Chemical element , one of the alkaline earth metal s, chemical symbol Ca, atomic number 20.
The most abundant metallic element in the human body, it is an essential part of bone s and teeth and has many physiological functions (see calcium deficiency ; tooth ). It is the fifth most abundant element in Earth's crust but does not occur naturally in the free state. In its compounds calcium has valence 2. It occurs in limestone , chalk , marble , dolomite , eggshells, pearls, coral, and many marine shells as calcium carbonate, or calcite ; in apatite as calcium phosphate; in gypsum as calcium sulfate; and in many other minerals. It is used as an alloying agent and in other metallurgical applications; its alloy with lead is used in cable sheathing and grids for bleach ), calcium sulfate (gypsum and plaster of paris ), and calcium phosphate (a plant food and stabilizer for plastics).