HALOGEN ELEMENT

Figure 1: Modern version of the periodic table of the elements. To see more information about an any of the five nonmetallic elements that comprise Group VIIa of the periodic table (see Figure). The halogen elements are fluorine (F), chlorine (Cl), bromine (Br), iodine (I), and astatine (At). They were given the name halogen from the Greek roots hal- (salt) and gen (to produce), because they all produce sodium salts of similar properties, of which sodium chloride, table salt, is the best known. Because of their great reactivity, the free halogen elements are not found in nature. In combined form, fluorine is the most abundant of the halogens in the Earth's crust. The percentages of the halogens in the igneous rocks of the Earth's crust are 0.06 fluorine, 0.031 chlorine, 0.00016 bromine, and 0.00003 iodine. Astatine does not occur in nature because it consists only of short-lived radioactive isotopes. The halogen elements show great resemblances to one another in their general chemical behaviour and in the properties of their compounds with other elements. There is, however, a progressive change in properties from fluorine through chlorine, bromine, and iodine to astatinethe difference between two successive elements being most pronounced with fluorine and chlorine. Fluorine is the most reactive of the halogens and, in fact, of all elements, and it has certain other properties that set it apart (see below General properties of the group). Chlorine is the best known of the halogen elements. The free element is widely used as a water purification agent, and it is employed in a number of chemical processes. Sodium chloride, of course, is one of the most familiar of chemical compounds. Fluorides are known chiefly for the controversy over their addition to public water supplies to prevent tooth decay, but organic fluorides are also used as refrigerants and lubricants. Iodine is most familiar as an antiseptic, and bromine is used chiefly to prepare a gasoline additive (ethylene dibromide) that prevents deposits of lead in engines. Additional reading R.B. Heslop and K. Jones, Inorganic Chemistry, rev. ed. (1976), contains a well-written chapter on the halogens. Viktor Gutmann, Coordination Chemistry in Non-Aqueous Solutions (1968), emphasizes the use of certain halides as solvents and the behaviour of various halides in certain solvents. Viktor Gutmann (ed.), Halogen Chemistry, 3 vol. (1967), is a collection of articles covering various aspects of halogen chemistry. Individual members of the family of halogen elements are treated in the following studies: R.N. Haszeldine and A.G. Sharpe, Fluorine and Its Compounds (1951), a readable, short text; Joel F. Liebman, Arthur Greenberg, and William R. Dolbier, Jr. (eds.), Fluorine-Containing Molecules (1988); George A. Olah, Richard D. Chambers, and F.K. Surya Prakash (eds.), Synthetic Fluorine Chemistry (1992); Joseph S. Thrasher and Steven H. Strauss (eds.), Inorganic Fluorine Chemistry (1994); Z.E. Jolles (ed.), Bromine and Its Compounds (1966), a useful account of bromine chemistry; and D. Price, B. Iddon, and B.J. Wakefield (eds.), Bromine Compounds: Chemistry and Applications (1988). A.P. Hagen (ed.), The Formation of Bonds to Halogens, 2 vol. (198991), covers halogenation and bond formation. Partially Halogenated Chlorofluorocarbons (Ethane Derivatives) (1992), published by the World Health Organization, discusses halogenated compounds to be substituted for those considered damaging to the ozone layer. Viktor Gutmann The Editors of the Encyclopdia Britannica