Chemistry - Halogens

The Halogens are in Group 7 in the periodic table because they have 7 electrons in the outer shell. Group 7 elements form salts when they react with metals. The term ‘halogen’ means ‘salt former’.

Properties

Halogens has properties of typical non metals i.e

• Poor conductors of electricity and heat

• Low density

• Low melting/ boiling point

Differentiating halogens

All of the halogens exist as diatomic molecules. This means that the elements are made up of pairs of atoms that are chemically joined together (for example, fluorine exists as F₂, chlorine as Cl₂, bromine as Br₂ and iodine as I₂).

Iodine forms a purple vapour when it is warmed.

Name | Formula | State at room temp | Colour of vapour | Melting point (C⁰⁾| Boiling point

Fluorine Fl_{2. Gas yellow -220 -188}

Chlorine Cl_{2. Gas yellow -101 -34}

Bromine Br_{2. Liquid orange/brown -7 +58}

Iodine I_{2. Solid purple/grey +114 +183}

To differentiate different halogens look at physical properties such as:

• State at room temp - There is a trend in state from gas to liquid to solid as you go down the group.

• Colour - The halogens become darker as you go down the group.

• Melting points - The melting and boiling points then increase as you go down the group.

Halogens with iron

Chlorine and iron

Needs small amount of heat to violently react producing Iron (III) chloride

Iron + chlorine → iron(III) chloride

2Fe + 3Cl₂ → 2FeCl₃

Bromine and iron

Immediately forms vapour, needs small amount of heat to react, quick but slower than chlorine and visible reaction producing Iron (III) bromide

Iron + bromine → iron(III) bromide

2Fe + 3Br₂ → 2FeBr₃

Iodide and iron

Produces purple vapour, needs several minutes of heating creating Iron (III) iodide

Iron + iodine → iron(III) iodide

2Fe + 3I₂ → 2FeI₃

Reactivity

Halogens decrease in reactivity as you go down the group. This is the opposite trend to that seen in the alkali metals in Group 1. Outer shell gets nearer to the nucleus so easier to attract one electron. Fluorine is the most reactive element of all in Group 7.

Displacement reactions

A more reactive halogen displaces a less reactive halide ion from its solution.

Uses of the Halogens

Chlorine - water supplies, swimming pools, bleaches

Bromide - plasters, sterilising sprays

Iodide - antiseptic

Safety precautions

• Fume cupboard used for reactions – halogens produce toxic vapours.