BROMINE

(Br), chemical element, deep-red fuming liquid, member of the halogen elements, or Group VIIa of the periodic table. A rare element, bromine is found in nature dispersed throughout the Earth's crust only in compounds, as soluble and insoluble bromides. Some enrichment occurs in ocean water (65 parts per million by weight), in the Dead Sea (approximately 5 grams per litre), in some thermal springs, and in rare, insoluble silver bromide minerals. The chief commercial source of bromine is ocean water, from which the element is extracted by means of chemical replacement (oxidation) by chlorine. The French chemist Antoine-Jrme Balard first isolated bromine from bitterns (saline liquors) left after the evaporation of Mediterranean Sea water and recognized it as an element in 1826. Bromine thereafter was manufactured as a by-product from bitterns left after the crystallization of the main salt products. Most of the bromine produced between 1928 and 1975 was used in the manufacture of ethylene dibromide, a liquid added to leaded gasolines to remove lead deposits from engine cylinders during combustion. The compound is also used to destroy nematodes and other pests in soil. Bromine has other uses, as in making various dyes and the compounds acetylene tetrabromide and bromoform (used as gauge liquids because of their high specific gravity). Bromides of potassium, sodium, calcium, strontium, lithium, and ammonium have been used widely in medicine because of their sedative action. Silver bromide (AgBr), an important component of photographic film, is, in common with silver chloride and iodide, light sensitive. Traces of potassium bromate (KBrO3) are added to wheat flour to improve baking characteristics. Other bromine compounds of significance include hydrogen bromide (HBr), a colourless gas used as a reducing agent and a catalyst in organic reactions. A solution of the gas in water is called hydrobromic acid, a strong acid that resembles hydrochloric acid in its activity toward metals or their oxides and hydroxides. Bromine has a pungent odour and is irritating to the skin, eyes, and respiratory system. Exposure to concentrated bromine vapour, even for a short time, may be fatal. Bromine exists in the diatomic form (Br2) over a wide range of temperatures. The vapour is amber in colour, and the liquid is reddish brown. A saturated solution of bromine in water is orange-red and on cooling yields a red crystalline hydrate (a clathrate, in which bromine molecules are trapped in cagelike spaces within the network of water molecules). Bromine is a strong oxidizing agent, so that it combines violently with certain elements, such as phosphorus, aluminum, and potassium, giving off light. In the presence of moisture, bromine attacks many metals to form bromides. The most stable oxidation state of the element is -1, in which bromine occurs naturally, but oxidation states of +1 (hypobromite, BrO-), +3 (bromite, BrO-2), +5 (bromate, BrO-3), and +7 (perbromate, BrO-4) are known. Bromine can be introduced into most organic molecules and can be displaced by other groups more readily than can chlorine. An organic bromo compound resembles the corresponding chloro derivative but is usually more dense, less volatile, less combustible, and less stable. Natural bromine is a mixture of two stable isotopes: bromine-79 (50.54 percent) and bromine-81 (49.46 percent). Of the 17 known radioactive isotopes of the element, bromine-77 has the longest half-life (57 hours). atomic number 35 atomic weight 79.909 melting point -7.2 C (19 F) boiling point 59 C (138 F) specific gravity 3.12 (20 C) oxidation states -1, +1, +3, +5, +7 electronic config. 2-8-18-7 or(Ar)3d104s24p5