HALOGEN

any of the five nonmetallic elements that comprise Group VIIa of the periodic table. The halogen elements are fluorine (F), chlorine (Cl), bromine (Br), iodine (I), and astatine (At). Most of the halogens are found in relatively small amounts in the Earth's crust. The single exception is astatine, which does not occur naturally because it consists exclusively of short-lived radioactive isotopes. The only practical source of this element is its synthesis by nuclear reactions. The halogens, particularly fluorine, are highly reactive, so that they never occur uncombined in nature. They readily react with most metals and many nonmetals to form a variety of compounds. The members of the halogen family closely resemble each other in general chemical behaviour and in the properties of their compounds. Characteristically, each halogen atom carries seven electrons in its outermost orbitals, which are of two different types. Potentially, the halogen atoms can hold one additional electron; in acquiring such an electron the atom acts as an oxidizing agent and in the process assumes an electrical charge and becomes an ion. Halogen elements exist in their salts as halide ions, which are very stable.