The Facts On File

DICTIONARYof

CHEMISTRY

The Facts On File

DICTIONARYof

CHEMISTRY

Edited byJohn Daintith

Fourth Edition

The Facts On File Dictionary of ChemistryFourth Edition

Copyright 2005, 1999 by Market House Books Ltd

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PREFACE

This dictionary is one of a series designed for use in schools. It is intended for stu-dents of chemistry, but we hope that it will also be helpful to other science studentsand to anyone interested in science. Facts On File also publishes dictionaries in avariety of disciplines, including biology, physics, mathematics, forensic science,weather and climate, marine science, and space and astronomy.

The Facts On File Dictionary of Chemistry was first published in 1980 and the thirdedition was published in 1999. This fourth edition of the dictionary has been exten-sively revised and extended. The dictionary now contains over 3,000 headwords cov-ering the terminology of modern chemistry. A totally new feature of this edition isthe inclusion of over 1,700 pronunciations for terms that are not in everyday use. Anumber of appendixes have been included at the end of the book containing usefulinformation, including a list of chemical elements and a periodic table. There is alsoa list of Web sites and a bibliography. A guide to using the dictionary has also beenadded to this latest version of the book.

We would like to thank all the people who have cooperated in producing this book.A list of contributors is given on the acknowledgments page. We are also grateful tothe many people who have given additional help and advice.

v

ACKNOWLEDGMENTS

Contributors

Julia Brailsford B.Sc.John Clark B.Sc.John Connor B.Sc.Derek Cooper Ph.D. F.R.I.C.Rich Cutler B.Sc.D.E. Edwards B.Sc. M.Sc.Richard Hadfield B.Sc.Valerie Illingworth B.Sc. M.Phil.Alan Isaacs B.Sc. Ph.D.Elizabeth Martin M.A.P.R. Mercer B.A. T.C.R.S. Smith B.Sc. C.Chem. M.R.I.C.Derek Stefaniw B.Sc. Ph.D.Elizabeth Tootil M.Sc.J. Truman B.Sc.David Eric Ward B.Sc. M.Sc. Ph.D.

Pronunciations

William Gould B.A.

NoteUnless otherwise stated, the melting and boiling points given in the dictionary areat standard pressure. Relative densities of liquids are at standard pressure with theliquid at 20C relative to water at 4C. Relative densities of gases are relative toair, both gases being at standard temperature and pressure.

The following abbreviations are used in the text:

p.n. proton number(atomic number)

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

vi

CONTENTS

Preface v

Acknowledgments vi

Guide to Using the Dictionary viii

Pronunciation Key x

Entries A to Z 1

Appendixes

I. Carboxylic Acids 297

II. Amino Acids 299

III. Sugars 302

IV. Nitrogenous Bases 303and Nucleosides

V. The Chemical Elements 305

VI. The Periodic Table 307

VII. The Greek Alphabet 308

VIII. Web Sites 309

Bibliography 310

vii

GUIDE TO USING THE DICTIONARY

The main features of dictionary entries are as follows.

HeadwordsThe main term being defined is in bold type:

acid A substance that gives rise to hy-drogen ions when dissolved in water.

PluralsIrregular plurals are given in brackets after the headword.

quantum (pl. quanta) A definite amountof energy released or absorbed in a process.

VariantsSometimes a word has a synonym or alternative spelling. This is placed in brackets afterthe headword, and is also in bold type:

promoter (activator) A substance thatimproves the efficiency of a catalyst.

Here, activator is another word for promoter. Generally, the entry for the synonym con-sists of a simple cross-reference:

activator See promoter.

AbbreviationsAbbreviations for terms are treated in the same way as variants:

electron spin resonance (ESR) A simi-lar technique to nuclear magnetic reso-nance, but applied to unpaired electrons ...

The entry for the synonym consists of a simple cross-reference:

ESR See electron spin resonance.

FormulasChemical formulas are also placed in brackets after the headword. These are in normaltype:

potassium iodide (KI) A white ionicsolid....

Here, KI is the chemical formula for potassium iodide.

viii

Multiple definitionsSome terms have two or more distinct senses. These are numbered in bold type

abundance 1. The relative amount of agiven element among others; for example,the abundance of oxygen in the Earthscrust is approximately 50% by mass.2. The amount of a nuclide (stable or ra-dioactive) relative to other nuclides of thesame element in a given sample.

Cross-referencesThese are references within an entry to other entries that may give additional useful in-formation. Cross-references are indicated in two ways. When the word appears in the de-finition, it is printed in small capitals:

boron nitride (BN) A compound formedby heating BORON in nitrogen

In this case the cross-reference is to the entry for `boron.

Alternatively, a cross-reference may be indicated by See, See also, or Compare, usu-ally at the end of an entry:

boron trifluoride (BF3) A colorless fum-ing gas made by See boron trichloride.

Hidden entriesSometimes it is convenient to define one term within the entry for another term:

charcoal An amorphous form of carbonmade by Activated charcoal is charcoalheated to

Here, activated charcoal is a hidden entry under charcoal, and is indicated by italic type.The entry for activated charcoal consists of a simple cross-reference:

activated charcoal See charcoal.

PronunciationsWhere appropriate pronunciations are indicated immediately after the headword, en-closed in forward slashes:

deuteride /dew-ter-d/ A compound ofdeuterium

Note that simple words in everyday language are not given pronunciations. Also head-words that are two-word phrases do not have pronunciations if the component words arepronounced elsewhere in the dictionary.

ix

/a/ as in back /bak/, active /ak-tiv/// as in abduct /b-dukt/, gamma /gam-//ah/ as in palm /pahm/, father /fah-ther/,/air/ as in care /kair/, aerospace /air--

spays//ar/ as in tar /tar/, starfish /star-fish/, heart

/hart//aw/ as in jaw /jaw/, gall /gawl/, taut /tawt//ay/ as in mania /may-ni/ ,grey /gray//b/ as in bed /bed//ch/ as in chin /chin//d/ as in day /day//e/ as in red /red/// as in bowel /bow-l//ee/ as in see /see/, haem /heem/, caffeine

/kaf-een/, baby /bay-bee//eer/ as in fear /feer/, serum /seer-m//er/ as in dermal /der-ml/, labour /lay-ber//ew/ as in dew /dew/, nucleus /new-klee-s//ewr/ as in epidural /ep-i-dewr-l//f/ as in fat /fat/, phobia /foh-bi/, rough

/ruf//g/ as in gag /gag//h/ as in hip /hip//i/ as in fit /fit/, reduction /ri-duk-shn//j/ as in jaw /jaw/, gene /jeen/, ridge /rij//k/ as in kidney /kid-nee/, chlorine /klor-

een/, crisis /kr-sis//ks/ as in toxic /toks-ik//kw/ as in quadrate /kwod-rayt//l/ as in liver /liv-er/, seal /seel//m/ as in milk /milk//n/ as in nit /nit/

/ng/ as in sing /sing//nk/ as in rank /rank/, bronchus /bronk-s//o/ as in pot /pot/// as in dog /dg/// as in buttock /but-k//oh/ as in home /hohm/, post /pohst//oi/ as in boil /boil//oo/ as in food /food/, croup /kroop/, fluke

/flook//oor/ as in pruritus /proor--tis//or/ as in organ /or-gn/, wart /wort//ow/ as in powder /pow-der/, pouch

/powch//p/ as in pill /pil//r/ as in rib /rib//s/ as in skin /skin/, cell /sel//sh/ as in shock /shok/, action /ak-shn//t/ as in tone /tohn//th/ as in thin /thin/, stealth /stelth//th/ as in then /then/, bathe /bayth//u/ as in pulp /pulp/, blood /blud/// as in typhus /t-fs/// as in pull /pl/, hook /hk//v/ as in vein /vayn//w/ as in wind /wind//y/ as in yeast /yeest/// as in bite /bt/, high /h/, hyperfine /h-

per-fn//yoo/ as in unit /yoo-nit/, formula /form-yoo-l//yoor/ as in pure /pyoor/, ureter /yoor-ee-ter//r/ as in fire /fr/

x

Bold type indicates a stressed syllable. In pronunciations, a consonant is sometimes dou-bled to prevent accidental mispronunciation of a syllable resembling a familiar word; forexample, /ass-id/ (acid), rather than /as-id/ and /ul-tr- sonn-iks/ (ultrasonics), rather than/ul-tr-son-iks/. An apostrophe is used: (a) between two consonants forming a syllable, asin /den-tl/ (dental), and (b) between two letters when the syllable might otherwise be mis-pronounced through resembling a familiar word, as in /the-r-pee/ (therapy) and /talk/(talc). The symbols used are:

Pronunciation Key

AAS See atomic absorption spec-troscopy.

absolute alcohol Pure alcohol (ethanol).

absolute configuration A particularmolecular configuration of a CHIRAL mole-cule, as denoted by comparison with a ref-erence molecule or by some sequence rule.There are two systems for expressing ab-solute configuration in common use: theDL convention and the RS convention.See optical activity.

absolute temperature Symbol: T Atemperature defined by the relationship:

T = + 273.15where 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 + )where V is the volume at temperature , V0the volume at 0, and the thermal expan-sivity of the gas. At low pressures (whenreal gases show ideal behavior) has thevalue 1/273.15. Therefore, at = 273.15the volume of the gas theoreticallybecomes zero. In practice, of course, sub-stances become solids at these tempera-tures. However, the extrapolation can beused for a scale of temperature on which273.15C corresponds to 0 (absolutezero). The scale is also known as the ideal-gas scale; on it temperature intervals werecalled degrees absolute (A) or degreesKelvin (K), and were equal to the Celsiusdegree. It can be shown that the absolutetemperature scale is identical to the ther-modynamic temperature scale (on whichthe unit is the kelvin).

absolute zero The zero value of ther-

modynamic temperature; 0 kelvin or273.15C.

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 material. Solids that absorb gases orliquids often have a porous structure. Theabsorption of gases in solids is sometimescalled sorption. Compare adsorption.

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 Earthscrust is approximately 50% by mass.2. The amount of a nuclide (stable or ra-dioactive) relative to other nuclides of thesame element in a given sample. The nat-ural abundance is the abundance of a nu-clide as it occurs naturally. For instance,chlorine has two stable isotopes of masses

1

A

accelerator

2

35 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.

accelerator A catalyst added to increasethe rate of cross-linking reactions in poly-mers.

acceptor /ak-sep-ter, -tor/ The atom orgroup to which a pair of electrons is do-nated in a coordinate bond. Pi-acceptorsare compounds or groups that accept elec-trons into pi, p or d orbitals.

accumulator (secondary cell; storage bat-tery) An electric cell or battery that canbe charged by passing an electric currentthrough it. The chemical reaction in the cellis reversible. When the cell begins to rundown, current in the opposite directionwill convert the reaction products backinto their original forms. The most com-mon example is the LEAD-ACID ACCUMULA-TOR, used in vehicle batteries.

acenaphthene /as--naf-theen, -nap-/(C12H10) A colorless crystalline deriva-tive of naphthalene, used in producingsome dyes.

acetal /ass--tal/ A type of organic com-pound formed by addition of an alcohol toan aldehyde. Addition of one alcohol mol-ecule gives a hemiacetal. Further additionyields the full acetal. Similar reactionsoccur with ketones to produce hemiketalsand ketals.

acetaldehyde /ass--tal-d-hd/ Seeethanal.

acetamide /ass--tam-d, -id; -set--md,-mid/ See ethanamide.

acetate /ass--tayt/ See ethanoate.

acetic acid /-see-tik, -set-ik/ Seeethanoic acid.

acetone /ass--tohn/ See propanone.

acetonitrile /ass--toh-n-trl, -see-toh-,-tril, -trl/ See methyl cyanide.

acetophenone /ass--toh-fee-nohn, -see-toh-/ See phenyl methyl ketone.

acetylation /-setl-ay-shn/ See acylation.

acetyl chloride /ass--tl, -see-tl/ Seeethanoyl chloride.

acetylene /-set--leen, -lin/ See ethyne.

acetyl group See ethanoyl group.

acetylide /-set--ld/ See carbide.

acetylsalicylic acid /ass--tl-sal--sil-ik,-see-tl-/ See aspirin.

Acheson process /ach--sn/ See carbon.

achiral /-kr-l/ Describing a moleculethat does not exhibit optical activity.

acid A substance that gives rise to hy-drogen ions when dissolved in water.Strictly, these ions are hydrated, known ashydroxonium or hydronium ions, and areusually given the formula H3O

+. An acid insolution will have a pH below 7. This def-inition does not take into account the com-petitive behavior of acids in solvents and itrefers only to aqueous systems. The LowryBrnsted theory defines an acid as a sub-stance that exhibits a tendency to release aproton, and a base as a substance thattends to accept a proton. Thus, when anacid releases a proton, the ion formed is theconjugate base of the acid. Strong acids(e.g. HNO3) react completely with waterto give H3O

+, i.e. HNO3 is stronger thanH3O

+ and the conjugate base NO3 is

weak. Weak acids (e.g. CH3COOH andC6H5COOH) are only partly dissociatedbecause H3O

+ is a stronger acid than the

1 2

Acenaphthene

free acids and the ions CH3COO and

C6H5COO are moderately strong bases.

The LowryBrnsted theory is named forthe English chemist Thomas Martin Lowry(18741936) and the Danish physicalchemist Johannes Nicolaus Brnsted(18791947). See also Lewis acid.

acid anhydride A type of organic com-pound of general formula RCOOCOR,where R and R are alkyl or aryl groups.They are prepared by reaction of an acylhalide with the sodium salt of a carboxylicacid, e.g.:

RCOCl + RCOONa+ RCOOCOR+ NaCl

Like the acyl halides, they are very reactiveacylating agents. Hydrolysis is to car-boxylic acids:

RCOOCOR + H2O RCOOH +RCOOH

See also acylation.

acidbase indicator An indicator thatis either 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.

acid dyes The sodium salts of organicacids used in the dyeing of silks and wool.They are so called because they are appliedfrom a bath acidified with dilute sulfuric orethanoic acid.

acid halide See acyl halide.

acidic Having a tendency to release aproton or to accept an electron pair from adonor. In aqueous solutions the pH is a

measure 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(VI)acid:

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

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

acidimetry /ass--dim--tree/ Volumet-ric analysis or acid-base titration in whicha standard solution of an acid is added tothe unknown (base) solution plus the indi-cator. Alkalimetry is the converse, i.e. thebase is in the buret.

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 are of the type NaHSO4 (sodium hy-drogensulfate) and Na3H(CO3)2.2H2O(sodium sesquicarbonate). For monobasicacids such as HF the acid salts are of theform KHF2 (potassium hydrogen fluoride).Although the latter were at one time for-mulated as a normal salt plus excess acid(i.e. KF.HF) it is preferable to treat these ashydrogen-bonded systems of the type K+

(FHF).

3

acid salt

RC O

O

C OR

Acid anhydride structure

acid value A measure of the free acidpresent in fats, oils, resins, plasticizers, andsolvents, defined as the number of mil-ligrams of potassium hydroxide requiredto neutralize the free acids in one gram ofthe substance.

acridine /ak-ri-deen/ (C12H9N) A color-less crystalline heterocyclic compoundwith three fused rings. Derivatives of acri-dine are used as dyes and biological stains.

Acrilan /ak-r-lan/ (Trademark) A syn-thetic fiber that consists of a copolymer of1-cyanoethene (acrylonitrile, vinylcyanide) and ethenyl ethanoate (vinyl ac-etate).

acrolein /-kroh-lee-in/ See propenal.

acrylic acid /-kril-ik/ See propenoicacid.

acrylic resin A synthetic resin made bypolymerizing an amide or ester derivativeof 2-propenoic acid (acrylic acid). Exam-ples of acrylic materials are Acrilan (frompropenonitrile) and Plexiglas (polymethyl-methacrylate). Acrylic resins are also usedin paints.

acrylonitrile /ak-r-loh-n-trl, -tril, -trl,-kril-oh-/ See propenonitrile.

actinic radiation /ak-tin-ik/ Radiationthat can cause a chemical reaction; for ex-ample, ultraviolet radiation is actinic.

actinides /ak-t-ndz/ See actinoids.

actinium /ak-tin-ee-m/ A soft silvery-white radioactive metallic element that isthe first member of the actinoid series. Itoccurs in minute quantities in uranium

ores and the metal can be obtained by re-ducing the trifluoride with lithium. It canbe produced by neutron bombardment ofradium and is used as a source of alphaparticles. The metal glows in the dark; it re-acts with water to produce hydrogen.

Symbol: Ac; m.p. 105050C; b.p.3200300C; r.d. 10.06 (20C); p.n. 89;most stable isotope 227Ac (half-life 21.77years).

actinoids /ak-t-noidz/ (actinides) Agroup of 15 radioactive elements whoseelectronic configurations display filling ofthe 5f level. As with the lanthanoids, thefirst member, actinium, has no f electrons(Ac [Rn]6d17s2) but other members alsoshow deviations from the smooth trend off-electron filling expected from simple con-siderations, e.g. thorium Th [Rn]6d27s2,berkelium Bk [Rn]5f86d17s2. The actinoidsare all radioactive and their chemistry isoften extremely difficult to study. In gen-eral, artificial methods using high-energybombardment are used to generate them.See also transuranic elements.

activated charcoal See charcoal.

activated complex The partiallybonded system of atoms in the transitionstate of a chemical reaction.

activation energy Symbol: Ea The min-imum energy a particle, molecule, etc.,must acquire before it can react; i.e. the en-ergy required to initiate a reaction regard-less of whether the reaction is exothermicor endothermic. Activation energy is oftenrepresented as an energy barrier that mustbe overcome if a reaction is to take place.See Arrhenius equation.

activator See promoter.

active mass See mass action; law of.

active site 1. a site on the surface of acatalyst at which catalytic activity occursor at which the catalyst is particularly ef-fective.

acid value

4

NNN

Acridine

2. The position on the molecule of an en-zyme that binds to the substrate when theENZYME acts as a catalyst.

activity 1. Symbol: a Certain thermody-namic properties of a solvated substanceare dependent on its concentration (e.g. itstendency to react with other substances).Real substances show departures fromideal behavior and a corrective concentra-tion term the activity has to be intro-duced into equations describing realsolvated systems.2. Symbol: A The average number of atomsdisintegrating per unit time in a radioactivesubstance.

activity coefficient Symbol: f A mea-sure of the degree of deviation from ideal-ity of 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 /ay-s-klik, -sik-lik/ Describing acompound that is not cyclic (i.e. a com-pound that does not contain a ring in itsmolecules).

acyl anhydride /ass-l, ay-sl/ See acidanhydride.

acylating agent /ass--layt-ing/ See acy-lation.

acylation /ass--lay-shn/ A reactionthat introduces the acyl group (RCO).Acylating agents are acyl halides (R.CO.X)and acid anhydrides (R.CO.O.CO.R),which react with such nucleophiles asH2O, ROH, NH3, and RNH2. In thesecompounds a hydrogen atom of a hydroxylor amine group is replaced by the RCOgroup. In acetylation the acetyl group(CH3CO) is used. In benzoylation thebenzoyl group (C6H5CO) is used. Acyla-tion is used to prepare crystalline deriva-tives of organic compounds to identifythem (e.g. by melting point) and to protectOH groups in synthetic reactions.

acyl group The group of atoms RCO.

acyl halide (acid halide) A type of or-ganic compound of the general formulaRCOX, where X is a halogen (acyl chlo-ride, acyl bromide, etc.).

Acyl halides can be prepared by the re-action of carboxylic acid with a halogenat-ing agent. Commonly, phosphorus halidesare used (e.g. PCl5) or a sulfur dihalideoxide (e.g. SOCl2):

RCOOH + PCl5 RCOCl + POCl3+ HCl

RCOOH + SOCl2 RCOCl + SO2+ HCl

The acyl halides have irritating vaporsand fume in moist air. They are very reac-tive to the hydrogen atom of compoundscontaining hydroxyl (OH) or amine(NH2) groups. See acylation.

addition polymerization See polymer-ization.

addition reaction A reaction in whichadditional atoms or groups of atoms are in-troduced into an unsaturated compound,such as an alkene or ketone. A simple ex-ample is the addition of bromine across thedouble bond in ethene:H2C:CH2 + Br2 BrH2CCH2Br

Addition reactions can be induced ei-ther by electrophiles or by nucleophiles.See also electrophilic addition; nucleophilicaddition.

adduct /-dukt/ See coordinate bond.

adenine /ad--neen, -nin, -nn/ A ni-trogenous base found in DNA and RNA. Itis also a constituent of certain coenzymesand when combined with the sugar ribose

5

adenine

O

CR Cl

Acyl halide: an acyl chloride

it forms the nucleoside adenosine found inAMP, ADP, and ATP. Adenine has apurine ring structure.

adenosine /-den--seen, -sin, ad-n--seen/ (adenine nucleoside) A NUCLEOSIDEformed from adenine linked to D-ribosewith a -glycosidic bond. It is widely foundin all types of cell, either as the free nucle-oside or in combination in nucleic acids.Phosphate esters of adenosine, such as ATP,are important carriers of energy in bio-chemical reactions.

adenosine diphosphate /d-fos-fayt/ SeeADP.

adenosine monophosphate /mon-oh-fos-fayt/ See AMP.

adenosine triphosphate /tr-fos-fayt/See ATP.

adiabatic change /ad-ee--bat-ik/ Achange during which no energy enters orleaves the system.

In an adiabatic expansion of a gas, me-chanical work is done by the gas as its vol-ume increases and the gas temperature falls.For an ideal gas undergoing a reversibleadiabatic change it can be shown that

pV = K1Tp1 = K2

and TV1 = K3where K1, K2, and K3 are constants and isthe ratio of the principal specific heat ca-pacities. Compare isothermal change.

adipic acid /-dip-ik/ See hexanedioicacid.

ADP (adenosine diphosphate) A NUCLEO-TIDE consisting of adenine and ribose withtwo phosphate groups attached. See alsoATP.

adsorbate /ad-sor-bayt, -zor-/ A sub-stance that is adsorbed on a surface. Seeadsorption.

adsorbent /ad-sor-bnt, -zor-/ The sub-stance on whose surface ADSORPTION takesplace.

adsorption /ad-sorp-shn, -zorp-/ Aprocess in which a layer of atoms or mole-cules of one substance forms on the surfaceof a solid or liquid. All solid surfaces takeup layers of gas from the surrounding at-mosphere. The adsorbed layer may be heldby chemical bonds (chemisorption) or byweaker van der Waals forces (physisorp-tion). Compare absorption.

adsorption indicator See absorptionindicator.

aerobic /air-oh-bik/ Describing a bio-chemical process that takes place only inthe presence of free oxygen. Compareanaerobic.

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 /ag-it, -ayt/ A hard microcrys-talline form of the mineral chalcedony (avariety of quartz). Typically it has greenishor brownish bands of coloration, and isused for making ornaments. Moss agate isnot banded, but has mosslike patterns re-sulting from the presence of iron and man-ganese oxides. Agate is used in instrumentbearings because of its resistance to wear.

agent orange A herbicide consisting ofa mixture of two weedkillers (2,4-D and2,4,5-T), which was formerly used in war-fare to defoliate trees where an enemy maybe hiding or to destroy an enemys crops. Italso contains traces of the highly toxicchemical dioxin, which may cause cancersand birth defects.

air The mixture of gases that surroundsthe Earth. The composition of dry air, byvolume, is:

nitrogen 78.08% oxygen 20.95% argon 0.93%

adenosine

6

carbon dioxide 0.03% neon 0.0018% helium 0.0005% krypton 0.0001% xenon 0.00001%Air also contains a variable amount of

water vapor, as well as particulate matter(e.g. dust and pollen), and small amountsof other gases.

air gas See producer gas.

alabaster A mineral form of gypsum(CaSO4.2H2O).

alanine /al--neen, -nn/ See aminoacids.

albumen /al-byoo-mn/ The white of anegg, which consists mainly of albumin. Seealbumin.

albumin /al-byoo-min/ A soluble proteinthat occurs in many animal fluids, such asblood serum and egg white.

alchemy An ancient pseudoscience thatwas the precursor of chemistry, datingfrom early Christian times until the 17thcentury. It combined mysticism and exper-imental techniques. Many ancient al-chemists searched for the Philosophersstone 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).

Alcohols have the OH group attachedto a carbon atom that is not part of an aro-matic ring: C6H5OH, in which the OHgroup is attached to the ring, is thus a phe-nol. Phenylmethanol (C6H5CH2OH) doeshave the characteristic properties of alco-hols.

Alcohols can have more than one OHgroup; those containing two, three or moresuch groups are described as dihydric, tri-hydric, and polyhydric respectively (as op-posed to those containing one OH group,

which are monohydric). Examples areethane-1,2-diol (ethylene glycol; (HOCH2-CH2OH) and propane-1,2,3-triol (glyc-erol; HOCH2CH(OH)CH2OH).

Alcohols are further classified accord-ing to the environment of the COHgrouping. If the carbon atom is attached totwo hydrogen atoms, the compound is aprimary alcohol. If the carbon atom is at-tached to one hydrogen atom and twoother groups, it is a secondary alcohol. Ifthe carbon atom is attached to three othergroups, it is a tertiary alcohol. Alcohols canbe prepared by:1. Hydrolysis of HALOALKANES using aque-

ous potassium hydroxide:RI + OH ROH + I

2. Reduction of aldehydes by nascent hy-drogen (from sodium amalgam in water):

RCHO +2[H] RCH2OHThe main reactions are:1. Oxidation by potassium dichromate(VI)

in sulfuric acid. Primary alcohols give

7

alcohol

primary (ethanol)

secondary (propan-2-ol)

tertiary (2-methylpropan-2-ol)

C

H OH

HCH3

C

H3C OH

HH3C

C

H3C OH

CH3H3C

Alcohols

aldehydes, which are further oxidized tocarboxylic acids:

RCH2OH RCHO RCOOH1. Secondary alcohols are oxidized to ke-

tones.2. Formation of esters with acids. The re-

action, which is reversible, is catalyzedby H+ ions:ROH + RCOOH RCOOR + H2O

3. Dehydration over hot pumice (400C)to alkenes:

RCH2CH2OH H2O RCH:CH24. Reaction with sulfuric acid. Two types

of reaction are possible. With excessacid at 160C dehdyration occurs togive an alkene:

RCH2CH2OH + H2SO4 H2O +RCH2CH2.HSO4

RCH2CH2.HSO4 RCH:CH2 +H2SO4

4. With excess alcohol at 140C an ether isformed:

2ROH ROR + H2OSee also acylation.

aldehyde /al-d-hd/ A type of organiccompound with the general formulaRCHO, where the CHO group (the alde-hyde group) consists of a carbonyl groupattached to a hydrogen atom. Simple ex-amples of aldehydes are methanal(formaldehyde, HCHO) and ethanal (ac-etaldehyde, CH3CHO).

Aldehydes are formed by oxidizing aprimary alcohol. In the laboratory potas-sium dichromate(VI) is used in sulfuricacid. They can be further oxidized to car-boxylic acids. Reduction (using a catalystor nascent hydrogen from sodium amal-gam in water) produces the parent alcohol.

Aldehydes undergo a number of reac-tions:1. They act as reducing agents, being oxi-

dized to carboxylic acids in the process.These reactions are used as tests foraldehydes using such reagents asFehlings solution and Tollens reagent(silver-mirror test).

2. They form addition compounds withhydrogen cyanide to give cyanohy-drins. For example, propanal gives 2-hydroxybutanonitrile:

C2H5CHO + HCN C2H5CH(OH)CN

3. They form addition compounds (bisul-fite addition compounds) with the hy-drogensulfate(IV) ion (hydrogensulfite;HSO3

):RCHO + HSO3

RCH(OH)(HSO3)4. They undergo CONDENSATION REACTIONS

with such compounds as hydrazine, hy-droxylamine, and their derivatives.

5. With alcohols they form hemiacetalsand ACETALS.

6. They polymerize readily. Polymethanalor methanal trimer can be formed frommethanal depending on the conditions.Ethanal gives ethanal trimer or ethanaltetramer.See also Cannizzaro reaction; ketone.

aldohexose /al-doh-heks-ohs/ An aldoseSUGAR with six carbon atoms.

aldol /al-dol, -dohl/ See aldol reaction.

aldol reaction A reaction in which twomolecules of aldehyde combine to give analdol i.e. a compound containing bothaldehyde and alcohol functional groups.The reaction is base-catalyzed; the reactionof ethanal refluxed with sodium hydroxidegives:2CH3CHO CH3CH(OH)CH2CHOThe mechanism is similar to that of theCLAISEN CONDENSATION: the first step is re-moval of a proton to give a carbanion,which subsequently attacks the carbon ofthe carbonyl group on the other molecule:CH3CHO + OH

CH2CHO + H2O.

aldopentose /al-doh-pen-tohs/ An al-dose SUGAR with five carbon atoms.

aldose /al-dohs/ A SUGAR containing analdehyde (CHO) or potential aldehydegroup.

aldehyde

8

O

HCR

Aldehyde

alginic acid /al-jin-ik/ (algin;(C6H8O6)n) A yellow-white organic solidthat is found in brown algae. It is a com-plex polysaccharide and produces, in evenvery dilute solutions, a viscous liquid. Al-ginic acid has various uses, especially in thefood industry as a stabilizer and textureagent.

alicyclic compound /al--s-klik, -sik-lik/ An aliphatic cyclic compound, suchas cyclohexane.

aliphatic compound /al--fat-ik/ Anorganic compound with properties similarto those of the alkanes, alkenes, andalkynes and their derivatives. Mostaliphatic compounds have an open chainstructure but some, such as cyclohexaneand sucrose, have rings. The term is used indistinction to aromatic compounds, whichare similar to benzene. Compare aromaticcompound.

alizarin /-liz--rin/ (1,2-dihydroxyan-thraquinone) An important orange-redorganic compound used in the dyestuffs in-dustry to produce red lakes. It occurs natu-rally in the root of the plant madder andmay also be synthesized from an-thraquinone.

alkali /al-k-l/ A water-soluble strongbase. Strictly the term refers to the hydrox-ides of the alkali metals (group 1) only, butin common usage it refers to any solublebase. Thus borax solution may be de-scribed 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 is

oxidizing). 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 whichfollow from this. For example, lithium re-acts in a fairly controlled way with water,sodium ignites, and potassium explodes.There is a general decrease in the follow-ing: melting points, heats of sublimation,lattice energy of salts, hydration energy ofM+, ease of decomposition of nitrates andcarbonates, and heat of formation of the -ide compounds (fluoride, hydride, oxide,carbide, chloride).

Lithium has the smallest ion and there-fore the highest charge/size ratio and is po-larizing with a tendency towards covalentcharacter in its bonding; the remaining ele-ments 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. For ex-ample, lithium hydroxide is much less sol-uble than the hydroxides of the othergroup 1 elements; lithium perchlorate issoluble in several organic solvents. Becauseof the higher lattice energies associatedwith smaller ions lithium hydride and ni-tride are fairly stable compared to NaH,which decomposes at 345C. Na2N, K3Netc., are not obtained pure and decomposebelow room temperature.

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 coordinationcompounds of the type MLn

+ apart fromsolvated species of very low correlationtimes. The group 1 elements form a varietyof organometallic compounds; the bonding

9

alkali metals

in lithium alkyls and aryls is essentially co-valent but the heavier elements form ioniccompounds. Organo-alkali metal com-pounds particularly the lithium com-pounds are widely used in syntheticorganic chemistry.

Francium is formed only by radioactivedecay and in nuclear reactions; all the iso-topes of francium have short half-lives, thelongest of which is 21 minutes (francium-223). The few chemical studies which havebeen carried out indicate that it would havesimilar properties to those of the other al-kali metals.

alkalimetry /al-k-lim--tree/ See acidime-try.

alkaline-earth metals (group 2 ele-ments) 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 ele-ments 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-wards metallic character as the group isdescended. For example, beryllium hy-droxide is amphoteric; magnesium hydrox-ide 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) andfor 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.Magnesium forms Grignard reagents(RMgX), which are important in organicsynthesis, and related compoundsR2Mg.MgX2 and R2Mg are known. Thefew organic compounds of Ca, Sr, and Baare ionic. All isotopes of radium are ra-dioactive and radium was once widely usedfor radiotherapy. The half-life of 226Ra(formed by decay of 238U) is 1600 years.

alkaloid /al-k-loid/ One of a group ofnatural organic compounds found inplants. They contain oxygen and nitrogenatoms; most are poisonous. However, theyinclude a number of important drugs withcharacteristic physiological effects, e.g.

alkalimetry

10

morphine, codeine, caffeine, cocaine, andnicotine.

alkane /al-kayn/ A type of hydrocarbonwith general formula CnH2n+2. Alkanes aresaturated compounds, containing no dou-ble or triple bonds. Methane (CH4) andethane (C2H6) are typical examples. Thealkanes are fairly unreactive (their formername, the paraffins, means small affin-ity). In ultraviolet radiation they reactwith chlorine to give a mixture of substitu-tion products. There are a number of waysof preparing alkanes:1. From a sodium salt of a carboxylic acid

treated with soda lime:RCOONa+ + NaOH RH + Na2CO3

2. By reduction of a haloalkane withnascent hydrogen from the action ofethanol on a zinccopper couple:

RX + 2[H] RH + HX3. By the Wurtz reaction i.e. sodium in

dry ether on a haloalkane:2RX + 2Na 2NaX + RR

4. By the Kolb electrolytic method:RCOO RR

5. By refluxing a haloalkane with magne-sium in dry ether to form a Grignardreagent:

RI + Mg RMgI5. With acid this gives the alkane:

RMgI + H RHThe main source of lower molecular

weight alkanes is natural gas (for methane)and crude oil.

alkene /al-keen/ A type of hydrocarbonwith the general formula CnH2n. Thealkenes (formerly called olefins) are unsat-urated compounds containing double car-boncarbon bonds. They can be obtainedfrom crude oil by cracking alkanes. Impor-tant examples are ethene (C2H4) andpropene (C3H6), both of which are used inplastics production and as starting materi-als for the manufacture of many other or-ganic chemicals.

The methods of synthesizing alkenesare:1. The elimination of HBr from a

haloalkane using an alcoholic solutionof potassium hydroxide:

RCH2CH2Br + KOH KBr + H2O +RCH:CH2

2. The dehydration of an alcohol by pass-ing the vapor over hot pumice (400C):

RCH2CH2OH RCH:CH2 + H2OThe reactions of alkenes include:1. Hydrogenation using a catalyst (usually

nickel at about 150C):RCH:CH2 + H2 RCH2CH3

2. Addition reactions with halogen acids togive haloalkanes:

RCH:CH2 + HX RCH2CH2XThe addition follows Markovnikoffsrule.

3. Addition reactions with halogens, e.g.RCH:CH2 + Br2 RCHBrCH2Br

4. Hydration using concentrated sulfuricacid, followed by dilution and warming:

RCH:CH2 + H2O RCH(OH)CH35. Oxidation by cold potassium perman-

ganate solutions to give diols:RCH:CH2 + H2O + [O]

RCH(OH)CH2OHEthene can be oxidized in air using asilver catalyst to the cyclic compoundepoxyethane (C2H4O).

6. Polymerization to polyethene (by theZiegler or Phillips process).

See also oxo process; ozonolysis.

alkoxide /al-koks-d/ An organic com-pound containing an ion of the type RO,where R is an alkyl group. Alkoxides aremade by the reaction of metallic sodium onan alcohol. Sodium ethoxide (C2H5O

Na+)is a typical example.

alkoxyalkane /al-koks-ee-al-kayn/ (Di-ethyl ether.) See ether.

alkylbenzene /al-kl-ben-zeen/ A typeof organic hydrocarbon containing one ormore alkyl groups substituted onto a ben-zene ring. Methylbenzene (C6H5CH3) and1,3-dimethylbenzene are simple examples.Alkylbenzenes can be made by a Friedel-Crafts reaction or by the Fittig reaction. In-dustrially, large quantities of methyl-benzene are made by the hydroforming ofcrude oil.

Substitution of alkylbenzenes can occurat the benzene ring. The alkyl group directsthe substituent into the 2- or 4-position.

11

alkylbenzene

Substitution of hydrogen atoms on thealkyl group can also occur.

alkyl group /al-kl/ A group obtainedby removing a hydrogen atom from analkane or other aliphatic hydrocarbon.

alkyl halide See haloalkane.

alkyl sulfide A thioether with the gen-eral formula RSR, where R and R arealkyl groups.

alkyne /al-kn/ A type of hydrocarbonwith the general formula CnH2n2. Thealkynes are unsaturated compounds con-taining triple carboncarbon bonds. Thesimplest member of the series is ethyne(C2H2), which can be prepared by the ac-tion of water on calcium dicarbide.

CaC2 + 2H2O Ca(OH)2 + C2H2The alkynes were formerly called theacetylenes.

In general, alkynes can be made by thecracking of alkanes or by the action of ahot alcoholic solution of potassium hy-droxide on a dibromoalkane, for example:

BrCH2CH2Br + KOH KBr +CH2:CHBr + H2O

CH2:CHBr + KOH CHCH + KBr +H2O

The main reactions of the alkynes are:1. Hydrogenation with a catalyst (usually

nickel at about 150C):C2H2 + H2 C2H4C2H4 + H2 C2H6

2. Addition reactions with halogen acids:C2H2 + HI H2C:CHI

H2C:CHI + HI CH3CHI23. Addition of halogens; for example, with

bromine in tetrachloromethane:C2H2 + Br2 BrHC:CHBr

BrHC:CHBr + Br2 Br2HCCHBr24. With dilute sulfuric acid at 6080C and

mercury(II) catalyst, ethyne formsethanal (acetaldehyde):

C2H2 + H2O H2C:C(OH)HThis enol form converts to the alde-hyde:

CH3COH5. Ethyne polymerizes if passed through a

hot tube to produce some benzene:3C2H2 C6H6

6. Ethyne forms unstable dicarbides(acetylides) with ammoniacal solutionsof copper(I) and silver(I) chlorides.

allotropy /-lot-r-pee/ The ability ofcertain elements to exist in more than onephysical form. Carbon, sulfur, and phos-phorus are the most common examples.Allotropy is more common in groups 14,15, and 16 of the periodic table than inother groups. See also enantiotropy;monotropy.

alloy A mixture of two or more metals(e.g. bronze or brass) or a metal with smallamounts of non-metals (e.g. steel). Alloysmay be completely homogeneous mixturesor may contain small particles of one phasein the other phase.

allyl group /al-l/ See propenyl group.

Alnico /al-m-koh/ (Trademark) Any ofa group of very hard brittle alloys used tomake powerful permanent magnets. Theycontain nickel, aluminum, cobalt, and cop-per in various proportions. Iron, titanium,and niobium can also be present. Theyhave a high remanence and coercive force.

alpha particle A He2+ ion emitted withhigh kinetic energy by a radioactive sub-stance. Alpha particles are used to causenuclear disintegration reactions.

alternating copolymer See polymer-ization.

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

M2SO4.M2(SO4)3.24H2OWhere M is a univalent metal or ion,

and M is a trivalent metal. Thus, alu-minum potassium sulfate (called potashalum, or simply alum) is

K2SO4.Al2(SO4)3.24H2OAluminum ammonium sulfate (called

ammonium alum) is(NH4)2SO4.Al2(SO4)3.24H2O

The name alum originally came fromthe presence of Al3+ as the trivalent ion, but

alkyl group

12

is also applied to other salts containingtrivalent ions, thus, Chromium(III) potas-sium sulfate (chrome alum) is

K2SO4.Cr2(SO4)3.24H2O

alumina See aluminum oxide.

aluminate See aluminum hydroxide.

aluminosilicate /-loo-m-noh-sil--kayt/See silicates.

aluminum A soft moderately reactivemetal; the second element in group 3 of theperiodic table. It was formerly classified insubgroup IIIA. Aluminum has the elec-tronic structure of neon plus three addi-tional outer electrons. There are numerousminerals of aluminum; it is the most com-mon metallic element in the Earths crust(8.1% by weight) and the third in order ofabundance. Commercially important min-erals are bauxite (hydrated Al2O3), corun-dum (anhydrous Al2O3), cryolite(Na3AlF6), and clays and mica (aluminosil-icates).

The metal is produced on a massivescale by the HallHeroult method in whichalumina, a non-electrolyte, is dissolved inmolten cryolite and electrolyzed. Thebauxite contains iron, which would conta-minate the product, so the bauxite is dis-solved in hot alkali, the iron oxide isremoved by filtration, and the pure alu-mina then precipitated by acidification.Molten aluminum is tapped off from thebase of the cell and oxygen evolved at theanode. The aluminum atom is much biggerthan boron (the first member of group 3)and its ionization potential is not particu-larly high. Consequently aluminum formspositive ions Al3+. However, it also hasnon-metallic chemical properties. Thus, itis amphoteric and also has a number of co-valently 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 vigorous 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 (alumina), but a varietyof polymorphs and hydrates are known. Itis relatively inert and has a high meltingpoint, and for this reason is widely used asa furnace lining and for general refractorybrick. Aluminum metal will react with al-kalis releasing hydrogen and producing ini-tially Al(OH)3 then Al(OH)4

.Aluminum reacts readily with the halo-

gens; in the case of chlorine thin sheets willburst into flame. The fluoride has a highmelting point (1290C) and is ionic. Theother halides are dimers in the vapor phase(two halogen bridges). Aluminum alsoforms a sulfide (Al2S3), nitride (AlN), andcarbide (Al4C), the latter two at extremelyhigh temperatures.

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

2 and AlCl4. A

number of very reactive aluminum alkylsare also known, some of which are impor-tant as polymerization catalysts.

Symbol: Al; m.p. 660.37C; b.p.2470C; r.d. 2.698 (20C); 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:

AlCl3 + 3H2O Al(OH)3 + 3HClIt is prepared by heating aluminum in drychlorine or dry hydrogen chloride. Vapor-density measurements show that its struc-ture is a dimer; it consists of Al2Cl6molecules in the vapor. The AlCl3 structurewould be electron-deficient. Aluminumchloride is used in FriedelCrafts reactionsin organic preparations.

13

aluminum chloride

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 dis-solving aluminum hydroxide in ethanoicacid and is used extensively as a mordant indyeing and as a size for paper and card-board products. 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 (Al(OH)3) Awhite powder prepared as a colorlessgelatinous precipitate by adding ammoniasolution or a small amount of sodium hy-droxide solution to a solution of an alu-minum salt. It is an amphoteric hydroxideand is used as a foaming agent in fire ex-tinguishers and as a mordant in dyeing.

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

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 cannot be pre-pared by the action of dilute nitric acid onaluminum since the metal is rendered pas-sive by a thin surface layer of oxide.

aluminum oxide (alumina; Al2O3) Awhite powder that is almost insoluble inwater. Because of its amphoteric nature itwill react with both acids and alkalis. Alu-minum oxide occurs naturally as bauxite,corundum, and white sapphire; it is manu-factured by heating aluminum hydroxide.It is used in the extraction by electrolysis of

aluminum, as an abrasive (corundum), infurnace linings (because of its refractoryproperties), and as a catalyst (e.g. in the de-hydration of alcohols).

aluminum potassium sulfate (potashalum; Al2(SO4)3.K2SO4.24H2O) A whitesolid, soluble in water but insoluble in al-cohol, prepared by mixing equimolecularquantities of solutions of ammonium andaluminum sulfate followed by crystalliza-tion. It is used as a mordant for dyes, as awaterproofing agent, and as a tanning ad-ditive.

aluminum sulfate (Al2(SO4)3.18H2O)A white crystalline solid. It is used as a sizefor paper, a precipitating agent in sewagetreatment, a foaming agent in fire control,and as a fireproofing agent. Its solutionsare acidic by hydrolysis, containing suchspecies as Al(H2O)5(OH)

2+.

aluminum trimethyl /tr-meth-l/ Seetrimethylaluminum.

amalgam /-mal-gm/ An alloy of mer-cury with one or more other metals. Amal-gams may be liquid or solid. An amalgamof sodium (Na/Hg) with water is used as asource of nascent hydrogen.

amatol /am--tol, -tohl/ A high explo-sive that consists of a mixture of ammo-nium nitrate and TNT (trinitrotoluene).

ambidentate See isomerism.

americium /am--rish-ee-m/ A highlytoxic radioactive silvery element of theactinoid series of metals. A transuranic ele-ment, it is not found naturally on Earth butis synthesized from plutonium. The ele-ment can be obtained by reducing the tri-fluoride with barium metal. It reacts withoxygen, steam, and acids. 241Am has beenused in gamma-ray radiography.

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

amethyst A purple form of the mineral

aluminum ethanoate

14

quartz (silicon(IV) oxide, SiO2) used as asemiprecious gemstone. The color comesfrom impurities such as oxides of iron.

amide /am-d, -id/ 1. A type of organiccompound of general formulae RCONH2(primary), (RCO)2NH (secondary), and(RCO)3N (tertiary). Amides are white,crystalline solids and are basic in nature,some being soluble in water. Amides can beformed by reaction of ammonia with acidchlorides or anhydrides:

(RCO)2O + 2NH3 RCONH2 +RCOONH4

+

Reactions of amides include: 1. Reaction with hot acids to give car-

boxylic acids:RCONH2 + HCl + H2O RCOOH +

NH4Cl 2. 2. Reaction with nitrous acid to give car-

boxylic acids and nitrogen:RCONH2 + HNO2 RCOOH + N2 +

H2O 3. 3. Dehydration by phosphorus(V) oxide to

give a nitrile:RCONH2 H2O RCN

See also Hofmann degradation.2. An inorganic salt containing the NH2

ion. They are formed by the reaction ofammonia with certain metals (such assodium and potassium). See sodamide.

amination /am--nay-shn/ The intro-duction of an amino group (NH2) into anorganic compound. An example is the con-version of an aldehyde or ketone into anamide by reaction with hydrogen and am-monia in the presence of a catalyst:

RCHO + NH3 + N2 RCH2NH2 +H2O

amine /-meen, am-in/ A compound con-taining a nitrogen atom bound to hydrogenatoms or hydrocarbon groups. They havethe general formula R3N, where R can behydrogen or an alkyl or aryl group. Aminescan be prepared by reduction of amides ornitro compounds.

An amine is classified according to thenumber of organic groups bonded to thenitrogen atom: one, primary; two, sec-ondary; three, tertiary. Since amines arebasic in nature they can form the quater-

nium ion, R3NH+. All three classes, plus a

quaternium salt, can be produced by theHofmann reaction (which occurs in asealed vessel at 100C):

RX + NH3 RNH3+ XRNH3

+ X + NH3 RNH2 + NH4XRNH2 + RX R2NH2+ X

R2NH2+ X + NH3 R2NH + NH4X

R2NH + RX R3NH+ XR3NH

+ X + NH3 R3N + NH4XR3N + RX R4N+X

Reactions of amines include:1. Reaction with acids to form salts:

R3N + HX R3NH+X2. Reaction with acid chlorides to give N-

substituted acid amides (primary andsecondary amines only):RNH2 + RCOCl RCONHR + HX

amine salt A salt similar to an ammo-nium salt, but with organic groups at-tached to the nitrogen atom. For example,

15

amine salt

primary (ethylamine)

secondary (diethylamine)

tertiary (triethylamine)

C2H5

H

H

N

C2H5

C2H5

H

N

C2H5

C2H5

C2H5

N

Amines

triethylamine ((C2H5)3N) will react withhydrogen chloride to give triethylammo-nium chloride:

(C2H5)3N + HCl (C2H3)3NH+ ClSometimes amine salts are named using thesuffix -ium. For instance, aniline(C6H5NH2) forms anilinium chlorideC6H5NH3

+ Cl. Often insoluble alkaloidsare used in medicine in the form of theiramine salt (sometimes referred to as thehydrochloride).

It is also possible for amine salts of thistype to have four groups on the nitrogenatom. For example, with chloroethane,tetraethylammonium chloride can beformed:

(C2H5)3N + C2H5Cl (C2H5)4N+Cl

amino acids /-mee-noh, am--/ Deriva-tives of carboxylic acids in which a hydro-gen atom in an aliphatic acid has beenreplaced by an amino group. Thus, fromethanoic acid, the amino acid 2-aminoethanoic acid (glycine) is formed. Allare white, crystalline, soluble in water (butnot in alcohol), and with the sole exceptionof the simplest member, all are optically ac-tive.

In the body the various proteins are as-sembled from the necessary amino acidsand it is important therefore that all the

amino acids should be present in sufficientquantities. In humans, twelve of the twentyamino acids can be synthesized by the bodyitself. Since these are not required in thediet they are known as nonessential aminoacids. The remaining eight cannot be syn-thesized by the body and have to be sup-plied in the diet. They are known asessential amino acids.

The amino acids that occur in proteinsall have the NH2 group and the COOHgroup attached to the same carbon atom.They are thus alpha amino acids, the car-bon atom being the alpha carbon. Theyhave complex formulae and are usually re-ferred to by their common names, ratherthan systematic names: alanine CH3CH(NH2)COOH arginine NH2C(NH)NH(CH2)3CH(NH2)-

COOH asparagine NH2COCH2CH(NH2)COOH aspartic acidCOOHCH2CH(NH2)COOH cysteine SHCH2CH(NH2)COOH cystine [HOOCCH(NH2)CH2S]2glutamic acid COOH(CH2)2CH(NH2)-

COOH glutamine NH2CH(CH2)2(CONH2)-

COOH glycine CH2(NH2)COOH histidine C3H3N2CH2CH(NH2)COOH isoleucine (CH3)CH2CH(CH3)CH(NH2)-

COOH leucine (CH3)2CHCH2CH(NH2)COOH lysine NH2(CH2)4CH(NH2)COOH methionine CH3S(CH2)2CH(NH2)COOH phenylalanine C6H5CH2CH(NH2)COOH proline NH(CH2)3CHCOOH serine CH2OHCH(NH2)COOH threonine CH3CHOHCH(NH2)-

COOH tryptophan C6H4NHC2HCH2CH(NH2)-

COOH tyrosine C6H4OHCH2CH(NH2)COOH valine (CH3)2CHCH(NH2)COOH

Note that proline is in fact a cyclicimino acid, with the nitrogen atom bondedto the alpha carbon.

See also optical activity.

aminobenzene /-mee-noh-ben-zeen, am--/ See aniline.

amino acids

16

H O

ONH3+

R CCC CCC

OH

R CCC C

NH2 O Hun-ionized

zwitterion

Amino acid

aminoethane /a-mee-noh-eth-ayn, am--/See ethylamine.

amino group The group NH2.

aminotoluine /-mee-noh-tol-yoo-een,am--/ See toluidine.

ammine /am-een, -meen/ A complex inwhich ammonia molecules are coordinatedto a metal ion; e.g. [Cu(NH3)4]

2+.

ammonia /-moh-nee-/ (NH3) A color-less gas with a characteristic pungent odor.On cooling and compression it forms a col-orless liquid, which becomes a white solidon further cooling. Ammonia is very solu-ble in water (a saturated solution at 0Ccontains 36.9% of ammonia): the aqueoussolution is alkaline and contains a propor-tion of free ammonia. Ammonia is also sol-uble in ethanol. It occurs naturally to asmall extent in the atmosphere, and is usu-ally produced in the laboratory by heatingan ammonium salt with a strong alkali.Ammonia is synthesized industrially fromhydrogen 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 also used commercially in

the manufacture of fertilizers, mainly am-monium nitrate, urea, and ammonium sul-fate. It is used to a smaller extent 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 marketed

as the liquid, compressed in cylinders (an-hydrous ammonia), or as aqueous solu-tions of various strengths. See alsoammonium hydroxide.

ammoniacal /am--n--kl/ Describ-ing a solution in aqueous ammonia.

ammonia-soda process See Solvayprocess.

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 heatingto ammonia, carbon dioxide, and water.The white solid sold commercially as am-monium carbonate is actually a double saltof both ammonium hydrogencarbonate(NH4HCO3) and ammoniumaminomethanoate (NH2CO2NH4). Thissalt is manufactured from ammoniumchloride and calcium carbonate. It decom-poses on exposure to air into ammoniumhydrogencarbonate and ammonia, and itreacts with ammonia to give the true am-monium carbonate. Commercial ammo-nium carbonate is used in baking powders,smelling salts, and in the dyeing and wool-scouring industries.

ammonium chloride (sal ammoniac;NH4Cl) A white crystalline solid with acharacteristic saline taste. It is very solublein water (37 g per 100 g of water at 20C).Ammonium chloride can be manufacturedby the action of ammonia on hydrochloricacid. It sublimes on heating because of theequilibrium:

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

nizing, as a flux for soldering, in dyeingand calico printing, and in the manufactureof Leclanch 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.A saturated aqueous solution of ammonia

17

ammonium hydroxide

has a relative density of 0.88 g cm3, and isknown as 880 ammonia. Ammonia solu-tion is a useful reagent and cleansing agent.

ammonium ion The ion NH4+, formedby coordination of NH3 to H

+. See alsoquaternary ammonium compound.

ammonium nitrate (NH4NO3) A col-orless crystalline solid that is very solublein water (871 g per 100 g of water at100C). It is usually manufactured by theaction of ammonia on nitric acid. It is usedin the manufacture of explosives and, be-cause of its high nitrogen content, as a fer-tilizer.

ammonium phosphate (triammoniumphosphate (V); (NH4)3PO4) A colorlesscrystalline salt made from ammonia andphosphoric(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. When heated carefully it gives am-monium hydrogensulfate, which onstronger heating yields nitrogen, ammonia,sulfur(IV) oxide (sulfur dioxide), andwater. Ammonium sulfate is manufacturedby the action of ammonia on sulfuric acid.It is the most important ammonium salt be-cause of its widespread use as a fertilizer.Its only drawback as a fertilizer is that ittends to leave an acidic residue in the soil.

amorphous /-mor-fs/ Describing asolid substance that has no long-rangeregular arrangement of atoms; i.e. is notcrystalline. Amorphous materials can con-sist of minute particles that possess orderover a very short distance. Glasses are alsoamorphous; the atoms in the solid have arandom arrangement. X-ray analysis hasshown that many substances that wereonce described as amorphous are com-posed of very small crystals. For example,charcoal, coke, and soot (all forms of car-bon) are made up of small graphite-likecrystals.

amount of substance Symbol: n Ameasure of the number of entities presentin a substance. See mole.

AMP (adenosine monophosphate) ANUCLEOTIDE consisting of adenine, ribose,and phosphate. See ATP.

ampere /am-pair/ Symbol: A The SI baseunit of electric current, defined as the con-stant current that, maintained in twostraight parallel infinite conductors of neg-ligible circular cross section placed onemeter apart in vacuum, would produce aforce between the conductors of 2 107newton per meter. The unit is named forthe French physicist and mathematicianAndr Marie Ampre (17751836).

amphiprotic /am-f-proh-tik/ See sol-vent.

ampholyte ion /am-f-lt/ See zwitte-rion.

amphoteric /am-f-te-rik/ A materialthat can display both acidic and basicproperties. The term is most commonly ap-plied to the oxides and hydroxides of met-als that can form both cations and complexanions. For example, zinc oxide dissolvesin acids to form zinc salts and also dis-solves in alkalis to form zincates,[Zn(OH)4]

2.

amu /ay-em-yoo/ See atomic mass unit.

amyl group /am-l/ See pentyl group.

amyl nitrite (C5H11ONO) A palebrown volatile liquid organic compound; anitrous acid ester of 3-methylbutanol (iso-amyl alcohol). It is used in medicine as aninhalant to dilate the blood vessels (andthereby prevent pain) in patients withangina pectoris.

amylopectin /am--lo-pek-tin/ The water-insoluble fraction of STARCH.

amylose /am--lohs/ A polymer of GLU-COSE, a polysaccharide sugar that is foundin starch.

ammonium ion

18

anabolism /-nab--liz-m/ All themetabolic reactions that synthesize com-plex molecules from more simple mole-cules. See also metabolism.

anaerobic /an-air-oh-bik/ Describing abiochemical process that takes place in theabsence of free oxygen. Compare aerobic.

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 the ques-tion how much of such and such a compo-nent is present? There is a vast number ofanalytical methods which can be applied,depending on the nature of the sample andthe purpose of the analysis. These includegravimetric, volumetric, and systematicqualitative analysis (classical wet meth-ods); and instrumental methods, such aschromatographic, spectroscopic, nuclear,fluorescence, and polarographic tech-niques.

Andrews experiment An investigation(1861) into the relationship between pres-sure and volume for a mass of carbon di-oxide at constant temperature. Theresulting isothermals showed clearly theexistence of a critical point and led togreater understanding of the liquefactionof gases. The experiment is named for theIrish physical chemist Thomas Andrews(18131885).

ngstrom /ang-strm/ Symbol: A unitof length defined as 1010 meter. Thengstrom is sometimes still used for ex-pressing wavelengths of light or ultravioletradiation or for the sizes of molecules, al-though the nanometer is preferred. Theunit is named for the Swedish physicist An-ders Jonas ngstrom (181474).

anhydride /an-h-drd/ A compoundformed by removing water from an acid or,less commonly, a base. Many non-metaloxides are anhydrides of acids: for exampleCO2 is the anhydride of H2CO3 and SO3 isthe anhydride of H2SO4. Organic anhy-drides are formed by removing H2O fromtwo carboxylic-acid groups, giving com-pounds with the functional groupCO.O.CO. These form a class of organiccompounds called ACID ANHYDRIDES (oracyl anhydrides).

anhydrite /an-h-drt/ See calcium sul-fate.

anhydrous /an-h-drs/ Describing asubstance that lacks moisture, or a saltwith no water of crystallization. For exam-ple, on strong heating, blue crystals of cop-per(II) sulfate pentahydrate, CuSO4.5H2O,form white anhydrous copper(II) sulfate,CuSO4.

aniline /an--lin, -ln/ (aminobenzene;phenylamine; C6H5NH2) A colorless oilysubstance made by reducing nitrobenzene(C6H5NO2). Aniline is used for making

19

aniline

OC

H3C

HO

OC

HO

H3C

ethanoic acid

OC

H3C

O

OC

H3C

ethanoic anhydride

Anhydride

dyes, pharmaceuticals, and other organiccompounds.

anion /an--n, -on/ A negativelycharged ion, formed by addition of elec-trons to atoms or molecules. In electrolysisanions are attracted to the positive elec-trode (the anode). Compare cation.

anionic detergent /an--on-ik/ See de-tergent.

anionic resin An ION-EXCHANGE mater-ial that can exchange anions, such as Cl

and OH, for anions in the surroundingmedium. Such resins are used for a widerange of analytical and purification pur-poses.

They are often produced by addition ofa quaternary ammonium group(N(CH3)3

+) or a phenolic group (OH)to a stable polyphenylethene resin. A typi-cal exchange reaction is:

resinN(CH3)3+Cl + KOH

resinN(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 /an--s-trop-ik/ A term de-scriptive of certain substances which haveone or more physical properties that differaccording to direction. Most crystals areanisotropic.

annealing /-neel-ing/ A type of heattreatment applied to metals to change theirphysical properties. The metal is heated to,and held at, an appropriate temperaturebefore being cooled at a suitable rate toproduce the desired grain structure. An-nealing is most commonly used to removethe stresses that have arisen during rolling,to increase the softness of the metal, and tomake it easier to machine. Objects made ofglass can also be annealed to removestrains.

annulene /an-y-leen/ A ring compoundcontaining alternating double and singleCC bonds. The compound C8H8, havingan 8-membered ring of carbon atoms, is

the first annulene larger than benzene. It isnot an AROMATIC COMPOUND because it isnot planar and does not obey the Hckelrule. C8H8 is called cyclo-octatetrane.Higher annulenes are designated by thenumber of carbon atoms in the ring. [10]-annulene obeys the Hckel rule but is notaromatic because it is not planar as a resultof interactions of the hydrogen atoms in-side the ring. There is some evidence that[18]-annulene, which is a stable red solid,has aromatic properties.

anode /an-ohd/ In electrolysis, the elec-trode that is at a positive potential with re-spect to the cathode. In any electricalsystem, such as a discharge tube or elec-tronic device, the anode is the terminal atwhich electrons flow out of the system.

anode sludge See electrolytic refining.

anodizing /an--dz-ing/ An industrialprocess for protecting aluminum with anoxide layer formed in an electrolytic cellcontaining an oxidizing acid (e.g. sulfu-ric(VI) acid). The layer of Al2O3 is porousand can be colored with certain dyes.

anomer /an--mer/ Either of two iso-meric forms of a cyclic form of a sugar thatdiffer in the disposition of the OH groupon the carbon next to the O atom of thering (the anomeric carbon). Anomers arediastereoisomers. They are designated or according to whether the OH isabove or below the ring respectively.

anthracene /an-thr-seen/ (C14H10) Awhite crystalline solid used extensively inthe manufacture of dyes. Anthracene isfound in the heavy- and green-oil fractionsof crude oil and is obtained by fractionalcrystallization. Its structure is benzene-like,having three six-membered rings fused to-

anion

20

Anthracene

gether. The reactions are characteristic ofAROMATIC COMPOUNDS.

anthracite /an-thr-st/ The highestgrade of coal, with a carbon content of be-tween 92% and 98%. It burns with a hotblue flame, gives off little smoke and leaveshardly any ash.

anthraquinone /an-thr-kwi-nohn, an-thr-kwin-ohn/ (C6H4(CO)2C6H4) A col-orless crystalline quinone used inproducing dyestuffs such as alizarin.

antibonding orbital See orbital.

anti-isomer /an-tee--s-mer/ See iso-merism.

antiknock agent /an-tee-nok/ A sub-stance added to gasoline to inhibit preigni-tion or knocking. A common example islead tetraethyl.

antimonic /an-t-mon-ik/ Designatingan antimony(IV) compound.

antimonous /an-t-moh-ns/ Designat-ing an antimony(III) compound.

antimony /an-t-moh-nee/ A metalloidelement existing in three allotropic forms;the most stable is a brittle silvery metal.Antimony belongs to group 15 (formerlyVB) of the periodic table. It is found inmany minerals, principally stibnite (Sb2S3).It is used in alloys small amounts of anti-mony can harden other metals. It is alsoused in semiconductor devices.

Symbol: Sb; m.p. 630.74C; b.p.1635C; 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-cipitate of antimony(III) chloride oxide(antimonyl chloride, SbOCl):

SbCl3 + H2O = SbOCl + 2HCl

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

antimonyl chloride /an-t-m-nil, an-tim--nil/ 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.

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.

antimony pentoxide /pen-toks-d/ Seeantimony(V) oxide.

antimony trichloride /tr-klor-d, -kloh-rd/ See antimony(III) chloride.

antimony trioxide /tr-oks-d/ See an-timony(III) oxide.

antioxidant /an-tee-oks--dnt/ A sub-stance that inhibits oxidation. Antioxi-dants are added to such products as foods,paints, plastics, and rubber to delay theiroxidation by atmospheric oxygen. Somework by forming chelates with metal ions,thus neutralizing the catalytic effect of theions in the oxidation process. Other typesremove intermediate oxygen free radicals.Naturally occurring antioxidants can limittissue or cell damage in the body. These in-clude vitamin E and -carotene.

antiparallel spins /an-tee-pa-r-lel/ Spinsof two neighboring particles in which themagnetic moments associated with the spinare aligned in opposite directions.

apatite /ap--tt/ A naturally occurringphosphate of calcium, CaF2.Ca3(PO4)3.

aprotic /-prot-ik, -proh-tik/ See solvent.

21

aprotic

aqua fortis /a-kw for-tis/ An old namefor nitric acid, HNO3.

aqua regia /ree-jee-/ A mixture of con-centrated nitric acid and three to four partsof hydrochloric acid. It dissolves all metalsincluding gold, hence the name. The mix-ture contains chlorine and NOCl (nitrosylchloride).

aqueous /ay-kwee-s, ak-wee-/ Describ-ing a solution in water.

aragonite /-rag--nt, a-r-g-nt/ Ananhydrous mineral form of calcium car-bonate, CaCO3, which occurs associatedwith limestone and in some metamorphicrocks. It is also the main ingredient ofpearls. It is not as stable as calcite, intowhich it may change over time.

arene /-reen/ An organic compoundcontaining a benzene ring; i.e. an aromatichydrocarbon or derivative.

argentic oxide /ar-jen-tik/ See silver(II)oxide.

argentous oxide /ar-jen-ts/ See silver(I)oxide.

arginine /ar-j-nn/ See amino acids.

argon /ar-gon/ An inert colorless odor-less monatomic element of the rare-gasgroup. It forms 0.93% by volume of air.Argon is used to provide an inert atmos-phere in electric and fluorescent lights, inwelding, and in extracting titanium and sil-icon. The element forms no known com-pounds.

Symbol: Ar; m.p. 189.37C; b.p.185.86C; d. 1.784 kg m3 (0C); p.n. 18;r.a.m. 39.95.

aromatic compound An organic com-pound containing benzene rings in itsstructure. Aromatic compounds, such asbenzene, have a planar ring of atoms linkedby alternate single and double bonds. Thecharacteristic of aromatic compounds isthat their chemical properties are not thoseexpected for an unsaturated compound;

they tend to undergo nucleophilic substitu-tion of hydrogen (or other groups) on thering, and addition reactions only occurunder special circumstances.

The explanation of this behavior is thatthe electrons in the double bonds are delo-calized over the ring, so that the six bondsare actually all identical and intermediatebetween single bonds and double bonds.The pi electrons are thus spread in a mole-cular orbital above and below the ring. Theevidence for this delocalization in benzeneis that: The bond lengths between carbonatoms in benzene are all equal and inter-mediate between single and double bondlengths. Also, if two hydrogen atoms at-tached to adjacent carbon atoms are sub-stituted by other groups, the compoundhas only one structure. If the bonds weredifferent two isomers would exist. Benzenehas a stabilization energy of 150 kJ mol1

over the Kekul structure.The delocalization of the electrons in

the pi orbitals of benzene accounts for theproperties of benzene and its derivatives,which differ from the properties of alkenesand other aliphatic compounds. The phe-nomenon is called aromaticity. A defini-tion of aromaticity is that it occurs incompounds that obey the Hckel rule: i.e.that there should be a planar ring with atotal of (4n + 2) pi electrons (where n is anyinteger). Using this rule as a criterion cer-tain non-benzene rings show aromaticity.Such compounds are called nonbenzenoidaromatics. Other compounds that have aring of atoms with alternate double andsingle bonds, but do not obey the rule (e.g.cyclooctotetraene, which has a non-planarring of alternating double and singlebonds) are called pseudoaromatics. Therule is named for the German chemist ErichArmand Arthur Joseph Hckel (18961980).

Compare aliphatic compound. See alsoresonance.

aromaticity /-roh-m-tis--tee/ Seearomatic compound.

Arrhenius equation /ah-ren-ee-s/ Anequation relating the rate constant of a

aqua fortis

22

chemical reaction and the temperature atwhich the reaction is taking place:

k = Aexp(Ea/RT)where A is a constant, k the rate constant,T the thermodynamic temperature inkelvins, R the gas constant, and Ea the ac-tivation energy of the reaction.

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

logek = logeA E/2.3RTThis equation enables the activation en-

ergy for a reaction to be determined. Theequation is named for the Swedish physicalchemist Svante August Arrhenius(18591927).

arsenate(III) /ar-s-nayt/ (arsenite) Asalt of the hypothetical arsenic(III) acid,formed by reacting arsenic(III) oxide withalkalis. Arsenate(III) salts contain the ionAsO3

3. Copper arsenate(III) is used as aninsecticide.

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 /ar-s-nik, ars-nik; adj. ars-sen-ik/ A toxic metalloid element existing inseveral allotropic forms; the most stable isa brittle gray metal. It belongs to group 15(formerly VA) of the periodic table. Ar-senic is found native and in several ores in-cluding mispickel (FeSAs), realgar (As4S4),and orpiment (As2S3). The element reactswith hot acids and molten sodium hydrox-ide but is unaffected by water and acidsand alkalis at normal temperatures. It isused in semiconductor devices, alloys, andgun shot. Various compounds are used inmedicines and agricultural insecticides andpoisons.

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

arsenic(V) oxide (arsenic oxide;As2O5) A white amorphous deliquescentsolid. It is an acidic oxide prepared by dis-solving arsenic(III) oxide in hot concen-trated nitric acid, followed bycrystallization then heating to 210C.

arsenide /ar-s-nd/ A compound of ar-senic and another metal. For example, withiron arsenic forms iron(III) arsenide,FeAs2, and gallium arsenide, GaAs, is animportant semiconductor.

arsenious chloride /ar-sen-ee-s/ Seearsenic(III) chloride.

arsenious oxide See arsenic(III) oxide.

arsenite /ar-s-nt/ See arsenate(III).

arsine /ar-seen, ar-seen, -sin/ (arsenic hy-dride; AsH3) A poisonous colorless gaswith an unpleasant smell. It decomposes toarsenic and hydrogen at 230C. It is pro-duced in the analysis for arsenic (Marshstest).

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

1237Al + 0

1n 1214Na + 24Herepresents the bombardment of aluminumwith neutrons to produce an isotope of

23

artificial radioactivity

sodium. All the transuranic elements(atomic numbers 93 and above) are artifi-cially radioactive since they do not occur innature.

aryl group /a-rl/ An organic group de-rived by removing a hydrogen atom froman aromatic hydrocarbon or derivative.

asbestos /ass-best-s/ A fibrous varietyof various rock-forming silicate minerals,such as the amphiboles and chrysotile. Ithas many uses that employ its properties ofheat-resistance and chemical inertness.Prolonged exposure to asbestos dust maycause asbestosis a form of lung cancer.

asparagine /-spa-r-jeen, -jin/ Seeamino acids.

aspartic acid /-spar-tik/ See aminoacids.

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

aspirin (acetylsalicylic acid; C9H8O4) Acolorless crystalline compound made bytreating salicylic acid with ethanoyl hy-dride. It is used as an analgesic and an-tipyretic drug, and small doses areprescribed for patients at risk of heart at-tack or stroke. It should not be given toyoung children.

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

astatine /ass-t-teen, -tin/ A radioactiveelement belonging to the halogen group. Itoccurs in minute quantities in uraniumores. Many short-lived radioisotopes areknown, all alpha-particle emitters.

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

asymmetric atom See chirality; iso-merism; optical activity.

atactic polymer See polymerization.

atmolysis /at-mol--sis/ The separationof gases by using their different rates of dif-fusion.

atmosphere A unit of pressure definedas 101 325 pascals (atmospheric pressure).The atmosphere is used in chemistry onlyfor rough values of pressure; in particular,for stating the pressures of high-pressureindustrial processes.

atom The smallest part of an elementthat can exist as a stable entity. Atoms con-sist of a small dense positively charged nu-cleus, 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 electrons(which is equal to the number of protons inthe nucleus). All atoms of a given elementhave the same number of protons (the pro-ton number). A given element may havetwo or more isotopes, which differ in thenumber of neutrons in the nucleus.

The electrons surrounding the nucleusare grouped into shells i.e. main orbitsaround the nucleus. Within these main or-bits there may be sub-shells. These corre-spond to atomic orbitals. An electron in anatom is specified by four quantum num-bers:1. The principal quantum number (n),

which specifies the main energy levels. ncan have values 1, 2, etc. The corre-sponding shells are denoted by letters K,L, M, etc., the K shell (n = 1) being thenearest to the nucleus. The maximumnumber of electrons in a given shell is2n2.

2. The orbital quantum number (l), whichspecifies the angular momentum. For agiven value of n, 1 can have possible val-ues of n1, n2, 2, 1, 0. For instance,the M shell (n = 3) has three sub-shellswith different values of l (0, 1, and 2).Sub-shells with angular momentum 0, 1,2, and 3 are designated by letters s, p, d,and f.

aryl group

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3. The magnetic quantum number (m).This can have values l, (l 1) 0 + (l + l), + 1. It determines the orienta-tion of the electron orbital in a magneticfield.

4. The spin quantum number (ms), whichspecifies the intrinsic angular momen-tum of the electron. It can have values+ and .Each electron in the atom has four

quantum numbers and, according to thePauli exclusion principle, no two electronscan have the same set of quantum num-bers. This explains the electronic structureof atoms. See also Bohr theory.

atomic absorption spectroscopy (AAS)A technique in chemical analysis in whicha sample is vaporized and an absorptionspectrum is taken of the vapor. The ele-ments present are identified by their char-acteristic absorption lines.

atomic emission spectroscopy (AES)A technique in chemical analysis thatinvolves vaporizing a sample of material at high temperature. Atoms in excitedstates decay to the ground state, 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 the probe is automaticallyraised and lowered to keep this force con-stant. Scanning the surface in this way en-ables a contour map of the surface to begenerated with the help of a computer. Anatomic force