Pronunciation: (kal'sē-um),[key] —n.Chem.,Biol. a silver-white divalent metal, occurring combined in limestone, chalk, gypsum, etc., occurring also in vertebrates and other animals, as a component of bone, skeletal mass, shell, etc., and as a necessary element in nerve conduction, heartbeat, muscle contraction, and many other physiological functions. Symbol: Ca; at. wt.: 40.08; at. no.: 20; sp. gr.: 1.55 at 20°C.