Halogens
The group 17 elements — fluorine through astatine — which are one electron short of a full outer shell.
Formula / equation
ns² np⁵
Unit
Dimensionless
In depth
All exist as diatomic molecules and are powerful oxidising agents, with reactivity falling down the group as atomic radius rises. Physical state changes from gas through liquid to solid down the group as London dispersion forces strengthen. Each displaces the halogens below it from their salts.
Examples in the real world
Chlorine gas bleaching dye, bromine as a dense red liquid, and iodine subliming to a violet vapour.