The halogens are a chemical series. They are the elements in Group 17 (old-style: VII or VIIA) of the periodic table: fluorine, chlorine, bromine, iodine, and astatine. The word comes from Greek roots meaning "salt" and "creator".

These elements are diatomic molecules in their natural form. They require one more electron to fill their outer electron shells, and so have a tendency to form a singly-charged negative ion. This negative ion is referred to as a halide ion; salts containing these ions are known as halides.

Halogens are highly reactive, and as such can be harmful or lethal to biological organisms in sufficient quantities. Chlorine and iodine are both used as disinfectants for such things as drinking water, swimming pools, fresh wounds, dishes, and surfaces. They kill bacteria and other potentially harmful microorganisms, a process known as sterilization. Their reactive properties are also put to use in bleaching. Chlorine is the active ingredient of most fabric bleaches and is used in the production of most paper products.

Halide ions combined with single hydrogen atoms form the hydrohalic acids (i.e., HF, HCl, HBr, HI), a series of particularly strong acids. (HAt, or "astatinic acid", could be called hydrohalic, but since all known isotopes of astatine are radioactive and have half-lives ranging from eight hours to less than a minute, it is not typically included in discussions of hydrohalic acids.)

They react with each other to form interhalogen compounds.

Many synthetic organic compounds such as plastic polymers, and a few natural ones, contain halogen atoms; these are known as halogenated compounds. Thyroid hormones contain iodide atoms. Chloride ions play a key role in brain function by mediating the action of the inhibitory transmitter GABA.

They show a number of trends when moving down the group - for instance, decreasing electronegativity and reactivity, increasing melting and boiling point.

See also

• pseudohalogen