry is ‘chem-istry beyond molecules’.

One type of supramolecule is a helicate,which has a double helix made of twochains held by copper ions along its axis.The structure is analogous to the doublehelix of DNA. See also host–guest chem-istry.

supramolecule See supramolecular chem-istry.

surfactant A substance that lowers sur-face tension and has properties of wetting,foaming, detergency, dispersion, and emul-sification, e.g. a soap or other detergent.

suspension A system in which smallparticles of a solid or liquid are dispersed ina liquid or gas.

sylvite See potassium chloride.

symbol, chemical See chemical symbol.

symmetry The property of an objectthat certain operations, called symmetryoperations, change the object to a state inwhich it is indistinguishable from its origi-nal state. Examples of symmetry opera-tions are rotation about an axis, reflectionthrough a plane, and inversion through apoint. A symmetry element is a geometricalfeature of the object with respect to whichthe symmetry operation is carried out. Forexample, an axis of rotation, a plane of re-flection, and a point through which inver-sion can be performed are symmetryelements. A symmetry operation has to beassociated with a symmetry element andvice versa. See also molecular symmetry.

syn-anti isomerism See isomerism.

synthesis The preparation of chemicalcompounds from simpler compounds.

synthesis gas A mixture of carbonmonoxide and hydrogen produced bysteam reforming of natural gas.

CH4 + H2O → CO + 3H2

Synthesis gas is a useful starting ma-terial for the manufacture of a number oforganic compounds.

synthon See supramolecular chemistry.

Système International d’Unités See SIunits.

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Système International d’Unités

talc (French chalk) An extremely softgreenish or white mineral, a hydrous sili-cate of magnesium (Mg3Si4O10(OH)2). Pu-rified it is sold as talcum powder. It is alsoused as a lubricant, an electrical insulator,a filler (in paint, rubber, and paper) and aningredient of some ceramics.

tantalum A hard ductile rare silverytransition metal, the third member ofgroup 5 (formerly VB) of the periodictable. It occurs chiefly in the mineralscolombite and tantalite, in association withniobium. It is strong, highly resistant tocorrosion, and easily worked. Tantalum isused in turbine blades and cutting tools,and in surgical and dental work. It is alsoused in electronic components and in fila-ments for light bulbs subject to shock andvibration.

Symbol: Ta; m.p. 2996°C; b.p. 5425 ±100°C; r.d. 16.654 (20°C); p.n. 73; mostcommon isotope 181Ta; r.a.m. 180.9479.

tartar emetic (potassium antimonyl tar-trate; KSbO(C4H4O6).¼H2O) A poiso-nous substance used as an emetic inmedicine and as a mordant in dyeing andas an insecticide.

Taube, Henry (1915– ) American in-organic chemist. Taube has developed anumber of experimental techniques forstudying the kinetics and mechanism of in-organic reactions, in particular electron-transfer reactions. Transition metals suchas iron, copper, cobalt, and molybdenumform coordination compounds of a typefirst described by Alfred WERNER. In a typ-ical coordination compound a metal ion isattached to a number of ligands, such aswater or ammonia. It was thought that theligands would keep the ions apart and in-

hibit electron transfer between ions. Taubeshowed experimentally that ligand bridgesform between interacting complexes, thusallowing electrons to be transferred. Forhis work in this field Taube was awardedthe 1983 Nobel Prize for chemistry.

tautomerism Isomerism in which eachisomer can convert into the other, so thatthe two isomers are in equilibrium. Theisomers are called tautomers. Tautomerismoften results from the migration of a hy-drogen atom.

technetium A silver-gray radioactivetransition metal, the second element ofgroup 7 (formerly VIIB) of the periodictable. It does not occur naturally on Earthalthough it has been detected in the spectraof stars. It is produced artificially by bom-barding molybdenum with deuterium nu-clei or neutrons, and also during the fissionof uranium. It is used in medicine as a ra-dioactive label.

Symbol: Tc; m.p. 2172°C; b.p. 4877°C;r.d. 11.5 (est.); p.n. 43; r.a.m. 98.9063(99Tc); most stable isotope 98Tc (half-life4.2 × 106 years).

telluride A compound of tellurium andanother element. See tellurium.

tellurium A brittle silvery semiconduct-ing metalloid element; the fourth memberof group 16 (formerly VIA) of the periodictable. It is found chiefly in combinationwith metals in minerals known as tel-lurides. Commercially it is recovered pri-marily during the refining of copper, lead,and gold. Tellurium is used mainly as anadditive to improve the qualities of stain-less steel and various metals. It is also usedin semiconductors, and as a catalyst in the

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T

petroleum industry. Tellurium compoundsare poisonous.

Symbol: Te; m.p. 449.5°C; b.p.989.8°C; r.d. 6.24 (20°C); p.n. 52; mostcommon isotope 128Te; r.a.m. 127.6.

temperature scale A practical scaleagainst which temperature can be mea-sured. A temperature scale is determinedby one or more fixed points, which are re-producible and are assigned an agreed tem-perature. On the Celsius scale, forexample, the fixed points are the tempera-ture of pure melting ice (the ice tempera-ture) and the temperature of pure boilingwater (the steam temperature). The differ-ence between these fixed points is subdi-vided into temperature interval units. TheInternational Practical Temperature Scalehas 11 fixed points that cover the range13.81 kelvin (K) to 1337.58 K.

temporary hardness A type of waterhardness caused by the presence of dis-solved calcium, iron, and magnesium hy-drogencarbonates. This form of hardnesscan be removed by boiling the water. Tem-porary hardness arises because rainwatercombines with carbon dioxide from the at-mosphere to form weak carbonic acid.This acid reacts with carbonate rocks, suchas limestone, to produce calcium hydro-gencarbonate, which then goes into solu-tion. When this solution is heated, thecomplete reaction sequence is reversed sothat calcium carbonate is precipitated. Ifthis compound is not removed, it will ac-cumulate as scale in boilers and hot-waterpipes and reduce their efficiency. Comparepermanent hardness. See also water soften-ing.

tera- Symbol: T A prefix used with SIunits denoting 1012. For example, 1 ter-awatt (TW) = 1012 watts (W).

terbium A soft ductile malleable silver-gray rare element of the lanthanoid seriesof metals. It occurs in association withother lanthanoids in minerals such asgadolinite, monazite, and xenotime. It isalso found in apatite. One of its few uses isas a dopant in solid-state devices. Terbium

compounds are also used in lasers and incolor television sets.

Symbol: Tb; m.p. 1356°C; b.p. 3123°C;r.d. 8.229 (20°C); p.n. 65; only natural iso-tope 159Tb; r.a.m. 158.92534.

ternary compound A chemical com-pound formed from three elements; e.g.Na2SO4 or LiAlH4.

tervalent (trivalent) Having a valence ofthree.

tesla Symbol: T The SI derived unit ofmagnetic flux density, equal to a flux den-sity of one weber of magnetic flux persquare meter. 1 T = 1 Wb m–2. It is namedfor the Croatian-born electrical engineerand inventor Nicola Tesla (1857–1943).

tetracarbonyl nickel(0) See nickel car-bonyl.

tetraethyl lead See lead tetraethyl.

tetragonal crystal See crystal system.

tetrahedral compound A compoundsuch as methane in which an atom has fourbonds directed toward the corners of a reg-ular tetrahedron. The angles between thebonds in such a compound are about 109°.

tetrahydrate A crystalline hydratedcompound containing four molecules ofwater of crystallization per molecule ofcompound. See also complex.

tetravalent (quadrivalent) Having a va-lence of four.

thallium A soft malleable bluish-whitemetallic element belonging to group 13(formerly IIIA) of the periodic table. It isfound in lead, zinc, and cadmium ores, andin pyrites (e.g. FeS2). Thallium and its com-pounds are highly toxic and were used pre-viously as a rodent and insect poison.Various compounds are now used in pho-tocells, infrared detectors, and low-meltingglasses.

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thallium

Symbol: Tl; m.p. 303.5°C; b.p. 1457°C;r.d. 11.85 (20°C); p.n. 81; most commonisotope 205Tl; r.a.m. 204.3833.

thermal dissociation The decomposi-tion of a chemical compound into compo-nent atoms or molecules by the action ofheat. Often it is temporary and reversible.

thermite (thermit) A mixture of alu-minum powder and (usually) iron(III)oxide. When ignited the oxide is reduced;the reaction is strongly exothermic andmolten iron is formed:

2Al + Fe2O3 → Al2O3 + 2FeThermite is used in incendiary bombs

and for welding steel. See also Gold-schmidt process.

thermochemistry The branch of chem-istry concerned with heats of reaction, sol-vation, etc.

thermodynamic engine See heat en-gine.

thermodynamics The study of heat andother forms of energy and the various re-lated changes in physical quantities such astemperature, pressure, density, etc.

The first law of thermodynamics statesthat the total energy in a closed system isconserved (constant). In all processes en-ergy is simply converted from one form toanother, or transferred from one system toanother.

A mathematical statement of the firstlaw is:

∆Q = ∆U + ∆WHere, ∆Q is the heat transferred to the

system, ∆U the change in internal energy(resulting in a rise or fall of temperature),and ∆W is the external work done by thesystem.

The second law of thermodynamics canbe stated in a number of ways, all of whichare equivalent. One is that heat cannot passfrom a cooler to a hotter body withoutsome other process occurring. Another isthe statement that heat cannot be totallyconverted into mechanical work, i.e. a heatengine cannot be 100% efficient.

The third law of thermodynamics statesthat the entropy of a substance tends tozero as its thermodynamic temperature ap-proaches zero.

Often a zeroth law of thermodynamicsis given: that if two bodies are each in ther-mal equilibrium with a third body, thenthey are in thermal equilibrium with eachother. This is considered to be more funda-mental than the other laws because they as-sume it. See also Carnot cycle; entropy.

thermodynamic temperature Symbol:T A temperature that measures changesin the entropy of a closed system with re-spect to heat energy. The thermodynamictemperature interval unit is the kelvin. Seealso absolute temperature.

thin-layer chromatography A tech-nique widely used for the analysis of mix-tures. Thin-layer chromatography employsa solid stationary phase, such as alumina orsilica gel, spread evenly as a thin layer on aglass plate. A base line is carefullyscratched near the bottom of the plateusing a needle, and a small sample of themixture is spotted onto the base line usinga capillary tube. The plate is then stood up-right in solvent, which rises up to the baseline and beyond by capillary action. Thecomponents of the spot of the sample willdissolve in the solvent and tend to be car-ried up the plate. However, some of thecomponents will cling more readily to thesolid phase than others and will not moveup the plate so rapidly. In this way, differ-ent fractions of the mixture eventually be-come separated. When the solvent hasalmost reached the top, the plate is re-moved and quickly dried. The plate is de-veloped to locate the positions of colorlessfractions by spraying with a suitable chem-ical or by exposure to ultraviolet radiation.The components are identified by compar-ing the distance they have traveled up theplate with standard solutions that havebeen run simultaneously, or by computingan RF value.

thionyl Describing a compound con-taining the group =SO.

thermal dissociation

224

thiosulfate A salt containing the ionS2O3

2–. See sodium thiosulfate(IV).

thixotropy The change of viscosity withmovement. Fluids that undergo a decreasein viscosity with increasing velocity aresaid to be thixotropic. Nondrip paint is anexample of a thixotropic fluid.

thoria See thorium dioxide.

thorium A soft ductile gray toxic ra-dioactive element of the actinoid series ofmetals. It has several long-lived radioiso-topes and is found in a variety of minerals,including monazite and pitchblende. Tho-rium is used in magnesium alloys, incan-descent gas mantles, refractory materials,and nuclear fuel elements.

Symbol: Th; m.p. 1750°C; b.p. 4790°C(approx.); r.d. 11.72 (20°C); p.n. 90;r.a.m. 232.0381; most stable isotope 232Th(half-life 1.41 × 1010 years).

thorium dioxide (thoria; ThO2) Awhite insoluble compound, used as a re-fractory, in incandescent gas mantles, andas a replacement for silica in some types ofoptical glass.

thulium A very rare soft malleable duc-tile silvery-gray element of the lanthanoidseries of metals. It occurs in associationwith other lanthanoids in minerals such asgadolinite and xenotime. It is used in arclighting. Thulium-170, an artificially cre-ated radioactive isotope, is used as a powersource in portable x-ray machines.

Symbol: Tm; m.p. 1545°C; b.p.1947°C; r.d. 9.321 (20°C); p.n. 69; onlynatural isotope 169Tm; r.a.m. 168.93421.

tin A white lustrous malleable metal; thefourth member of group 14 (formerly IVA)of the periodic table. Tin is the first dis-tinctly metallic element of the group eventhough it retains some amphoteric proper-ties. Its electronic structure has outer s2p2

electrons ([Kr]4d105s25p2). The element isof low abundance in the Earth’s crust(0.004%) but is widely distributed, largelyas the mineral cassiterite (SnO2). The metalhas been known since early Bronze Age civ-

ilizations when the ores used were rela-tively rich. Currently worked ores are aslow as 1–2% tin and considerable concen-tration must be carried out before roasting.The metal itself is obtained by reductionusing carbon:

SnO2 + C → Sn + CO2Tin is an expensive metal and several

processes are therefore used to recover itfrom scrap tin-plate. These may involvedry chlorination to give the volatile SnCl4,or electrolytic methods using an alkalineelectrolyte:

Sn + 4OH– → Sn(OH)42– + 2e– (anode)

Sn(OH)42– → Sn2+ + 4OH– (cathode)

Sn2+ + 2e → Sn (cathode)Tin does not react directly with hydro-

gen but an unstable hydride, SnH4, can beprepared by reduction of SnCl4. The lowstability is due to the rather poor overlap ofthe diffuse orbitals of the tin atom with thesmall H-orbitals. Tin forms both tin(II)oxide and tin(IV) oxide. Both are ampho-teric, dissolving in acids to give tin(II) andtin(IV) salts, and in bases to form stannitesand stannates,

SnO + 4OH– → [SnO3]4– + 2H2Ostannite (relatively unstable)

SnO2 + 4OH– → [SnO4]4– + 2H2Ostannate

The halides, SnX2, may be prepared bydissolving tin metal in the hydrogen halideor by the action of heat on SnO plus the hy-drogen halide. Tin(IV) halides may be pre-pared by direct reaction of halogen withthe metal. Although tin(II) halides are ion-ized in solution their melting points are alllow, suggesting considerable covalence inall but the fluoride. The tin(IV) halides arevolatile and essentially covalent with slightpolarization of the bonds. Tin(II) com-pounds are readily oxidized to tin(IV) com-pounds and are therefore good reducingagents for general laboratory use.

Tin has three crystalline modificationsor allotropes, α-tin or ‘gray tin’ (diamondstructure), stable below 13°C; β-tin or‘white tin’, stable between 13°C and160°C; and γ-tin, stable only above 160°C.The latter two are metallic, with close-packed structures. Tin also has several iso-topes. It is used in a large number of alloysincluding Babbit metal, bell metal, Britan-

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tin

nia metal, bronze, gun metal, tin plate, andpewter as well as several special solders.

Symbol: Sn; m.p. 232°C; b.p. 2270°C;r.d. 7.31 (20°C); p.n. 50; most commonisotope 118Sn; r.a.m. 118.710.

tincal A naturally occurring form ofborax. See disodium tetraborate decahy-drate.

tin(II) chloride (stannous chloride;SnCl2) A transparent solid made by dis-solving tin in hydrochloric acid. Tin(II)chloride is a reducing agent, it combineswith ammonia, and forms hydrates withwater. It is used as a mordant.

tin(IV) chloride (stannic chloride; SnCl4)A colorless fuming liquid. Tin(IV) chlorideis soluble in organic solvents but is hy-drolyzed by water. It dissolves sulfur,phosphorus, bromine, and iodine, and itdissolves in concentrated hydrochloric acidto give the anion SnCl62–.

tin(IV) hydride See stannane.

tin(II) oxide (stannous oxide; SnO) Adark green or black solid. It can also be ob-tained in an unstable red form, which turnsblack on exposure to air. Tin(II) oxide canbe made by precipitating the hydratedoxide from a solution containing tin(II)ions and dehydrating the product at100°C. It is used in the manufacture of cos-metics and ceramics.

tin(IV) oxide (stannic oxide; tin dioxide;SnO2) A colorless crystalline solid, whichis usually discolored owing to the presenceof impurities. It occurs in nature as themineral cassiterite. It exists as hexagonalor rhombic crystals or in an amorphousform. Tin(IV) oxide is insoluble in water. Itis used for polishing glass and metal and incosmetics.

tin(II) sulfide (SnS) A gray solid thatcan be prepared from tin and sulfur. Above265°C tin(II) sulfide slowly turns intotin(IV) sulfide and tin:

2SnS(s) → SnS2(s) + Sn(s)

tin(IV) sulfide (SnS2) A yellowish solidthat can be precipitated by reacting hydro-gen sulfide with a solution of a solubletin(IV) salt. A crystalline form sometimesknown as mosaic gold is obtained by heat-ing a mixture of tin filings, sulfur, and am-monium chloride. Tin(IV) sulfide is used asa pigment.

titania See titanium(IV) oxide.

titanic chloride See titanium(IV) chlor-ide.

titanic oxide See titanium(IV) oxide.

titanium A silvery lustrous light strongtransition metal, the first element of group4 (formerly subgroup IVB) of the periodictable. It occurs in various ores as tita-nium(IV) oxide and also in combinationwith iron and oxygen. The most commer-cially important minerals are rutile (TiO2)and ilmenite (FeTi)3). The mineral titanite((CaTiO)SlO4) is often used for gemstones.Titanium is extracted by conversion of ti-tanium(IV) oxide to the chloride, which isreduced to the metal by heating withsodium. Titanium is chiefly reactive only athigh temperatures. Its strength, low den-sity, and resistance to corrosion make it es-pecially useful for alloys used in theaviation and aerospace industries. Tita-nium pins, plates, and joints are also usedin dentistry and medicine to replace dam-aged teeth or bones. It forms compoundswith oxidation states +4, +3, and +2, the+4 state being the most stable. Titaniumcompounds are used in pigments and cos-metics.

Symbol: Ti; m.p. 1660°C; b.p. 3287°C;r.d. 4.54 (20°C); p.n. 22; most commonisotope 48Ti; r.a.m. 47.867.

titanium(IV) chloride (titanic chloride;titanium tetrachloride; TiCl4) A volatilecolorless liquid formed by heating tita-nium(IV) oxide with carbon in a stream ofdry chlorine at 700°C. The product is puri-fied by fractional distillation. Titanium(IV)chloride fumes in moist air forming theoxychlorides of titanium. It undergoes hy-drolysis in water but this can be prevented

tincal

226

by the presence of excess hydrochloricacid. Crystallization of such a solutiongives crystals of the di- and pentahydrates.Titanium(IV) chloride is used in the prepa-ration of pure titanium and as an interme-diate in the production of titaniumcompounds from minerals.

titanium dioxide See titanium(IV) oxide.

titanium(IV) oxide (titania; titanic oxide;titanium dioxide; TiO2) A white ionicsolid that occurs naturally in three crys-talline forms: rutile (tetragonal), brookite(orthorhombic), and anatase (tetragonal).It reacts slowly with acids and is attackedby the halogens at high temperatures. Onheating it decomposes to give titanium(III)oxide and oxygen. Titanium(IV) oxide isamphoteric: with hot concentrated sulfuricacid, it forms titanyl sulfate, TiOSO4;when fused with alkalis, it forms titanates.It is used as a white pigment.

titanium tetrachloride See titanium(IV)chloride.

titrant See titration.

titration A procedure in volumetricanalysis in which a solution of known con-centration (called the titrant) is added to asolution of unknown concentration from aburette until the equivalence point or endpoint of the titration is reached. See volu-metric analysis.

tonne Symbol: t An m.k.s. unit of massequal to 103 kilograms in SI units (i.e. onemegagram).

topaz A hydrous aluminosilicate min-eral containing some fluoride ions, used formaking refractories, glasses, and glazes. Aclear pale yellow or brown form is used asa gemstone.

torr A unit of pressure equal to onemmHg. In SI units it is equal to 133.322pascals. It was named for Italian physicistEvangelista Torricelli (1609–47).

tracer An isotope of an element used to

investigate chemical reactions or physicalprocesses (e.g. diffusion). See isotopes.

transactinide elements Those elementsfollowing 103Lw (i.e. following the actinideseries). To date, element 104 (ruther-fordium) through to element 112 and ele-ment 114 have been synthesized; so far,elements 111, 112, and 114 are unnamed.All the transactinides are unstable andhave very short half-lives. There is specula-tion that islands of stable nuclei with veryhigh mass numbers exist, because theoreti-cal calculations predict unusual stabilityfor elements 114 and 118.

trans-isomer See isomerism.

transition elements (group 3–12 ele-ments) A class of elements occurring inthe periodic table in three series: from scan-dium to zinc; from yttrium to cadmium;and from lanthanum to mercury. The tran-sition elements are all metals. They owetheir properties to the presence of d elec-trons in the atoms. In the first transition se-ries, calcium has the configuration3s23p64s2. The next element scandium has3s23p63d14s2, and the series is formed byfilling the d levels up to zinc at 3d104s2. Theother two transition series occur by fillingof the 4d and 5d levels. Often the elementsscandium, yttrium, and lanthanum areconsidered with the lanthanoids ratherthan the transition elements, and zinc, cad-mium, and mercury are also treated as aseparate group. Transition elements havecertain characteristic properties resultingfrom the existence of d levels:1. They have variable valences – i.e. they

can form compounds with the metal indifferent oxidation states (Fe2+ and Fe3+,etc.).

2. They form a vast number of inorganiccomplexes, in which coordinate bondsare formed to the metal atom or ion.

3. Compounds of transition elements areoften colored.See also d-block elements; lanthanoids;

metals; zinc group.

transition state (activated complex) Sym-bol: ‡ A short-lived high-energy mol-

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transition state

ecule, radical, or ion formed during a reac-tion between molecules possessing the nec-essary activation energy. The transitionstate decomposes at a definite rate to yieldeither the reactants again or the final prod-ucts. The transition state can be consideredto be at the top of the potential energy pro-file.

For the reaction,X + YZ = X…Y…Z‡ → XY + Z

the sequence of events is as follows. X ap-proaches YZ and when it is close enoughthe electrons are rearranged producing aweakening of the bond between Y and Z.A partial bond is now formed between Xand Y producing the transition state. De-pending on the experimental conditions,the transition state either then breaksdown to form the products or reverts backto the reactants.

transition state theory An alternativename for ACTIVATED COMPLEX THEORY.

transition temperature A temperatureat which some definite physical change oc-curs in a substance. Examples of such tran-sitions are change of state, change ofcrystal structure, and change of magneticbehavior.

transmutation A change of one elementinto another by radioactive decay or bybombardment of the nuclei with particles.

transport number Symbol: t In an elec-trolyte, the transport number of an ion isthe fraction of the total charge carried bythat type of ion in conduction.

transuranic elements Those elementsof the ACTINOID series that have higheratomic numbers than uranium, i.e. neptu-nium through lawrencium. Thetransuranic elements are formed by addingneutrons to the lower actinoids by high-en-ergy bombardment. They generally havesuch short half-lives that macroscopic iso-lation is impossible. Neptunium, pluto-nium, americium, and possibly curium arealso formed in trace quantities by naturalneutron bombardment from spontaneousfission of 235U.

triammonium phosphate See ammo-nium phosphate.

triatomic Denoting a molecule, radical,or ion consisting of three atoms. For exam-ple, O3 and H2O are triatomic molecules.

tribasic acid An acid with three replace-able hydrogen atoms (such as phos-phoric(V) acid, H3PO4). See acid.

triclinic crystal See crystal system.

tridymite See silicon(IV) oxide.

trigonal bipyramid See complex.

trigonal crystal See crystal system.

trihydrate A crystalline hydrated com-pound that contains three molecules ofwater of crystallization per molecule ofcompound.

triiron tetroxide (ferrosoferric oxide;magnetic iron oxide; Fe3O4) A blacksolid prepared by passing either steam orcarbon dioxide over red-hot iron. It mayalso be prepared by passing steam overheated iron(II) sulfide. Triiron tetroxideoccurs in nature as the mineral magnetite.It is insoluble in water but will dissolve inacids to give a mixture of iron(II) andiron(III) salts in the ratio 1:2. Generally itis chemically unreactive; it is, however, afairly good conductor of electricity. Seealso iron(II) oxide; iron(III) oxide.

trimer A molecule (or compound)formed by addition of three identical mol-ecules.

trimethylaluminum (aluminum tri-methyl; (CH3)3Al) A colorless liquid pro-duced by the sodium reduction of dimethylaluminum chloride. It ignites sponta-neously on contact with air and reacts vio-lently with water, acids, halogens,alcohols, and amines. Aluminum alkyls areused in the Ziegler process for the manu-facture of high-density polyethene.

trimolecular Describing a reaction or

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step that involves three molecules interact-ing simultaneously with the formation of aproduct. For example, the final step in re-action between hydrogen peroxide andacidified potassium iodide is trimolecular:

HOI + H+ + I– → I2 + H2OIt is uncommon for reactions to take

place involving trimolecular steps. The ox-idation of nitrogen(II) oxide tonitrogen(IV) oxide,

2NO + O2 → 2NO2is often classified as a trimolecular reactionbut many chemists believe it to involve twobimolecular reactions.

trioxoborix(III) acid See boric acid.

trioxosulfuric(IV) acid See sulfurousacid.

trioxygen See ozone.

triple bond A covalent bond formed be-tween two atoms in which three pairs ofelectrons contribute to the bond. One pairforms a sigma bond (equivalent to a singlebond) and two pairs give rise to two pibonds. It is conventionally represented asthree lines, thus H–C≡C–H. See multiplebond.

triple point The only point at which thegas, solid, and liquid phases of a substancecan coexist in equilibrium. The triple pointof water (273.16 K at 101 325 Pa) is usedto define the kelvin.

trisodium phosphate(V) (sodium or-thophosphate; Na3PO4) A white solidprepared by adding sodium hydroxide todisodium hydrogenphosphate. On evapo-ration white hexagonal crystals of the do-decahydrate (Na3PO4.12H2O) may beobtained. These crystals do not effloresceor deliquesce. They dissolve readily inwater to produce alkaline solutions owingto salt hydrolysis. Trisodium phosphate isused as a water softener.

tritiated See tritium.

tritium Symbol: T, 3H A radioactive iso-tope of hydrogen of mass number 3. Thenucleus contains 1 proton and 2 neutrons.Tritium decays with emission of low-en-ergy beta radiation to give 3He. The half-life is 12.3 years. It is useful as a tracer instudies of chemical reactions. Compoundsin which 3H atoms replace the usual 1Hatoms are said to be tritiated. A positive tri-tium ion, T+, is a triton.

triton See tritium.

trivalent (tervalent) Having a valence ofthree.

tungsten A hard gray malleable transi-tion metal, the third element of group 6(formerly VIB) of the periodic table. It oc-curs naturally in the minerals wolframite((Fe,Mn)WO4) and scheelite (CaWO4). Itwas formerly called wolfram. It is used asthe filaments in electric lamps and in vari-ous steel alloys. Tungsten has the highestmelting point of all the metals. Tungstensalts are used in paints and in tanning.

Symbol: W; m.p. 3410 ± 20°C; b.p.5650°C; r.d. 19.3 (20°C); p.n. 74; mostcommon isotope 184W; r.a.m. 183.84.

tungsten carbide Either of two carbides(W2C and WC) produced by heating pow-dered tungsten with carbon. The carbidesare extremely hard and are used in industryto make cutting tools or as an abrasive.WC has a very high melting point(2770°C) and will conduct electricity. W2Calso has a very high melting point(2780°C) but is a less efficient conductor ofelectricity. It is very resistant to chemicalattack and behaves in a manner very simi-lar to that of tungsten. It is strongly at-tacked by chlorine to give tungstenhexachloride, WCl6.

turquoise A naturally occurring hy-drated copper aluminum phosphate, usedas a green-blue gemstone.

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turquoise

ultracentrifuge A high-speed centrifugeused for separating out very small parti-cles. Because the sedimentation rate de-pends on the particle size, theultracentrifuge can be used to measure themolecular mass of colloidal particles andlarge molecules (e.g. proteins).

ultrahigh vacuum See vacuum.

ultraviolet (UV) A form of electromag-netic radiation, shorter in wavelength thanvisible light. Ultraviolet wavelengths rangebetween about 1 nm and 400 nm. Ordi-nary glasses are not transparent to thesewaves; quartz is a much more effective ma-terial for making lenses and prisms for usewith ultraviolet. Like light, ultraviolet radi-ation is produced by electronic transitionsbetween the outer energy levels of atoms.However, having a higher frequency, ultra-violet photons carry more energy thanthose of light and can induce photolysis ofcompounds. See also electromagnetic radi-ation.

unimolecular Describing a reaction (orstep) in which only one molecule is in-volved. For example, radioactive decay is aunimolecular reaction:

Ra → Rn + αOnly one atom is involved in each disinte-gration.

In a unimolecular chemical reaction,the molecule acquires the necessary energyto become activated and then decomposes.The majority of reactions involve only uni-or BIMOLECULAR steps. The following reac-tions are all unimolecular:

N2O4 → 2NO2PCl5 → PCl3 + Cl2

CH3CH2Cl → C2H4 + HCl

unit A reference value of a quantity,used to express other values of the samequantity. See also SI units.

unit cell The smallest group of atoms,ions, or molecules that, when repeated atregular intervals in three dimensions, willproduce the lattice of a crystal system.

unit processes (chemical conversions)The recognized steps used in chemicalprocesses, e.g. alkylation, distillation, hy-drogenation, pyrolysis, nitration, etc.

univalent (monovalent) Having a va-lence of one.

universal gas constant See gas con-stant.

universal indicator (multiple-range indi-cator) A mixture of indicator dyestuffsthat shows a gradual change in color overa wide pH range. A typical formulationcontains methyl orange, methyl red, bro-mothymol blue, and phenolphthalein andchanges through a red, orange, yellow,green, blue, and violet sequence betweenpH 3 and pH 10. Several commercialpreparations are available as both solu-tions and test papers.

unsaturated solution See saturated so-lution.

unsaturated vapor See saturated vapor.

unun- A prefix used in the names of newchemical elements. Synthetic elements withhigh proton numbers were given tempo-rary names based on the proton number,pending official agreement on the actual

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U

name to be used. These names were formedfrom the word elements in the table above. The suffix -ium was also used (because theelements were metals). So element 104, forexample, had the temporary systematicname unnilquadium (un + nil + quad +ium). The names for elements using thissystem are shown in the table below.

uraninite See pitchblende.

uranium A toxic radioactive silvery ele-ment of the actinoid series of metals. Itsthree naturally occurring radioisotopes,238U (99.283% in abundance), 235U(0.711%), and 234U (0.005%), are foundin numerous minerals including the ura-nium oxides pitchblende, uraninite, andcarnotite. The readily fissionable 235U is amajor nuclear fuel and nuclear explosive,while 238U is a source of fissionable 239Pu.Spent uranium fuel rods have been used tomake armor-piercing shells.

Symbol: U; m.p. 1132.5°C; b.p.3745°C; r.d. 18.95 (20°C); p.n. 92; moststable isotope U238; r.a.m. 238.0289.

uranium hexafluoride (UF6) A crys-talline volatile compound, used in separat-ing uranium isotopes by differences in therates of gas diffusion.

uranium–lead dating A technique fordating certain rocks depending on thedecay of the radioisotope 238U to 206Pb(half-life 4.5 × 109 years) or of 235U to207Pb (half-life 7.1 × 108 years). The decayof 238U releases alpha particles and an esti-mate of the age of the rock can be made bymeasuring the amount of helium present.Another method is to measure the ratio ofradioactive lead present to the amount ofnonradioactive lead. See also radioactivedating.

uranium(IV) oxide (uranium dioxide;urania; UO2) An extremely poisonousradioactive black crystalline solid. It occursin pitchblende (uraninite) and is used in ce-ramics, photographic chemicals, pigments,and as a nuclear fuel.

uranium(VI) oxide (uranium trioxide;UO3) An extremely poisonous radioac-tive orange solid, used as a pigment in ce-ramics and in uranium refining.

uranyl The radical UO22+, as in uranyl

nitrate UO2(NO3)2.

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uranyl

Element Number Symbol nil 0 n un 1 u bi 2 b tri 3 t quad 4 q pent 5 p hex 6 h sept 7 s oct 8 0 enn 9 e

P.N. Temporary name Symbol Name Symbol 101 unnilunium Unu mendelevium Md 102 unnilbiium Unb nobelium No 103 unniltriium Unt lawrencium Lr 104 unnilquadium Unq rutherfordium Rf 105 unnilpentium Unq dubnium Db 106 unnilhexium Unh seaborgium Sg 107 unnilseptium Uns bohrium Bh 108 unniloctium Uno hassium Hs 109 unnilennium Une meitnerium Mt 110 ununnilium Uun darmstadtium Ds111 unununium Uuu unnamed112 ununbiium Uut unnamed

vacancy See defect.

vacuum A space containing gas belowatmospheric pressure. A perfect vacuumcontains no matter at all, but for practicalpurposes soft (low) vacuum is usually de-fined as down to about 10–2 pascal, andhard (high) vacuum as below this. Ultra-high vacuum is lower than 10–7 pascal.

vacuum distillation The distillation ofliquids under a reduced pressure, so thatthe boiling point is lowered. Vacuum dis-tillation is a common laboratory techniquefor purifying or separating compoundsthat would decompose at their ‘normal’boiling point.

valence (valency) The combining powerof an element or radical, equal to the num-ber of hydrogen atoms that will combinewith or displace one atom of the element.For simple covalent molecules the valenceis obtained directly, for example C in CH4is tetravalent; N in NH3 is trivalent. Forions the valence is regarded as equivalentto the magnitude of the charge; for exam-ple Ca2+ is divalent, CO3

2– is a divalentradical. The rare gases are zero-valent be-cause they do not form compounds undernormal conditions. Because the valence formany elements is constant, the valence ofsome elements can be deduced without ref-erence to compounds formed with hydro-gen. Thus, because the valence of chlorinein HCl is 1, the valence of aluminum inAlCl3 is 3; because oxygen is divalent(H2O), silicon in SiO2 is tetravalent. Theproduct of the valence and the number ofatoms of each element in a compound mustbe equal. For example, in Al2O3 for thetwo aluminum atoms (valence 3) the prod-

uct is 6 and for the three oxygen atoms (va-lence 2) the product is also 6.

The valence of an element is generallyequal to either the number of valence elec-trons or eight minus the number of valenceelectrons. Transition metal ions displayvariable valence.

valence band The highest energy levelin a semiconductor or nonconductor thatcan be occupied by electrons. See energylevel; semiconductor.

valence-bond theory A technique forcalculating the electronic structure of mol-ecules. In valence-bond theory the startingpoint is to consider the atoms in a moleculeand the ways in which valence electrons inthe atoms can pair up in chemical bonds.Each way of pairing up the electrons iscalled a valence bond structure. A WAVE-FUNCTION is written for each valence bondstructure. The total wavefunction Ψ for themolecule is a linear combination of thewavefunctions for the valence-bond struc-tures. The wavefunction incorporates RES-ONANCE between the various valence bondstructures.

valence electron An outer electron inan atom that can participate in formingchemical bonds.

valency See valence.

vanadium A strong silvery toxic transi-tion metal, the first element of group 5(formerly VB) of the periodic table. It oc-curs in complex ores, especially those ofiron, lead, and uranium, in small quanti-ties. It is used in alloy steels to give themadded strength and heat resistance. Vana-dium forms compounds with oxidation

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V

states +5, +4, +3, and +2. It forms coloredions.

Symbol: V; m.p. 1890°C; b.p. 3380°C;r.d. 6.1 (20°C); p.n. 23; most common iso-tope 51V; r.a.m. 50.94.

vanadium(V) oxide (vanadium pentox-ide; V2O5) An oxide of vanadium exten-sively used as a catalyst in oxidationprocesses, as in the contact process.

van der Waals, Johannes Diderik(1837–1923) Dutch physicist. In his doc-toral thesis of 1873 entitled On the Conti-nuity of the Liquid and Gaseous States vander Waals studied the kinetic theory ofgases and liquids. This work provided thefoundation for most of his subsequentwork. He derived an equation of state forgases which is more realistic than the idealgas laws discovered by Robert BOYLE andothers because it takes into account boththe non-zero sizes of the molecules and theexistence of attractive intermolecularforces. The equation of state he found iscalled the van der Waals equation and theintermolecular forces are called van derWaals forces. This work resulted in van derWaals winning the 1910 Nobel Prize forphysics.

van der Waals equation An equationof state for real gases. For n moles of gasthe equation is

(p + n2a/V2)(V – nb) = nRTwhere p is the pressure, V the volume, andT the thermodynamic temperature. a and bare constants for a given substance and R isthe gas constant. The equation gives a bet-ter description of the behavior of real gasesthan the perfect gas equation (pV = nRT).

The equation contains two corrections:b is a correction for the non-negligible sizeof the molecules; a/V2 corrects for the factthat there are attractive forces between themolecules, thus slightly reducing the pres-sure from that of an ideal gas. See also gaslaws; kinetic theory.

van der Waals force An intermolecularforce of attraction arising from weak elec-trostatic interactions between molecules.Such forces are considerably weaker than

those of chemical bonds and typically haveenergies of less than one joule per mole.

The van der Waals force, named forDutch physicist Johannes van der Waals(1837–1923), arises because of three ef-fects: permanent dipole–dipole interactionsfound for any polar molecule; dipole–in-duced dipole interactions, where one di-pole causes a slight charge separation inbonds that have a high polarizability; anddispersion forces, which result from tem-porary polarity arising from an asymmetri-cal distribution of electrons around thenucleus. Even atoms of the rare gases ex-hibit dispersion forces.

van’t Hoff factor Symbol: i The ratioof the number of particles present in a so-lution to the number of undissociated mol-ecules added. It is used in studies ofcolligative properties, which depend on thenumber of entities present. For example, ifn moles of a compound are dissolved anddissociation into ions occurs, then thenumber of particles present will be in. Os-motic pressure (π), for instance, will begiven by the equation

πV = inRTIt is named for the Dutch chemist Jacobusvan’t Hoff (1852–1911).

van’t Hoff isochore The equation:d(logeK)/dT = ∆H/RT2

showing how the equilibrium constant, K,of a reaction varies with thermodynamictemperature, T. ∆H is the enthalpy of reac-tion and R is the gas constant.

vapor A gas formed by the vaporizationof a solid or liquid. Some particles near thesurface of a liquid acquire sufficient energyin collisions with other particles to escapefrom the liquid and enter the vapor; someparticles in the vapor lose energy in colli-sions and re-enter the liquid. At a giventemperature an equilibrium is established,which determines the vapor pressure of theliquid at that temperature.

vapor density The ratio of the mass of acertain volume of a vapor to the mass of anequal volume of hydrogen (measured at thesame temperature and pressure). Determi-

233

vapor density

nation of vapor densities is one method offinding the relative molecular mass of acompound (equal to twice the vapor den-sity). Victor Meyer’s method, Dumas’method, or Hofmann’s method can beused.

vaporization The process by which aliquid or solid is converted into a gas orvapor by heat. Unlike boiling, which oc-curs at a fixed temperature, vaporizationcan occur at any temperature. Its rate in-creases as the temperature rises.

vapor pressure The pressure exerted bya vapor. The saturated vapor pressure isthe pressure of a vapor in equilibrium withits liquid or solid. It depends on the natureof the liquid or solid and the temperature.

verdigris Any of various greenish basicsalts of copper. True verdigris is basic cop-per acetate, a green or blue solid of variablecomposition used as a paint pigment. Theterm is often applied to green patinasformed on metallic copper. Copper cook-ing vessels can form a coating of basic cop-per carbonate, CuCO3.Cu(OH)2. Theverdigris formed on copper roofs anddomes is usually basic copper sulfate,CuSO4.3Cu(OH)2.H2O, while basic cop-per chloride, CuCl2.Cu(OH)2, may form inregions near the sea.

vermilion See mercury(II) sulfide.

vesicant A substance that causes blister-ing of the skin. Mustard gas is an example.

vibrational spectroscopy The spectro-scopic investigation of the vibrational en-ergy levels of molecules. In the infraredregion of the electromagnetic spectrum vi-brational transitions are accompanied byrotational transitions. Infrared spectra ofmolecules are series of bands, with eachband being associated with a vibrationaltransition and every line in that band beingassociated with a rotational transition thataccompanies the vibrational transition.

Some features of vibrational spectra canbe analyzed by regarding the vibrations assimple harmonic motion but a realistic ac-

count of molecular vibrations requires thatanharmonicity is taken into account. A di-atomic molecule can have a vibrational-rotational spectrum only if it has a perma-nent dipole moment. A polyatomic mol-ecule can have a vibrational-rotationalspectrum only if the normal modes of vi-bration cause the molecule to have an os-cillating dipole moment. The analysis ofvibrational–rotational spectra is greatly fa-cilitated by the use of group theory. Thistype of spectroscopy provides informationabout interatomic distances and the forceconstants of chemical bonds.

vicinal positions Positions in a mol-ecule at adjacent atoms. For example, in1,2-dichloroethane the chlorine atoms arein vicinal positions, and this compoundcan thus be named vic-dichloroethane.

viscosity Symbol: η The resistance toflow of a fluid.

vitreous Resembling glass, or having thestructure of a glass.

volatile Easily converted into a vapor.

volt Symbol: V The SI derived unit ofelectrical potential, potential difference,and e.m.f., defined as the potential differ-ence between two points in a circuit be-tween which a constant current of oneampere flows when the power dissipated isone watt. 1 V = 1 W A–1. It is named for theItalian physicist Alessandro Volta (1745–1827).

voltaic cell See cell.

voltameter See coulombmeter.

volume strength See hydrogen perox-ide.

volumetric analysis One of the classi-cal wet methods of quantitative analysis. Itinvolves measuring the volume of a solu-tion of accurately known concentrationthat is required to react with a solution ofthe substance being determined. The solu-tion of known concentration (the standard

vaporization

234

solution) is added in small portions from aburette. The process is called a titrationand the observed point of complete reac-tion is called the end point. End points areobserved with the aid of indicators or byinstrumental methods, such as via conduc-tion or light absorption. Volumetric analy-sis can also be applied to gases. The gas istypically held over mercury in a graduatedtube, and volume changes are measured onreaction or after absorption of componentsof a mixture.

VSEPR (valence-shell electron-pair re-

pulsion) A method of predicting the shapeof molecules. In this theory one atom istaken to be the central atom and pairs ofvalence electrons are drawn round the cen-tral atom. The shape of the molecule is de-termined by minimizing the Coulombrepulsion between pairs and the repulsionbetween pairs due to the Pauli exclusionprinciple. Thus, for three pairs of electronsan equilateral triangle is favored and forfour pairs of electrons a tetrahedron is fa-vored. The VSEPR theory has had consid-erable success in predicting molecularshapes. See lone pair.

235

VSEPR

washing soda See sodium carbonate.

water (H2O) A colorless liquid thatfreezes at 0°C and, at atmospheric pres-sure, boils at 100°C. In the gaseous statewater consists of single H2O molecules.Due to the presence of two lone pairs theatoms do not lie in a straight line, the anglebetween the central oxygen atom and thetwo hydrogen atoms being 105°; the dis-tance between each hydrogen atom and theoxygen atom is 0.099 nm. When ice forms,hydrogen bonds some 0.177 nm long de-velop between the hydrogen atom and oxy-gen atoms in adjacent molecules, giving iceits tetrahedral crystalline structure with adensity of 916.8 kg m–3 at STP. Differentice structures develop under higher pres-sures. When ice melts to form liquid water,the tetrahedral structure breaks down, butsome hydrogen bonds continue to exist;liquid water consists of groups of associ-ated water molecules, (H2O)n, mixed withsome monomers and some dimers. Thismixture of molecular species has a higherdensity than the open-structured crystals.The maximum density of water, 999.97 kgm–3, occurs at 3.98°C. This accounts forthe ability of ice to float on water and forthe fact that water pipes burst as ice ex-pands on freezing.

Although water is predominantly a co-valent compound, a very small amount ofionic dissociation occurs (H2O ˆ H+ +OH–). In every liter of water at STP there isapproximately 10–7 mole of each ionicspecies. It is for this reason that, on the pHscale, a neutral solution has a value of 7.

As a polar liquid, water is the mostpowerful solvent known. This is partly aresult of its high dielectric constant andpartly its ability to hydrate ions. This latterproperty also accounts for the incorpora-

tion of water molecules into some ioniccrystals as water of crystallization.

Water is decomposed by reactive metals(e.g. sodium) when cold and by less activemetals (e.g. iron) when steam is passedover the hot metal. It is also decomposedby electrolysis.

water gas A mixture of carbon monox-ide and hydrogen produced when steam ispassed over red-hot coke or made to com-bine with hydrocarbons, e.g.

C(s) + H2O(g) → CO(g) + H2(g)CH4(g) + H2O(g) → CO(g) + 3H2(g)

The production of water gas usingmethane is an important step in the prepa-ration of hydrogen for ammonia synthesis.Compare producer gas.

water glass See sodium silicate.

water of crystallization Water presentin definite proportions in crystalline com-pounds. Compounds containing water ofcrystallization are called HYDRATES. Exam-ples are copper(II) sulfate pentahydrate(CuSO4.5H2O) and sodium carbonate dec-ahydrate (Na2CO3.10H2O). The water canbe removed by heating.

When hydrated crystals are heated thewater molecules may be lost in stages. Forexample, copper(II) sulfate pentahydratechanges to the monohydrate (CuSO4.H2O)at 100°C, and to the anhydrous salt(CuSO4) at 250°C. The water molecules incrystalline hydrates may be held by hydro-gen bonds (as in CuSO4.H2O) or, alterna-tively, may be coordinated to the metal ionas a complex aquo ion.

water softening The removal fromwater of dissolved calcium, magnesium,and iron compounds, thus reducing the

236

W

HARDNESS of the water. The compounds arepotentially damaging because they can ac-cumulate in pipes and boilers. They alsoreact with, and therefore waste, soap.Temporary hardness can be removed byboiling the water. Permanent hardness canbe removed in a number of ways: by distil-lation; by the addition of sodium carbon-ate (which causes dissolved calcium, forexample, to precipitate out as calcium car-bonate); and by the use of ion-exchangeproducts such as Permutit (utilizes zeolites)and Calgon (utilizes polyphosphates).

watt Symbol: W The SI unit of power,defined as a power of one joule per second.1 W = 1 J s–1.

wavefunction Symbol ψ. A functionthat specifies the state of a system in theWAVE MECHANICS formulation of quantummechanics. Thus, a wavefunction appearsin the SCHRÖDINGER EQUATION, with eachwavefunction which is a solution to theSchrödinger equation for a given quantummechanical system being an EIGENFUNC-TION of the equation. The physical inter-pretation of ψ, known as the Borninterpretation since it was put forward byMax Born in 1926, is that the square of thewavefunction is a measure of the probabil-ity of finding a particle in a small volumeelement at a given point.

waveguide See microwaves.

wavelength Symbol: λ The distance be-tween the ends of one complete cycle of awave. Wavelength is related to the speed(c) and frequency (v) thus:

c = vλ

wave mechanics See quantum theory.

wave number Symbol: σ The reciprocalof the wavelength of a wave. It is the num-ber of wave cycles in unit distance, and isoften used in spectroscopy. The unit is themeter–1 (m–1). The circular wave number(symbol: k) is given by:

k = 2πσ

weak acid An acid that is not fully dis-sociated in solution.

weak base A base that is only partly orincompletely dissociated into its compo-nent ions in solution.

weber Symbol: Wb The SI unit of mag-netic flux, equal to the magnetic flux that,linking a circuit of one turn, produces ane.m.f. of one volt when reduced to zero atuniform rate in one second. 1 Wb = 1 V s.It is named for German physicist WilhelmWeber (1804–91).

Werner, Alfred (1866–1919) French-born Swiss chemist. Werner’s main contri-bution to chemistry consists of hiselucidation of structure and valence in in-organic molecules. In a series of papersWerner put forward a theory of what hecalled ‘coordination compounds’. He dis-tinguished between the primary and sec-ondary valence of a metal, with thesecondary valence being associated withhow many ligands can surround a metalatom. Werner put forward his ideas inNew Ideas on Inorganic Chemistry (1911).He won the 1913 Nobel Prize for chem-istry for his work on coordination com-pounds.

Weston cadmium cell (cadmiumcell) A standard cell that produces a con-stant e.m.f. of 1.0186 volts at 20°C. It con-sists of an H-shaped glass vessel containinga negative cadmium-mercury amalgamelectrode in one leg and a positive mercuryelectrode in the other. The electrolyte, asaturated cadmium sulfate solution, fillsthe horizontal bar of the vessel to connectthe two electrodes. The e.m.f. of the cellvaries very little with temperature, beinggiven by the equation E = 1.0186 – 0.000037 (T – 293), where T is the thermody-namic temperature.

wet cell A type of cell, such as a car bat-tery, in which the electrolyte is a liquid so-lution.

white arsenic See arsenic(III) oxide.

237

white arsenic

white cast iron See cast iron.

white gold See palladium.

white lead See lead(II) carbonate hy-droxide.

white phosphorus See phosphorus.

white spirit A liquid hydrocarbon re-sembling kerosene obtained from petro-leum, used as a solvent and in themanufacture of paints and varnishes.

Wilkinson, Sir Geoffrey (1921–96)British inorganic chemist. Wilkinson isnoted for his studies of inorganic com-plexes. He shared the Nobel Prize forchemistry in 1973 with Ernst FISCHER forwork on sandwich compounds. A theme ofWilkinson’s work in the 1960s was thestudy and use of complexes containing ametal–hydrogen bond. Thus complexes ofrhodium with triphenyl phosphine((C6H5)3P) can react with molecular hy-drogen. The compound RhCl(P(C6H5)3),known as Wilkinson’s catalyst, was thefirst such complex to be used as a homoge-neous catalyst for adding hydrogen to thedouble bonds of alkenes (hydrogenation).This type of compound can also be used asa catalyst for the reaction of hydrogen andcarbon monoxide with alkenes (hydro-formylation). It is the basis of industrial

low-pressure processes for making aldehy-des from ethene and propene.

witherite See barium carbonate.

wolfram A former name for tungsten.

Wood’s metal A fusible alloy contain-ing 50% bismuth, 25% lead, and 12.5%tin and cadmium. Its low melting point(70°C) leads to its use in fire-protection de-vices.

Woodward–Hoffmann rules A set ofrules concerning the course of chemical re-actions proposed by the American chemistsRobert Burns Woodward and Roald Hoff-mann in the late 1960s. These rules can bestated in terms of molecular orbital theory(particularly FRONTIER ORBITAL THEORY).This gives rise to the Woodward–Hoffmann rules being referred to as theconservation of orbital symmetry. Therules were initially developed for organicreactions but can also be applied to inor-ganic reactions.

work function See photoelectric effect.

wrought iron A low-carbon steel ob-tained by refining the iron produced in ablast furnace. Wrought iron is processedby heating and hammering to reduce theslag content and to ensure its even distrib-ution.

white cast iron

238

xenon A colorless odorless monatomicgas, the fifth member of the rare-gases; i.e.group 18 (formerly VIIIA or 0) of the peri-odic table. It occurs in trace amounts in airfrom which it is recovered by fractionaldistillation. Xenon is used in electrontubes, strobe lighting, arc lamps, andlasers. Xenon compounds with fluorineand oxygen are known.

Symbol: Xe; m.p. –111.9°C; b.p.–107.1°C; mass density 5.8971 (0°C)kg m–3; p.n. 54; most common isotope132Xe; r.a.m. 131.29.

x-radiation An energetic form of elec-tromagnetic radiation. The wavelengthrange is 10–11 m to 10–8 meter. X-rays arenormally produced when high-energy elec-trons are absorbed in matter. The radiationcan pass through matter to some extent,hence its use in medicine and industry forinvestigating internal structures. It can bedetected with photographic emulsions anddevices like the Geiger-Müller tube.

X-ray photons result from electronictransitions between the inner energy levelsof atoms. When high-energy electrons areabsorbed by matter, an x-ray line spectrumresults. The structure depends on the sub-stance and is thus used in x-ray spec-troscopy. The line spectrum is alwaysformed in conjunction with a continuousbackground spectrum. The minimum (cut-off) wavelength λ0 corresponds to the max-imum x-ray energy, Wmax. This equals themaximum energy of electrons in the beamproducing the x-rays. Wavelengths in thecontinuous spectrum above λ0 are causedwhen electrons decelerate and lose energyrapidly, such as when they collide with anuclear target or pass through an electricfield. X-radiation emitted in this process is

called bremsstrahlung (German for brak-ing radiation).

x-ray crystallography The study of theinternal structure of crystals using the tech-nique of x-ray diffraction.

x-ray diffraction A technique used todetermine crystal structure by directing x-rays at the crystals and examining the dif-fraction patterns produced. At certainangles of incidence a series of spots are pro-duced on a photographic plate; these spotsare caused by interaction between the x-rays and the planes of the atoms, ions, ormolecules in the crystal lattice. The posi-tions of the spots are consistent with theBragg equation nλ = 2dsinθ.

x-rays See x-radiation.

yellow phosphorus See phosphorus.

yocto- Symbol: y A prefix used with SIunits, denoting 10–24. For example, 1 yoc-tometer (ym) = 10–24 meter (m).

yotta- Symbol: Y A prefix used with SIunits, denoting 1024. For example, 1 yot-tameter (Ym) = 1024 meter (m).

ytterbium A soft malleable ductile sil-very element belonging to the lanthanoidseries of metals. It occurs in associationwith other lanthanoids in minerals such asgadolinite, monazite, and xenotime. Ytter-bium has been used to improve the me-chanical properties of steel. It is also usedin ceramics.

Symbol: Yb; m.p. 824°C; b.p. 1193°C;r.d. 6.965 (20°C); p.n. 70; most commonisotope 174Yb; r.a.m. 173.04.

239

XYZ

yttrium A silvery metallic element, thesecond element of group 3 (formerly IIIB)of the periodic table. It is found in almostevery lanthanoid mineral, particularlymonazite. Yttrium is used in various alloys,in yttrium–aluminum garnets used in theelectronics industry and as gemstones, as acatalyst, and in superconductors. A mix-ture of yttrium and europium oxides iswidely used as the red phosphor on televi-sion screens.

Symbol: Y; m.p. 1522°C; b.p. 3338°C;r.d. 4.469 (20°C); p.n. 39; r.a.m.88.90585.

Zeeman effect The splitting of atomicspectral lines by a magnetic field. This ef-fect was found by the Dutch physicistPieter Zeeman (1865–1943) in 1896. Someof the patterns of line splitting that be ex-plained both by classical electron theoryand the BOHR THEORY of electrons inatoms. The Zeeman splitting that can beexplained in these ways is known as thenormal Zeeman effect. There exist morecomplicated Zeeman splitting patterns thatcannot be explained either by classical elec-tron theory or the Bohr theory. This morecomplicated type of Zeeman effect isknown as the anomalous Zeeman effect. Itwas subsequently realized that the anom-alous Zeeman effect occurs because of elec-tron spin and that the normal Zeemaneffect occurs only for transitions betweensinglet states.

At very high magnetic fields a splittingpattern known as the Paschen–Back effectoccurs. In this pattern, named for the Ger-man spectroscopists Friedrich Paschen andErnst Back in 1912, the basic pattern re-turns to that of the normal Zeeman effect,but with each line split up into a set ofclosely spaced lines. This occurs becausethe total orbital and spin angular momen-tum vectors of the atom, denoted L and Srespectively, precess independently aboutthe direction of the magnetic field.

Zeise’s salt A complex of platinum andethane (ethylene) first synthesized by W. C.Zeise in 1827. It has the formulaPtCl3(CH2CH2) and was the first complexin which the metal ion was known to coor-

dinate to a pi-electron system rather thanto individual atoms.

zeolite A member of a group of hydratedaluminosilicate minerals, which occur innature and are also manufactured for theirion-exchange and selective-absorptionproperties. They are used for water soften-ing and for sugar refining. The zeoliteshave an open crystal structure and can beused as molecular sieves.

See also ion exchange; molecular sieve.

zepto- Symbol: z A prefix used with SIunits, denoting 10–21. For example, 1 zep-tometer (zm) = 10–21 meter (m).

zero order Describing a chemical reac-tion in which the rate of reaction is inde-pendent of the concentration of a reactant;i.e.

rate = k[X]0

The concentration of the reactant re-mains constant for a period of time al-though other reactants are beingconsumed. The hydrolysis of 2-bromo-2-methylpropane using aqueous alkali has arate expression,

rate = k[2-bromo-2-methylpropane]i.e. the reaction is zero order with re-

spect to the concentration of the alkali. Therate constant for a zero reaction has theunits mol dm–3 s–1.

zero point energy The energy pos-sessed by the atoms and molecules of a sub-stance at absolute zero (0 K).

zetta- Symbol: Z A prefix used with SIunits, denoting 1021. For example, 1zettameter (Zm) = 1021 meter (m).

Zewail, Ahmed H. (1946– ) Egyptian-born American chemist. Zewail haspioneered the technique called femto-chemistry for studying chemical reactions.Zewail and his colleagues have studied thedetailed dynamics of many chemical reac-tions using this technique. This has justi-fied the picture of chemical reactions givenby Svante ARRHENIUS, Henry EYRING, andothers as well as giving many surprising

yttrium

240

discoveries. Zewail won the 1999 NobelPrize for chemistry.

zinc A bluish-white hard brittle reactivetransition metal, the first element of group12 (formerly IIB) of the periodic table. Itoccurs naturally chiefly as the sulfide (zincblende) and carbonate (smithsonite). It isextracted by roasting the ore in air andthen reducing the oxide to the metal usingcarbon. Zinc is used to galvanize iron, inalloys (e.g. brass), and in dry batteries. Itreacts with acids and alkalis but only cor-rodes on the surface in air. Zinc ions areidentified in solution by forming white pre-cipitates with sodium hydroxide or ammo-nia solution, both precipitates beingsoluble in excess reagent. Zinc compoundsare used in paints, cosmetics, and medi-cines. See also zinc group.

Symbol: Zn; m.p. 419.58°C; b.p.907°C; r.d. 7.133 (20°C); p.n. 30; mostcommon isotope 64 Zn; r.a.m. 65.39.

zincate A salt containing the ionZnO2

2–.

zinc-blende (sphalerite) structure Aform of crystal structure that consists of azinc atom surrounded by four sulfur atomsarranged tetrahedrally; each sulfur atom issimilarly surrounded by four atoms of zinc.Zinc sulfide crystallizes in the cubic sys-tem. Covalent bonds of equal strength andlength result in the formation of a giantmolecular structure. If the zinc and sulfuratoms are replaced by carbon atoms the di-amond structure is produced. Wurtzite(another form of zinc sulfide) is similar butbelongs to the hexagonal crystal system.

zinc carbonate See calamine.

zinc chloride (ZnCl2) A white crys-talline solid prepared by the action of dryhydrogen chloride or chlorine on heatedzinc. The anhydrous product undergoessublimation. The hydrated salt may be pre-pared by the addition of excess zinc to di-lute hydrochloric acid and crystallizationof the solution. Hydrates with 4, 3, 5/2,3/2, and 1 molecule of water exist. Zincchloride is extremely deliquescent and dis-

solves easily in water. It is used in dentistry,as a flux, and as a dehydrating agent in or-ganic reactions.

zinc group A group of metallic elementsin the periodic table, consisting of zinc(Zn), cadmium (Cd), and mercury (Hg).The elements all occur at the ends of thethree transition series and all have outerd10s2 configurations; Zn [Ar]3d104s2, Cd[Kr]4d105s2, Hg[Xn]5d106s2. The elementsall use only outer s-electrons in reactionand in combination with other elementsunlike the coinage metals, which immedi-ately precede them. Thus all form diposi-tive ions, and oxidation states higher than+2 are not known. In addition mercuryforms the mercurous ion, sometimes writ-ten as Hg(I), but actually the ion +Hg-Hg+.Although the elements fall naturally at theend of the transition series their propertiesare more like those of main-group elementsthan of transition metals:

1. The melting points of the transitionelements are generally high whereas thoseof the zinc group are low (Zn 419.5°C, Cd329.9°C, Hg –38.87°C).

2. The zinc-group ions are diamagneticand colorless whereas most transition ele-ments have colored ions and many para-magnetic species.

3. The zinc group does not display thevariable valence associated with transitionmetal ions. However the group does havethe transition-like property of formingmany complexes or coordination com-pounds, such as [Zn(NH3)4]2+ and[Hg(CN)4]2–.

Within the group, mercury is anom-alous because of the Hg2

2+ ion mentionedpreviously and its general resistance to re-action. Zinc and cadmium are electroposi-tive and will react with dilute acids torelease hydrogen whereas mercury willnot. When zinc and cadmium are heated inair they burn to give the oxides, MO,which are stable to further strong heating.In contrast mercury does not react readilywith oxygen (slow reaction at the boilingpoint), and mercuric oxide, HgO, decom-poses to the metal and oxygen on furtherheating. All members of the group form di-alkyls and diaryls, R2M.

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zinc group

zincite (spartalite) A red-orange mineralform of zinc oxide, ZnO, often containingalso some manganese. It is an importantore of zinc. See zinc oxide.

zinc oxide (ZnO) A compound pre-pared by the thermal decomposition ofzinc nitrate or carbonate; it is a white pow-der when cold, yellow when hot. Zincoxide is an amphoteric oxide, almost insol-uble in water, but dissolves readily in bothacids and alkalis. If mixed with powderedcarbon and heated to red heat, it is reducedto the metal. Zinc oxide is used in the paintand ceramic industries. Medically it is usedin zinc ointment.

zinc sulfate (ZnSO4) A white crystallinesolid prepared by the action of dilute sulfu-ric acid on either zinc oxide or zinc car-bonate. On crystallization the hydratedsalt is formed. The heptahydrate(ZnSO4.7H2O) is formed below 30°C, thehexahydrate (ZnSO4.6H2O) above 30°C,and the monohydrate (ZnSO4.H2O) at100°C; at 450°C the salt is anhydrous.Zinc sulfate is extremely soluble in water.It is used in the textile industry.

zinc sulfide (ZnS) A compound that canbe prepared by direct combination of zincand sulfur. Alternatively it can be prepared

as a white amorphous precipitate by bub-bling hydrogen sulfide through an alkalinesolution of a zinc salt or by the addition ofammonium sulfide to a soluble zinc salt so-lution. Zinc sulfide occurs naturally as themineral zinc blende. It dissolves in diluteacids to yield hydrogen sulfide. Impurezinc sulfide is phosphorescent. It is used asa pigment and in the coatings on lumines-cent screens.

zircon See zirconium.

zirconium A hard lustrous silvery tran-sition element the second element of group4 (formerly IVB) of the periodic table. Itoccurs chiefly in the mineral zircon(ZrSiO4) some deposits of which are ofgemstone quality. It is extracted by chlori-nation, followed by reduction with mag-nesium. It is used in some strong alloysteels and as a protective coating. It is very corrosion-resistant.

Symbol: Zr; m.p. 1850°C; b.p. 4380°C;r.d. 6.506 (20°C); p.n. 40; most commonisotope 90Zr; r.a.m. 91.224.

zwitterion (ampholyte ion) An ion thathas both a positive and negative charge onthe same species. Zwitterions occur when amolecule contains both a basic group andan acidic group.

zincite

242

APPENDIXES

244

Appendix I

1

1

H2

He

1 2 3 4 5 6

3

Li

4

Be

5

B6

C7

N8

O9

F1

0 Ne

11 N

a1

2 Mg

13 A

l1

4 Si1

5

P1

6

S1

7 Cl

18 A

r

19 K

20 C

a2

1 Sc2

2 Ti

23 V

24 C

r2

5 Mn

26 F

e2

7 Co

28 N

i2

9 Cu

30 Z

n3

1 Ga

32 G

e3

3 As

34 Se

35 B

r3

6 Kr

37 R

b3

8 Sr3

9 Y4

0 Zr

41 N

b4

2 Mo

43 T

c4

4 Ru

45 R

h4

6 Pd

47 A

g4

8 Cd

49 In

50 Sn

51 Sb

52 T

e5

3

I5

4 Xe

55 C

s5

6 Ba

57-7

1

La-

Lu

72 H

f7

3 Ta

74 W

75 R

e7

6 Os

77 Ir

78 P

t7

9 Au

80 H

g8

1 Tl

82 P

b8

3 Bi

84 P

o8

5 At

86 R

n

87 F

r8

8 Ra

89-1

03

Ac-

Lr

10

4 Rf

10

5 Db

10

6 Sg1

07 Bh

10

8 Hs

10

9 Mt

11

0 Ds

11

1

Uu

u1

12

Uu

b1

13

11

4

Uu

q1

15

11

6

Uu

h

23

45

67

89

10

11

12

13

14

15

16

17

18

Peri

odic

Tab

le o

f th

e E

lem

ents

- gi

vin

g gr

ou

p,

ato

mic

nu

mb

er,

and

ch

emic

al s

ymb

ol

Lan

than

ides

Act

inid

es

Mo

der

n f

orm

Eu

rop

ean

con

ven

tio

nN

. A

mer

ican

con

ven

tio

n

Th

e ab

ove

is

the

mo

der

n r

eco

mm

end

ed f

orm

of

the

tab

le u

sin

g 1

8gr

ou

ps.

Old

er g

rou

p d

esig

nat

ion

s ar

e sh

ow

n b

elo

w.

Period

6 7

57 L

a

89 A

c

58 C

e

90 T

h

59 P

r

91 P

a

60 N

d

92 U

61 P

m

93 N

p

62 Sm

94 P

u

63 E

u

95 A

m

64 G

d

96 C

m

65 T

b

97 B

k

66 D

y

98 C

f

67 H

o

99 E

s

68 E

r

10

0 Fm

69 T

m

10

1 Md

70 Y

b

10

2 No

71 L

u

10

3 Lr

12

34

56

78

91

01

11

21

31

41

51

61

71

8

IAII

AII

IAIV

AV

AV

IAV

IIA

VII

I (o

r V

IIIA

)IB

IIB

IIIB

IVB

VB

VIB

VII

B0

(o

r V

IIIB

)

IAII

AII

IBIV

BV

BV

IBV

IIB

VII

I (o

r V

IIIB

)IB

IIB

IIIA

IVA

VA

VIA

VII

AV

IIIA

(or

0)

7

245

Appendix IIThe Chemical Elements

(* indicates the nucleon number of the most stable isotope)

Element Symbol p.n. r.a.m Element Symbol p.n. r.a.m

actinium Ac 89 227*

aluminum Al 13 26.982

americium Am 95 243*

antimony Sb 51 112.76

argon Ar 18 39.948

arsenic As 33 74.92

astatine At 85 210

barium Ba 56 137.327

berkelium Bk 97 247*

beryllium Be 4 9.012

bismuth Bi 83 208.98

bohrium Bh 107 262*

boron B 5 10.811

bromine Br 35 79.904

cadmium Cd 48 112.411

calcium Ca 20 40.078

californium Cf 98 251*

carbon C 6 12.011

cerium Ce 58 140.115

cesium Cs 55 132.905

chlorine Cl 17 35.453

chromium Cr 24 51.996

cobalt Co 27 58.933

copper Cu 29 63.546

curium Cm 96 247*

darmstadtium Ds 110 269*

dubnium Db 105 262*

dysprosium Dy 66 162.50

einsteinium Es 99 252*

erbium Er 68 167.26

europium Eu 63 151.965

fermium Fm 100 257*

fluorine F 9 18.9984

francium Fr 87 223*

gadolinium Gd 64 157.25

gallium Ga 31 69.723

germanium Ge 32 72.61

gold Au 79 196.967

hafnium Hf 72 178.49

hassium Hs 108 265*

helium He 2 4.0026

holmium Ho 67 164.93

hydrogen H 1 1.008

indium In 49 114.82

iodine I 53 126.904

iridium Ir 77 192.217

iron Fe 26 55.845

krypton Kr 36 83.80

lanthanum La 57 138.91

lawrencium Lr 103 262*

lead Pb 82 207.19

lithium Li 3 6.941

lutetium Lu 71 174.967

magnesium Mg 12 24.305

manganese Mn 25 54.938

meitnerium Mt 109 266*

mendelevium Md 101 258*

mercury Hg 80 200.59

molybdenum Mo 42 95.94

neodymium Nd 60 144.24

246

Appendix II

The Chemical ElementsElement Symbol p.n. r.a.m Element Symbol p.n. r.a.m

neon Ne 10 20.179

neptunium Np 93 237.048

nickel Ni 28 58.69

niobium Nb 41 92.91

nitrogen N 7 14.0067

nobelium No 102 259*

osmium Os 76 190.23

oxygen O 8 15.9994

palladium Pd 46 106.42

phosphorus P 15 30.9738

platinum Pt 78 195.08

plutonium Pu 94 244*

polonium Po 84 209*

potassium K 19 39.098

praseodymium Pr 59 140.91

promethium Pm 61 145*

protactinium Pa 91 231.036

radium Ra 88 226.025

radon Rn 86 222*

rhenium Re 75 186.21

rhodium Rh 45 102.91

rubidium Rb 37 85.47

ruthenium Ru 44 101.07

rutherfordium Rf 104 261*

samarium Sm 62 150.36

scandium Sc 21 44.956

seaborgium Sg 106 263*

selenium Se 34 78.96

silicon Si 14 28.086

silver Ag 47 107.868

sodium Na 11 22.9898

strontium Sr 38 87.62

sulfur S 16 32.066

tantalum Ta 73 180.948

technetium Tc 43 99*

tellurium Te 52 127.60

terbium Tb 65 158.925

thallium Tl 81 204.38

thorium Th 90 232.038

thulium Tm 69 168.934

tin Sn 50 118.71

titanium Ti 22 47.867

tungsten W 74 183.84

uranium U 92 238.03

vanadium V 23 50.94

xenon Xe 54 131.29

ytterbium Yb 70 173.04

yttrium Y 39 88.906

zinc Zn 30 65.39

zirconium Zr 40 91.22

247

The Greek Alphabet

A α alphaB β betaΓ γ gamma∆ δ deltaE ε epsilonZ ζ zetaH η etaΘ θ thetaI ι iotaK κ kappaΛ λ lambdaM µ mu

N ν nuΞ ξ xiO ο omikronΠ π piP ρ rhoΣ σ sigmaT τ tauΥ υ upsilonΦ φ phiX χ chiΨ ψ psiΩ ω omega

Appendix III

Fundamental Constants

Appendix IV

speed of light c 2.997 924 58 × 108 m s–1

permeability of free space µo 4π × 10–7

= 1.256 637 0614 × 10–6 H m–1

permittivity of free space ε0=µ0–1c–2 8.854 187 817 × 10–12 F m–1

charge of electron or proton e ±1.602 177 33 × 10–19 Crest mass of electron me 9.109 39 × 10–31 kgrest mass of proton mp 1.672 62 × 10–27 kgrest mass of neutron mn 1.674 92 × 10–27 kgelectron charge-to-mass ratio e/m 1.758 820 × 1011 C kg–1

electron radius re 2.817 939 × 10–15 mPlanck constant h 6.626 075 × 10–34 J sBoltzmann constant k 1.380 658 × 10–23 J K–1

Faraday constant F 9.648 531 × 104 C mol–1

248

Chemical society webpages include:

American Chemical Society www.chemistry.org

Royal Society of Chemistry www.rsc.org

The International Union of Pure and www.iupac.orgApplied Chemistry

Information on nomenclature is available at:

Queen Mary College, London www.chem.qmul.ac.uk/iupac

Advanced Chemistry Development, Inc www.acdlabs.com/iupac/nomenclature

The definitive site for the chemical elements is:

WebElements Periodic Table www.webelements.com

Webpages

Appendix V

BibliographyThere are a number of comprehensive texts covering inorganic chemistry. These include:

Cotton, F. A.; Murillo, C.; Wilkinson, G.; Bochmann, M. & Grimes, R. Advanced Inor-ganic Chemistry. 6th ed. New York: Wiley, 1999

Greenwood, N. N. & Earnshaw, A. Chemistry of the Elements. Oxford, U.K.: Butter-worth-Heinemann, 1997

Shriver, D. F. & Atkins, P. W. Inorganic Chemistry. 3rd ed. Oxford, U.K.: Oxford Uni-versity Press, 1999

Additional useful sources are:

Emsley, J. Nature's Building Blocks – An A-Z Guide to the Elements. Oxford, U.K.:Oxford University Press, 2001

King, R.B. (ed.) Encyclopedia of Inorganic Chemistry. New York: Wiley, 1994