Group 7 elements are called the Halogens. They are all diatomic molecules and the melting temperature increases down the group. When two molecules are close together they experience
London forces (instantaneous dipole-induced
dipole forces). As you go down the group there are more electrons so there are stronger London forces and the melting point increases. The halogens exist as diatomic molecules X
2 at room temperature and pressure.
Fluoride is added to drinking water and toothpaste to prevent tooth decay.
Chlorine is added to water to kill bacteria. Chlorine is also an extremely toxic gas. It is a respiratory irritant in small concentrations and fatal in large concentrations.
Bromine is used in flame retardants. When the substance burns it releases bromine atoms that remove the reactive radicals in the flame, slowing combustion. Potassium bromide is used as an anticonvulsant.
Iodine is used as an antiseptic to clean wounds and is needed by the thyroid gland in trace amounts.
| Halogen | Number of electrons | Boiling point °C | Appearance and state at RTP |
| | | | |
| F2 | 18 | -188 | pale yellow gas |
| Cl2 | 34 | -34 | pale green gas |
| Br2 | 70 | 59 | red-brown liquid |
| I2 | 106 | 184 | shiny grey-black solid |
| At2 | 170 | 230 | never been seen |