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Physical properties

The arrangement of the periodic table is such that trends can be analysed both across a period and down a group.

Group 2 of the periodic table is shown here. Trends that can be analysed down the group include atomic radius, first ionization energy and melting point.

Elements in the same group also undergo similar chemical reactions.

Trend in atomic radius

Explaining the trend in atomic radius

The atomic radius of the elements increases down group 2 from beryllium to barium.

The increase in radius is due to higher principle energy levels being filled, whose orbitals are located further from the nucleus.

ElementAtomicradius(nm)

beryllium

magnesium

calcium

strontium

barium

The number of protons increases down the group; however, so does the number of shielding electrons. Effective nuclear charge therefore remains approximately constant.

Trend in first ionization energy

First ionization energies in group 2

Trend in melting points

Explaining the trend in melting points

The melting points of the elements decrease down group 2, with the exception of magnesium to calcium. beryllium

magnesium

calcium

strontium

barium

Element Melting point (K)

A metal’s melting point depends on the strength of its metallic bonds. This decreases down the group because the atomic radius increases, resulting in a weaker attraction between the nucleus and delocalized electrons.

The melting point of magnesium is lower than expected due to variation in how its atoms pack in the metallic crystal.

Physical properties summary

First ionization energy of group 2 metals

Solubilities of group 2 hydroxides

The solubility of the group 2 hydroxides increases down the group. Magnesium hydroxide is considered to be sparingly soluble and the hydroxides of the lower members of the groups are all considered to be soluble.

As the solubility of the group 2 hydroxides increases, so does the pH of the solutions formed. This is because the more of the hydroxide that dissolves, the greater the concentration of hydroxide ions (OH-) in the solution formed.

Mg(OH)2

Ca(OH)2

Sr(OH)2

Ba(OH)2

Group 2hydroxide

Solubility

sparingly soluble

slightly soluble

soluble

Applications of group 2 hydroxides

A suspension of magnesium hydroxide is commonly called milk of magnesia. It is used in medicine as a laxative and to relieve acid indigestion.

Calcium hydroxide, also called slaked lime, is used in agriculture to raise the pH of soils. Soil pH is an important factor in agriculture.

Solubilities of group 2 sulfates

The solubility of the group 2 sulfates decreases down the group. Magnesium and calcium sulfate are considered to be soluble, whereas strontium and barium sulfate are considered to be insoluble.

Group 2hydroxide

Solubility

soluble

slightly soluble

insoluble

Note that this decrease in solubility down the group is the opposite of the trend for the solubility of the group 2 hydroxides.

Testing for sulfate ions

Applications of group 2 sulfates

Barium sulfate is used as a radiocontrast agent to help take X-ray images of the digestive system. It is sometimes known as a ‘barium meal’.

Barium sulfate is insoluble, so is not absorbed by the body when swallowed. However, barium is a very good absorber of X-rays and it helps to define structures of the digestive system to aid in diagnosis.

Applications of group 2 compounds

Chemical properties summary

Reaction with oxygen

Flame tests

When group 2 metals are burned in oxygen, coloured flames are produced. This is due to the presence of metal ions. Flame tests exploit this fact.

magnesium – bright white

calcium – brick red/orange

strontium – red/crimson

barium – pale green/yellow-green

The presence of certain metal ions can be identified by noting the characteristic flame colour that results from burning. The colours for group 2 metal ions are:

Explaining flame tests

When heated, some electrons in an atom or ion are excited to higher energy levels. When they fall back to their initial levels, energy is emitted; sometimes seen as visible light.

Electrons may be excited by different amounts into different energy levels and drop back at different times. The colour of the flame is a combination of all these energy emissions.

heatlight

Flame test colours

Redox reaction with oxygen

When group 2 metals react with oxygen, they form the metal oxide. For example:

2Mg(s) + O2(g) 2MgO(s)

The oxidation state of magnesium has increased from 0 in its elemental form to +2 when it is in magnesium oxide. This means the magnesium has been oxidized.

The oxidation state of oxygen has decreased from 0 in its elemental form to -2 when it is in magnesium oxide. This means the oxygen has been reduced.

0 +20 -2oxidation

states

Redox reaction with chlorine

When group 2 metals react with chlorine, they form the metal chloride. For example:

Ca(s) + Cl2(g) CaCl2(s)

0 0 +2 -1oxidation

states

The oxidation state of calcium has increased from 0 in its elemental form to +2 when it is in calcium chloride. This means the calcium has been oxidized.

The oxidation state of chlorine has decreased from 0 in its elemental form to -1 when it is in calcium chloride. This means the chlorine has been reduced.

Reaction with water

Redox reaction with water

When group 2 metals react with water they form the metal hydroxide and hydrogen gas. For example:

Sr(s) + 2H2O(l) → Sr(OH)2(aq) + H2(g)

The oxidation state of strontium has increased from 0 in its elemental form to +2 when it is in strontium hydroxide. This means the strontium has been oxidized.

The oxidation state of hydrogen has decreased from +1 in water to 0 when it is in its elemental form. The means the hydrogen has been reduced.

0 +2+1 0oxidation

states

Explaining the trend in reactivity

The reactivity of the elements down group 2 from beryllium to barium increases.

Although increased shielding cancels the increased nuclear charge down the group, the increase in atomic radius results in a decrease in the attractive force between the outer electrons and the nucleus.

This is because it is successively easier to remove electrons to form the 2+ ion.

Reaction of oxides with water

When group 2 metal oxides react with water they form the metal hydroxide. For example:

SrO(s) + H2O(l) Sr(OH)2(aq)

Similar to the reaction between the metal and water, the resulting solution has high pH due to the hydroxide ions from the metal hydroxide. Reactivity is as follows:

beryllium

magnesium

calcium

strontium, barium

does not reactreacts slowly to form alkaline suspension

reacts to form alkaline suspension

react to form alkaline solutions

Oxide Reaction

Decomposition of group 2 carbonates

When heated, the group 2 metal carbonates decompose to form the metal oxide and carbon dioxide gas. Splitting compounds using heat is called thermal decomposition.

magnesium carbonate: MgCO3

calcium carbonate: CaCO3

strontium carbonate: SrCO3

barium carbonate: BaCO3

increasing stability

The group 2 carbonates become more stable to thermal decomposition going down the group:

MCO3(s) MO(s) + CO2(g)

Decomposition of group 2 nitrates

Thermal decomposition of group 2 metal nitrates forms the metal oxide, nitrogen dioxide and oxygen.

2M(NO3)2(s) 2MO(s) + 4NO2(g) + O2(g)

Like the group 2 metal carbonates, the nitrates become more stable to thermal decomposition down the group.

magnesium nitrate: Mg(NO3)2

calcium nitrate: Ca(NO3)2

strontium nitrate: Sr(NO3)2

barium nitrate: Ba(NO3)2

increasing stability

Explaining the trend in thermal stability

Metal ions become larger down group 2 but have the same charge. This means their charge density is reduced.

A metal ion with a high charge density has strong polarizing power. It can therefore polarize the carbonate ion, making it more likely to split into O2- and CO2 when heated. polarization

A metal ion with a low charge density has weak polarizing power, meaning the carbonate ion is less polarized and therefore more thermally stable.

Equations for reactions

Stability of group 2 carbonates

Glossary

What’s the keyword?

Multiple-choice quiz

What are the halogens?

The halogens are the elements in Group 7 of the periodic table.

The name halogen comes from the Greek words for salt-making.

Physical properties of halogens

Trends in boiling point

Halogen molecules increase in size down the group. This leads to greater van der Waals forces between molecules, increasing the energy needed to separate the molecules and therefore higher melting and boiling points.

fluorineatomic radius = 42 × 10-12

mboiling point = -118 °C

iodine atomic radius = 115 × 10-12

mboiling point = 184 °C

van der Waals forces

Trends in electronegativity

Electronegativity of the halogens decreases down the group due to an increase in atomic radius.

fluorineatomic radius = 42 × 10-12

melectronegativity = 4.0

iodineatomic radius = 115 × 10-12

melectronegativity = 2.5

Increased nuclear charge has no significant effect because there are more electron shells and more shielding. Iodine atoms therefore attract electron density in a covalent bond less strongly than fluorine.

Astatine

The name astatine comes from the Greek word for unstable.

It was first made artificially in 1940, by bombarding 209Bi with -radiation. What do you predict for these properties of astatine?

Astatine exists in nature in only very tiny amounts. It is estimated that only 30 grams of astatine exist on Earth at any one time. This is because it is radioactive, and its most stable isotope (210At) has a half-life of only 8 hours.

electronegativity.

state at room temperature

colour

Halogens: true or false?

Reactions of the halogens

Halogens react with metals such as sodium and iron:

They also take part in displacement reactions with halide ions, such as the reaction that is used to make bromine from potassium bromide in seawater:

halogen + hydrogen hydrogen halide

They also react with non-metals such as hydrogen:

halogen + sodium sodium halide

chlorine +potassiumbromide

potassiumchloride

bromine +

Reaction with iron

Reactions with hydrogen

Chlorine and hydrogen explode in bright sunlight but react slowly in the dark.

The halogens react with hydrogen gas to product hydrogen halides. For example:

Cl2(g) + H2(g) 2HCl(g)

Iodine combines partially and very slowly with hydrogen, even on heating.

Bromine and hydrogen react slowly on heating with a platinum catalyst.

Redox reactions of halogens

What is the reactivity of the halogens?

The reactions of the halogens with iron and hydrogen show that their reactivity decreases down the group.

How do you think fluorine and astatine would react with iron wool and hydrogen?

Iron wool burns and glows brightly.

Iron wool has a very slight glow.

Iron wool glows but less brightly than with chlorine.

chlorine

bromine

iodine

Halogen Reaction with iron wool

Reaction with hydrogen

Explodes in sunlight, reacts slowly in the dark.

Reacts slowly on heating with catalyst.

Reacts partially and very slowly.

Electron structure and reactivity

Halogen displacement reactions

Halogen displacement reactions are redox reactions.

Cl2 + 2KBr 2KCl + Br2

To look at the transfer of electrons in this reaction, the following two half equations can be written:

Chlorine has gained electrons, so it is reduced to Cl- ions.

What has been oxidized and what has been reduced?

2Br- Br2 + 2e-Cl2 + 2e- 2Cl-

Bromide ions have lost electrons, so they have been oxidized to bromine.

Oxidizing ability of halogens

fluorine

iodine

bromine

chlorine

In displacement reactions between halogens and halides, the halogen acts as an oxidizing agent.

This means that the halogen:

What is the order of oxidizing ability of the halogens?

is reduced to form the halide ion.

gains electrons

oxidizes the halide ion to the halogen

Oxidizing ability of halogens

Chlorine and disproportionation

Reaction of chlorine with water

Chlorination of drinking water raises questions about individual freedom because it makes it difficult for individuals to opt out.

Chlorine is used to purify water supplies because it is toxic to bacteria, some of which can cause disease. Adding it to water supplies is therefore beneficial for the population.

However, chlorine is also toxic to humans, so there are risks associated with gas leaks during the chlorination process. There is also a risk of the formation of chlorinated hydrocarbons, which are also toxic.

Bleach and the chlorate(I) ion

Household bleach commonly contains the chlorate(I) ion, ClO-, in the form of sodium chlorate(I), NaOCl.

ClO- + H2O + Cl- + 2OH-

The chlorine has been reduced because it has gained electrons. Its oxidation state has decreased from +1 in ClO- to –1 in Cl-.

How many electrons are needed to balance this equation?

The chlorate(I) ion behaves as an oxidizing agent. It oxidizes the organic compounds in food stains, bacteria and dyes.

Has the chlorine been oxidized or reduced in the reaction?

Redox reactions of chlorate ions

Halides

When halogens react with metals, they form compounds called halides. Many naturally-occurring halides have industrial, household and medical applications.

caesium chloride

sodiumhexafluoroaluminate

titanium(IV) chloride

lithium iodide

potassium bromide

Halide Formula Uses

NaAlF6

TiCl4LiI

Extraction and separation of DNA

Electrolysis of aluminium oxide

Extraction of titanium

Electrolyte in batteries

Epilepsy treatment in animals

Identifying halide ions

Halides can be identified by their reaction with acidified silver nitrate solution to form silver halide precipitates.

Silver chloride has a low solubility in water, so it forms a white precipitate: the positive result in the test for chloride ions.

KCl(aq) + AgNO3(aq) KNO3(aq)

+ AgCl(s)

potassium

chloride

silver chloride

+potassium

nitratesilver

nitrate+

Uses of halides in photography

Silver halides are used in photography.

Ag+ + e- Ag

Photographic film coated with a silver halide is exposed to light, causing the halide to decompose to form silver. This appears as a black precipitate on the photographic film.

paper coated in

silver halide

silver precipitate

white paper under mask

William Fox Talbot

William Fox Talbot (1800–1877) was a British scientist and mathematician. He was one of the key figures in the development of the use of silver halides in photography.

Fox Talbot adapted the process by removing any unreacted silver halide by washing with sodium thiosulfate solution. This meant that the print could be used repeatedly in the way that photographic negatives can be today.

A French scientist called Louis Daguerre developed the use of silver halides on copper plates. These were effective at producing prints, but could only be used once.

Hydrogen halides

The hydrogen halides are colourless gases at room temperature.

Hydrogen fluoride has an unexpectedly high boiling point compared to the other hydrogen halides. This is due to hydrogen bonding between the H–F molecules.

Hydrogen halide Boiling point (°C)

Halides as reducing agents

A substance that donates electrons in a reaction (i.e. is oxidized) is a reducing agent because it reduces the other reactant.

fluoride

iodide

bromide

chloride

The larger the halide ion, the easier it is for it to donate electrons and therefore the more reactive it is.

This is because its outermost electrons are further from the attraction of the nucleus and more shielded from it by other electrons. The attraction for the outermost electrons is therefore weaker.

Halides: true or false?

Sodium halides and sulfuric acid

The sodium halides react with concentrated sulfuric acid.

The reactions of sodium halides with concentrated sulfuric acid demonstrate the relative strengths of the halide ions as reducing agents.

During this reaction two things can happen to the sulfuric acid. It can

act as an acid.

be reduced

Oxidation states

Glossary

What’s the keyword?

Multiple-choice quiz

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