Mercury (element) - chemistry.

Mercury (element) - chemistry. I INTRODUCTION Mercury (element), symbol Hg, Latin, hydrargyrum, "liquid silver," metallic element that is a free-flowing liquid at room temperature. Mercury is one of the transition elements of the periodic table (see Periodic Law). The atomic number of mercury is 80. Mercury, once known as liquid silver and as quicksilver, was known to ancient civilizations, and no single individual is credited with having discovered it. It was studied by the alchemists (see Alchemy). It was first distinguished as an element by French chemist Antoine Laurent Lavoisier, who burned mercury and other substances in his experiments to determine the composition of air. The element was named after the planet Mercury. II PROPERTIES AND OCCURRENCE At ordinary temperatures mercury is a shining, mobile liquid, silvery-white in color. Slightly volatile at room temperature, mercury becomes solid when subjected to a pressure of 7,640 atmospheres (7.7 million millibars), and this pressure is used as a standard in measuring extremely high pressures. The metal dissolves in nitric or concentrated sulfuric acid but is resistant to alkalis. When cooled to sufficiently low temperatures, mercury exhibits superconductivity (the ability to conduct electrical currents with zero resistance). Mercury has a freezing point of about -39°C (about -38°F), a boiling point of about 357°C (about 674°F), and a density of 13.55 grams per cu cm. The atomic weight of mercury is 200.59. Mercury ranks about 67th in natural abundance among the elements in crustal rocks. It occurs in its pure form or combined with silver in small amounts but is found most often in the ore cinnabar, a mineral consisting of mercuric sulfide (HgS). The metal is obtained from cinnabar by heating the ore in air until the mercuric sulfide breaks down, yielding pure mercury metal. III USES Mercury is used in thermometers because its coefficient of expansion is nearly constant--that is, the change in volume for each degree of rise or fall in temperature is the same. It is also used in other types of scientific apparatuses, such as vacuum pumps, barometers, and electric rectifiers and switches. Mercury-vapor lamps are used as a source of ultraviolet light and for sterilizing water. Because of the extremely toxic effects of mercury, the use of the metal and its compounds has been reduced in several industries, including pharmaceuticals, dentistry, and agriculture. The use of mercury in fluorescent lamps and mercury batteries has also been significantly reduced as alternatives have been developed. Perhaps most significant is the substitution of diaphragm cells for traditional mercury cells in chlorine-alkali production, which once accounted for a large percentage of total mercury consumption. Mercury combines with all the common metals except iron and platinum to form alloys that are called amalgams. In one method of extracting gold and silver from their ores, the metals are combined with mercury to make them dissolve; the mercury is then removed by distillation. This method is no longer commonly used, however. Mercury forms monovalent and divalent compounds. Among the commercially important compounds of mercury are mercuric sulfide, a common antiseptic also used as the pigment vermilion; mercurous chloride, or calomel, used for electrodes, and formerly used as a cathartic; mercuric chloride, or corrosive sublimate; and organic compounds used as disinfectants, germicides, and antiseptics, known as mercurials. IV MERCURY POISONING Mercury is acutely hazardous as a vapor and in the form of its water-soluble salts, which corrode membranes of the body. Mercury vapor is a more toxic form than liquid mercury because the fumes easily enter and poison the body through inhalation. The fine gray mercury powder, which is easily produced when liquid mercury is agitated with substances such as grease and chalk, is potentially more dangerous than the liquid metal because it is less readily recognized. Short-term, limited contact with mercury can cause acute symptoms such as bleeding gums, vomiting, and stomach pain. Mercury poisoning is potentially fatal and can cause irreversible brain, liver, and kidney damage. Because it is difficult for the body to eliminate, mercury acts as a cumulative poison; it can eventually accumulate in the body to dangerous levels. Chronic mercury poisoning occurs when small amounts of the metal or its fat-soluble salts, particularly methylmercury, are repeatedly ingested, either orally or by absorption through the skin, over a long period of time. Ingestion can occur through contamination of the food chain. Because organic mercury compounds such as dimethylmercury were once widely used, significant quantities of mercury have been found in whales and some species of fish. Concern regarding uncontrolled industrial discharge of mercury into the environment, particularly from coal-fired power plants, has led to stricter environmental regulations in many countries. See Environmental and Occupational Diseases. Concern over the potential health hazards of mercury levels in fish led the United States Food and Drug Administration (FDA) and the Environmental Protection Agency (EPA) to issue consumption guidelines in March 2004. The two agencies said pregnant women, children, nursing mothers, and women who may become pregnant should not eat shark, swordfish, king mackerel, or tilefish. The agencies also recommended that this group reduce its consumption of tuna, especially albacore tuna, which is often sold as white tuna. The recommended levels were 6 oz (170 g) of albacore tuna per week and 12 oz (340 g) of light tuna per week. The European Union's food safety authority also recommended that pregnant women limit their consumption of fish, particularly swordfish and tuna. Scientific studies have shown that unsafe levels of mercury can cause neurological damage, particularly in young children and fetuses. Some consumer and environmental groups recommended even lower consumption of tuna. Microsoft ® Encarta ® 2009. © 1993-2008 Microsoft Corporation. All rights reserved.