Calcium
. Calcium, symbol Ca, reactive, silvery-white metallic element. In group 2 (or IIa) of the periodic table (see Periodic Law ), calcium is one of the alkaline earth metals . The atomic number of calcium is 20. The British chemist Sir Humphry Davy isolated calcium in 1808 by means of electrolysis. PROPERTIES AND OCCURRENCE Calcium has six stable and several radioactive isotopes. A malleable and ductile metal, calcium rapidly tarnishes to yellow on exposure to air. Calcium melts at about 839° C (about 1542° F), boils at about 1484° C (about 2703° F), and has a specific gravity of 1.54; its atomic weight is 40.08. Calcium is fifth in abundance among the elements in the earth's crust, but it is not found uncombined in nature. It occurs in many highly useful compounds, such as calcium carbonate, of which calcite , marble , limestone , and chalk are composed; calcium sulfate in alabaster or gypsum ; calcium fluoride in fluorite ; calcium phosphate in rock phosphate; and in many silicate