F · Cl · Br · I · At · Ts
Halogens are usually found in compounds rather than as free elements. Chloride is abundant in seawater and table salt, while fluorides, bromides and iodides occur in minerals, waters and living systems in very different amounts.
Their physical appearance changes down the group. Fluorine and chlorine are gases under ordinary conditions, bromine is a liquid and iodine is a solid. This visible progression reflects stronger attractions between larger molecules.
Halogen chemistry is important in disinfection, medicines, polymers and many industrial processes. The same reactivity that makes these elements useful also means the pure substances require careful handling.