Halogen: definition in chemistry
Halogen: Any element in group 17 of the periodic table: fluorine, chlorine, bromine, iodine, astatine and tennessine. The name means salt-former.
Halogen atoms have seven valence electrons, so they readily gain one electron to form halide ions with a −1 charge, such as Cl⁻. They exist as diatomic molecules (F₂, Cl₂, Br₂ and I₂) and are very reactive nonmetals, with reactivity decreasing down the group. A more reactive halogen displaces a less reactive one from its compounds, as when chlorine turns a colorless potassium bromide solution orange by releasing bromine.
Examples
- Fluorine — the most reactive of all nonmetals
- Chlorine — used to disinfect drinking water and swimming pools
- Bromine — a red-brown liquid that gives off choking vapor
- Iodine — a gray-black solid used as an antiseptic
On the periodic table
The halogens form group 17, the second column from the right, just before the noble gases.