halogen Elements (fluorine, chlorine, bromine, iodine and astatine) belonging to group VII of the periodic table. They react with most other elements and with organic compounds. Fluorine is the most reactive and radioactive astatine is the least reactive. The halogens are highly electronegative; they react strongly because they require only one electron to achieve the ‘stable 8’ inert gas configuration. They produce crystalline salts (halides) containing negative ions of the type F⁻ and Cl⁻.

hal·o·gen / ˈhaləjən/ • n. Chem. any of the elements fluorine, chlorine, bromine, iodine, and astatine, occupying group VIIA (17) of the periodic table. They are reactive nonmetallic elements that form strongly acidic compounds with hydrogen, from which simple salts can be made. ∎  [as adj.] denoting lamps and radiant heat sources using a filament surrounded by the vapor of iodine or another halogen: halogen headlights.DERIVATIVES: hal·o·gen·ic / ˌhaləˈjenik/ adj.

halogen (hal-ŏ-jen) n. any one of the related elements fluorine, chlorine, bromine, or iodine.