Fluorine - chemistry.

Fluorine - chemistry. I INTRODUCTION Fluorine (Latin fluo, "flow"), symbol F, chemically reactive, poisonous gaseous element. In group 17 (or VIIa) of the periodic table (see Periodic Law), fluorine is one of the halogens. The atomic number of fluorine is 9. The element was first isolated in 1886 by the French chemist Henri Moissan. II PROPERTIES AND OCCURRENCE Fluorine is a pale, greenish-yellow gas, slightly heavier than air, poisonous, corrosive, and of penetrating and disagreeable odor. Its atomic weight is 18.998. Fluorine melts at -219.61°C (-363.30°F), boils at -188.13°C (-306.63°F), and has a specific gravity of 1.51 in its liquid state at its boiling point. It is the most chemically active of the nonmetallic elements. It combines directly with most elements and indirectly with nitrogen, chlorine, and oxygen. Nearly all compounds are decomposed by fluorine to form fluorides that are among the most stable of all chemical compounds. Fluorine occurs naturally in the combined form as fluorite, cryolite, and apatite. Fluorite, from which most fluorine compounds are generally derived, is commonly mined in the United States from large deposits in northern Kentucky and southern Illinois. Fluorine also occurs as fluorides in seawater, rivers, and mineral springs, in the stems of certain grasses, and in the bones and teeth of animals. It is the 17th element in order of abundance in Earth's crust. The preparation of fluorine as a free element is difficult and seldom done, since free fluorine is very reactive. However, gaseous fluorine can be prepared by electrolytic techniques, and liquid fluorine may be prepared by passing the gas through a metal or rubber tube surrounded by liquid air. III FLUORINE COMPOUNDS Hydrofluoric acid (hydrogen fluoride, HF or H2F2), one of the most important fluorine compounds, is prepared by heating calcium fluoride in sulfuric acid. The aqueous solution of this acid, generally used commercially, is obtained by passing the anhydrous hydrogen fluoride vapors into a leaden receiver containing distilled water, thus yielding the acid in dilute form. Hydrofluoric acid is extremely corrosive and must be preserved in lead, steel, or plastic containers. Hydrofluoric acid dissolves glass, making the acid useful for various forms of glass etching, such as marking the divisions on thermometer tubes and etching designs into glassware and ceramics. Another fluorine compound, hydrofluosilicic acid, combines with such bases as sodium and potassium to form salts called fluosilicates or silicofluorides. Fluorine and many fluorides, such as hydrogen fluoride and sodium fluoride, are extremely poisonous. Drinking water containing excessive amounts of fluorides causes tooth enamel to become brittle and to chip off, leaving a stained or mottled effect. The proper proportion of fluorides in drinking water, however, has been found to greatly reduce tooth decay (see Dentistry). IV USES Fluorine compounds have many applications. The chlorofluorocarbons, odorless and nonpoisonous liquids or gases such as Freon, are used as a dispersing agent in aerosol sprays and as a refrigerant. In 1974, however, some scientists suggested that these chemicals reached the stratosphere and were destroying Earth's ozone layer. With confirmation of these findings by the late 1980s, the production of these chemicals began to be phased out (see Environment). Another chemical, Teflon, a fluorine plastic that is very resistant to most chemical action, is widely used to make such products as motor gaskets and dashboard accessories in the automobile industry. Teflon is also used as a coating on the inner surface of frying pans and other kitchen utensils to reduce the need for fat in cooking. Many organic fluorine compounds developed during World War II (1939-1945) showed extensive commercial potential. For example, the liquid fluorinated hydrocarbons derived from petroleum are useful as highly stable lubricating oils. Uranium hexafluoride, the only volatile compound of uranium, is used in the gaseous diffusion process to provide fuel for atomic power plants. Microsoft ® Encarta ® 2009. © 1993-2008 Microsoft Corporation. All rights reserved.