Copper (element) - chemistry.

Copper (element) - chemistry. I INTRODUCTION Copper (element), symbol Cu, brownish-red metallic element that is one of the most widely used of metals. Copper is one of the transition elements of the periodic table (see Periodic Law). The atomic number of copper is 29. Copper was known to prehistoric people and was probably the first metal from which useful articles were made. Copper objects have been found among the remains of many ancient civilizations, including those of Egypt, Asia Minor, China, southeastern Europe, Cyprus (from which the word copper is derived), and Crete (Kríti). It was known to Native Americans, and American ores were found by the European explorers. It is also found in the pure state. II PROPERTIES AND USES Copper melts at about 1085°C (about 1985°F), boils at about 2562°C (about 4644°F), and has a specific gravity of 8.95. The atomic weight of copper is 63.546. Because of its many desirable properties, such as its conductivity of electricity and heat, its resistance to corrosion, its malleability and ductility, and its beauty, copper has long been used in a wide variety of applications. The principal uses are electrical, because of copper's extremely high conductivity, which is second only to that of silver. Because copper is very ductile, it can be drawn into wires of any diameter from about 0.025 mm (about 0.001 in) upward. The tensile strength of drawn copper wire is about 4200 kg/sq cm (about 60,000 lb/sq in); it can be used in outdoor power lines and cables, as well as in house wiring, lamp cords, and electrical machinery such as generators, motors, controllers, signaling devices, electromagnets, and communications equipment. Copper has been used for coins throughout recorded history and has also been fashioned into cooking utensils, vats, and ornamental objects. Copper was at one time used extensively for sheathing the bottom of wooden ships to prevent fouling. Copper can easily be electroplated, alone or as a base for other metals. Large amounts are used for this purpose, particularly in making electrotypes, reproductions of type for printing. The metallurgy of copper varies with the composition of the ore. Native copper is crushed, washed, and cast in bars. Oxides and carbonates are reduced with carbon. The most important ores, the sulfides, contain not more than 12 percent, sometimes as little as 1 percent, of copper; they must first be crushed and concentrated by flotation. The concentrates are smelted in a reverberatory furnace, which yields crude metallic copper, approximately 98 percent pure. Crude copper is further purified by electrolysis, yielding bars exceeding 99.9 percent purity. Pure copper is soft but can be hardened somewhat by being worked. Alloys of copper, which are far harder and stronger than the pure metal, have higher resistance and so cannot be used for electrical purposes. They do, however, have corrosion resistance almost as good as that of pure copper and are very easily worked in machine shops. The two most important alloys are brass, a zinc alloy, and bronze, a tin alloy. Both tin and zinc are sometimes added to the same alloy, and no sharp dividing line can be drawn between brass and bronze. Both are used in enormous quantities. Copper is also alloyed with gold, silver, and nickel, and is an important constituent of such alloys as Monel metal, gunmetal, and German silver. Copper forms two series of chemical compounds: cuprous, in which the copper has a valence of 1, and cupric, in which the copper has a valence of 2. Cuprous compounds are easily oxidized to cupric, in many cases by mere exposure to air; cupric compounds are stable. Certain copper solutions have the power of dissolving cellulose, and large quantities of copper are for this reason used in the manufacture of rayon. Copper is also used in many pigments and in such insecticides as Paris green and such fungicides as Bordeaux mixture, although it is being largely replaced by synthetic organic chemicals for these purposes. III OCCURRENCE Native copper occurs in the vicinity of Lake Superior in northern Michigan, where it is mined in economically important quantities; it occurs also in small amounts in other parts of the world. Overall, copper is about the 25th most abundant element in crustal rocks. It is usually found admixed with other metals, such as gold, silver, bismuth, and lead, and exists in small specks in rock, but individual masses weighing as much as 420 metric tons have been found. The principal sources of copper are chalcopyrite and bornite, mixed sulfides of copper and iron. Other important ore minerals are chalcocite and covellite, sulfides of copper, which are found in Arizona and Nevada in the United States and in Cornwall, England. Enargite, a sulfarsenate of copper, is found in various parts of the United States. Azurite, a basic carbonate of copper, is found in France and Australia, and malachite, also a basic carbonate of copper, in the Ural Mountains. Tetrahedrite, a sulfantimonide of copper and various other metals, and chrysocolla, a copper silicate, are both widely distributed. Cuprite, an oxide, is found in Cuba, and atacamite, a basic chloride, in Peru. U.S. production of copper in 1991 was about 18.5 percent of the estimated world production of 8.8 million metric tons. The leading ore-producing states are Arizona, New Mexico, and Utah. Microsoft ® Encarta ® 2009. © 1993-2008 Microsoft Corporation. All rights reserved.