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.