Chemical properties of the halogens

Atoms of group 7 (IUPAC group 17) elements all have seven electrons in their outer shell. This means that the halogens all have similar chemical properties.

Reactions with metals

The halogens react with metals to produce salts (the word ‘halogen’ means ‘salt former’). For example, chlorine reacts with sodium:

sodium + chlorine → sodium chloride

2Na(s) + Cl2(g) → 2NaCl(s)

Sodium and chlorine react vigorously when heated, giving an orange flame and clouds of white sodium chloride.

The halogens become less reactive going down group 7. The following table describes what is seen when halogens react with iron wool.

HalogenReaction with iron wool
FluorineCold iron wool burns to produce white iron(III) fluoride
ChlorineHot iron wool burns vigorously to produce orange-brown iron(III) chloride
BromineHot iron wool burns quickly to produce red-brown iron(III) bromide
IodineHot iron wool reacts slowly in iodine vapour to produce grey iron iodide

Write a balanced equation for the reaction of iron with chlorine to produce solid iron(III) chloride, FeCl3. Include state symbols.

2Fe(s) + 3Cl2(g) → 2FeCl3(s)

Reactions with hydrogen

The halogens react with hydrogen to produce compounds called hydrogen halides. For example, chlorine reacts with hydrogen:

hydrogen + chlorine → hydrogen chloride

H2(g) + Cl2(g) → 2HCl(g)

The hydrogen halides are gases at room temperature. They dissolve in water to produce acidic solutions. Hydrogen chloride dissolves in water to produce hydrochloric acid, HCl(aq).

The table describes what is seen when halogens react with hydrogen.

HalogenReaction with hydrogen
FluorineExplodes in the cold and dark, forming hydrogen fluoride
ChlorineExplodes with a flame or in sunlight, forming hydrogen chloride
BromineVigorous reaction with burning hydrogen, forming hydrogen bromide
IodineVery slow reaction when heated strongly, forming some hydrogen iodide

Astatine is placed below iodine in group 7. Use the information to predict the reaction of astatine with hydrogen.

Astatine should react very slowly with hydrogen, even when heated. A little hydrogen astatide should form, which should dissolve in water to form an acidic solution.