Calcium, a soft, silver-white metallic chemical element. Calcium is malleable (can be hammered or rolled into a thin sheet) and ductile (can be drawn into wire), and can combine with many other metals to form alloys. At ordinary temperatures it is chemically stable in dry air, but is oxidized rapidly in moist air. Calcium is a strong reducing agent, and is capable of converting most metallic oxides and halides into their metals. It can combine with most nonmetallic elements. At high temperatures, calcium is capable of absorbing oxygen, nitrogen, and other gases.

Calcium is essential to animal and plant life. It is the most abundant metallic element in animals, and is necessary to build strong bones, teeth, and shells. It also occurs in body fluids and cells, where it plays a role in many activities, including muscle contraction, blood clotting, transmission of nerve impulses, and stimulation of many enzymatic reactions. Calcium is necessary for cell division in plants.

Calcium is used mainly in preparing such metals as thorium and uranium that are difficult to obtain from their compounds. It is also used to remove nonmetallic impurities from various metals and alloys, and to remove oxygen and other gases from vacuum tubes. Calcium is alloyed with lead to make cable coverings.

Calcium forms many useful compounds. Calcium carbonate is the chief constituent of a number of common rocks (such as limestone) and minerals (such as dolomite) that are used in metallurgy, in building construction, and as soil conditioners. Calcium oxide, also called lime or quicklime, is used in metallurgy, in water treatment, and in the manufacture of chemicals, paper, mortar, and many other substances. Other calcium compounds are used as food additives, in medicines, and in the manufacture of lubricants, glass, detergents, and plastics.

Calcium was discovered in 1808 by the British chemist Sir Humphry Davy. It is the fifth most abundant element in the earth's crust, and is always found combined with other elements. It occurs chiefly as carbonate, sulfate, and phosphate in various rocks and minerals. Most commercial calcium oxide is produced by heating limestone or dolomite. Calcium metal is usually obtained either by heating calcium oxide with aluminum in a vacuum or by electrolysis of molten calcium chloride.

Symbol: Ca. Atomic number: 20. Atomic weight: 40.08. Specific gravity: 1.55. Melting point: 1,542 F. (839 C.). Boiling point: 2,703 F. (1,484 C.). Calcium has six stable isotopes: Ca-40, Ca-42, Ca-43, Ca-44, Ca-46, and Ca-48. Calcium is an alkaline-earth metal belonging to Group IIA of the Periodic Tableand has a valence of +2.