halogen (hălˈəjĕn) [key] [Gr., = salt-bearing], any of the chemically active elements found in Group 17 of the periodic table; the name applies especially to fluorine (symbol F), chlorine (Cl), bromine (Br), and iodine (I). Astatine (At), formerly known as alabamine, is a radioactive element also classed as a halogen; its most stable isotope (which does not occur in nature) has a half-life of less than 81/2 hr. The chemical and physical properties of astatine are not well known; it is believed to resemble iodine. The halogens are the best-defined family of chemical elements. Chemically they closely resemble one another; they are nonmetallic and form monovalent negative ions. They also exhibit an almost perfect gradation of physical properties. Fluorine, a pale yellow gas, is the least dense and chemically the most active, displacing the other halogens from their compounds and even displacing oxygen from water. Chlorine, a yellow-green gas, is more dense and less reactive than fluorine. Bromine is a dark red liquid. Iodine is a grayish black solid and is the least chemically active of the four; however, among the nonmetals only oxygen is more reactive than iodine. Pure halogens exist as diatomic molecules, e.g., Cl2; they form interhalogen compounds, i.e., compounds between two halogens. The halogens form numerous compounds with other elements. With hydrogen they form hydrogen halides, whose water solutions are called hydrohalic acids, e.g., the water solution of hydrogen chloride is called hydrochloric acid. They form numerous metal halides, or salts, e.g., sodium chloride, common table salt. They also form halocarbons, compounds with carbon and often other elements such as hydrogen and oxygen. Chloroform, iodoform, and carbon tetrachloride are halocarbons. Some other halogen compounds are calomel (mercurous chloride), fluorite, sal ammoniac (ammonium chloride), corrosive sublimate (mercuric chloride), and chlorine bleaches.