Halogen

Halogen is a chemical term referring to any element in group 17 (formerly VIIA) of the periodic table. The halogens are fluorine, chlorine, bromine, iodine, astatine, and the synthetic tennessine. They are highly reactive nonmetals that typically form salts when combined with metals and hydrogen halides when bonded to hydrogen. In their elemental state, halogens exist as diatomic molecules (F2, Cl2, Br2, I2; At2 and Ts2 are less well characterized).

Common properties include high electronegativity and seven valence electrons, which drive their strong tendency to attract electrons and to form -1 oxidation state in many compounds. Reactivity generally decreases down the group as atomic size increases and bond strength to hydrogen or metals changes. Halogens readily form a wide range of compounds, from simple salts such as sodium chloride (NaCl) to interhalogen compounds (ClF, BrF) and halogen oxides. They also form hydrogen halides (HF, HCl, HBr, HI) and acids when dissolved in water, collectively called hydrohalic acids.

Halogens occur in various minerals and seawater salts. Industrially important applications include chlorine for water disinfection and the production of PVC, fluorine for toothpaste and specialty chemicals, bromine for flame retardants and drilling fluids, and iodine for medical uses. Astatine is rare and highly radioactive, and tennessine is a synthetic element produced in very small amounts.

Safety and handling reflect their reactivity: halogens can corrode, oxidize, and form toxic gases; proper containment and ventilation are essential in laboratory and industrial settings.