FLUORINE, (Lat. fluo, "flow"), element, symbol F, one of the Halogens in group 17 (or VIIa) of the periodic table; at.no. 9, at.wt. 18.998. Fluorine melts at -219.61° C (-363.30° F); boils at -188.13° C (-306.63° F); and has a sp.gr. of 1.51 in its liquid state at its boiling point.

Fluorine was first isolated (1886) by the French chemist Henri Moissan. He obtained the element by electrolyzing anhydrous hydrogen fluoride dissolved in acid potassium fluoride, which conducted the electric current. Although Moissan originally used vessels of platinum, containers made of other metals, such as copper, may be used.

Properties and Occurrence

Fluorine is a pale, greenish-yellow gas, slightly heavier than air, poisonous, corrosive, and of penetrating and disagreeable odor. 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 (or fluorspar), Cryolite, and Apatite. Fluorite, from which most fluorine compounds are generally derived, is commonly mined in the U.S. 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 the crust of the earth. According to one recent suggestion by astrophysicists, the relative abundance of fluorine in interstellar space may be linked to processes initiated by supernova explosions.

The preparation of fluorine as a free element is difficult, and fluorine is rarely employed in that form. Gaseous fluorine may be prepared by the Moissan electrolytic method; liquid fluorine may be prepared by passing the gas through a metal or rubber tube surrounded by liquid air.

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 or steel containers. It has the property of dissolving glass, and this property is used in a common test for the presence of a fluoride; hydrofluoric acid is also used extensively in various forms of glass etching, such as the marking of divisions on thermometer tubes and the etching of designs on glassware, and in other forms of ceramic etching, such as pottery decoration.

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.

Uses

Fluorine compounds have many applications. The chlorofluorocarbons (CFCs), 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, it was suggested that these chemicals reached the stratosphere and were destroying the earth's Ozone Layer. With confirmation of these findings by the late 1980s, the production of these chemicals began to be phased out. After 1995 the U.S. and many European countries banned the manufacture and use of CFCs due to their dangerous effects on the 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 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.