Halogen
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.
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.)
Many synthetic organic compounds, 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.