The halogen family
Fluorine, chlorine, bromine, and iodine are group 17 nonmetals. Their simple substances contain diatomic molecules. Halogen atoms gain one electron to form halide ions X⁻. Down the group, atomic radius and polarizability increase, melting and boiling points generally rise, and colour deepens. Oxidizing ability decreases: F₂ > Cl₂ > Br₂ > I₂.
Displacement and redox
A more reactive halogen oxidizes the halide ion of a less reactive halogen. For example, Cl₂ + 2Br⁻ → 2Cl⁻ + Br₂. Chlorine is reduced; bromide is oxidized. The reverse reaction is not spontaneous under comparable aqueous conditions.