calcium

Soft, silvery-white metallic element, atomic number 20, relative atomic mass 40.08. It is one of the alkaline-earth metals. It is the fifth most abundant element (the third most abundant metal) in the Earth's crust. It is found mainly as its carbonate CaCO₃, which occurs in a fairly pure condition as chalk and limestone (see calcite). Calcium is an essential component of bones, teeth, shells, milk, and leaves, and it forms 1.5% of the human body by mass.

Calcium ions in animal cells are involved in regulating muscle contraction, blood clotting, hormone secretion, digestion, and glycogen metabolism in the liver. It is acquired mainly from milk and cheese, and its uptake is facilitated by vitamin D. Calcium deficiency leads to chronic muscle spasms (tetany); an excess of calcium may lead to the formation of stones (calculi) in the kidney or gall bladder.

The element was discovered and named by the English chemist Humphry Davy in 1808. Its compounds include slaked lime (calcium hydroxide, Ca(OH)₂); plaster of Paris (calcium sulphate, CaSO₄.1/2H₂O); calcium phosphate (Ca₃(PO₄)₂), the main constituent of animal bones; calcium hypochlorite (CaOCl₂), a bleaching agent; calcium nitrate (Ca(NO₃)₂.4H₂O), a nitrogenous fertilizer; calcium carbide (CaC₂), which reacts with water to give ethyne (acetylene); calcium cyanamide (CaCN₂), the basis of many pharmaceuticals, fertilizers, and plastics, including melamine; calcium cyanide (Ca(CN)₂), used in the extraction of gold and silver and in electroplating; and others used in baking powders and fillers for paints.