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Zinc, cadmium, and mercury

zinc blende. It is never found in its pure state in nature. It is the fourth most common metal in industrial use (after iron, copper, and alumi­num).

Zinc is rarely used alone, but large amounts are employed in alloys. In die-casting (using molds to form objects out of liquid metal), zinc is combined with aluminum and small amounts of copper and magnesium. To make brass, 3 to 45 per cent zinc is combined with copper. The addition of tin to the zinc and copper makes bronze. In galvanizing, steel is hot-dipped in molten zinc. The thin layer of zinc protects the steel from corrosion. Galva­nized metal is used in such products as roof gutters and tank linings. Zinc alloys are used to protect ships and buried steel structures. United States pennies are made mostly from a zinc alloy with a thin coating of copper.

Zinc is also used in making paint pigments, cosmetics, soaps, skin ointments, and various plastics. A compound of zinc is used to coat the inside of television screens and fluores­cent lamps. Other compounds are used in dry batteries and in embalming and to protect wood from decay and from insects.

Zinc is an essential trace element in the human body in red blood cells. It is also an im­portant ingredient of the hormone insulin. In plants and animals, zinc is necessary for nor­mal growth and healing. Its deficiency can cause leaf disease in trees.

Cadmium

Cadmium is a soft, silvery-white metal that is found only in small quantities, usually in com­bination with zinc minerals. It is similar to zinc and is obtained as a by-product from zinc, lead, and copper ores.

Cadmium is used mainly as a protective plating on steel and other metals. It is used in many alloys. For instance, it improves the strength and maleability of copper for electri­cal contacts and terminals. It is also used for many types of bearings, especially when they operate in high temperatures. Cadmium is a component of rechargeable batteries that can be used in watches, calculators, and other small devices. It is also used in television screens, photoelectric devices, and solar cells.

Cadmium is very poisonous. Inhalation of cadmium-oxide fumes, for example, during welding of cadmium-plated steel without ade­quate ventilation, can be fatal. Inhalation of cadmium dust can also result in serious illness or death. Even small amounts of cadmium en­tering the body over a prolonged period of time can deform bones or damage the kid­neys. Thus, widespread industrial use of the metal is strictly controlled in order to mini­mize the chances of cadmium polluting the environment.

Mercury

Also called quicksilver, mercury is a dense, sil­very metal, the only elemental metal that is liq­uid at room temperature. It is less common in nature than other metals. It usually occurs in



Major groups of elements: Zinc, cadmium, and mercury 33




Date: 2015-12-11; view: 246


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