[1. A metallic bivalent element; atomic no. 20, atomic wt. 40.078, density 1.55, melting point 842°C. The oxide of calcium is an alkaline earth, CaO, quicklime, which on the addition of water becomes calcium hydrate, Ca(OH)2, slaked lime. For some organic calcium salts not listed below, see the name of the organic acid portion. Many calcium salts have crucial uses in metabolism and in medicine. Calcium salts are responsible for the radiopacity of bone, calcified cartilage, and arteriosclerotic plaques in arteries.