Any of the five chemical elements in group VIIa of the periodic table. Fluorine, Chlorine, Bromine, Iodine, and the radioactive Astatine are non-metallic, monovalent negative ions and exist in pure form as diatomic molecules.

The first four elements exhibit an almost perfect gradation of physical properties. Fluorine is the least dense and chemically the most active, displacing other halogens from their compounds and oxygen from water. Iodine is the least active.

The halogens form numerous compounds with each other, and with other elements, such as hydrogen halides, metal halides salts), and halocarbons.