Halogens

The salt-formers

Halogen means “salt-former” in Greek. Each halogen has seven electrons in its outer shell, one short of a full set, and grabs an electron from almost any metal it meets. The result is a salt: sodium chloride, table salt, is the best-known example, but fluorides, bromides and iodides are all formed the same way.

Gas, liquid and solid in one column

The halogens are the only group in the periodic table that includes elements in all three states at room temperature. Fluorine is a pale yellow gas, chlorine a yellow-green gas, bromine a dark red liquid that gives off orange fumes, and iodine a shiny purple-black solid that turns straight into violet vapour when warmed.

The change comes from the size of the molecules. Larger halogen molecules have more electrons, so the weak attractions between them grow stronger down the group, raising melting and boiling points.

Reactivity falls down the group

Fluorine is the most reactive element of all. It attacks glass, reacts violently with water and even combines with the noble gas xenon. Chlorine is less aggressive, bromine less again, and iodine is mild enough to be dabbed on cuts. A more reactive halogen will displace a less reactive one from its salts: bubble chlorine through a solution of potassium bromide and the solution turns orange as bromine is freed.

In your water, your teeth and your thyroid

Halogens also feature in the history of environmental damage. Chlorofluorocarbons, once used in fridges and aerosol cans, released chlorine high in the atmosphere that destroyed the ozone layer. The 1987 Montreal Protocol phased them out, and the ozone layer is slowly recovering.

• Chlorine disinfects drinking water and swimming pools and is used to make PVC
• Fluoride strengthens tooth enamel and is added to toothpaste
• Iodine is essential for thyroid hormones; iodised salt prevents deficiency
• Bromine compounds are used as flame retardants and in some medicines