Calcium - chemistry.

Calcium - chemistry. I INTRODUCTION 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. II 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 842°C (1548°F), boils at 1484°C (2703°F), and has a specific gravity of 1.55; its atomic weight is 40.08. Calcium is fifth in abundance among the elements in Earth's crust, but it is not found uncombined in nature. It occurs in many highly useful compounds, such as calcium carbonate (CaCO3), of which calcite, marble, limestone, and chalk are composed; calcium sulfate (CaSO4) in alabaster or gypsum; calcium fluoride (CaF2) in fluorite; calcium phosphate (Ca3(PO4)2) in rock phosphate; and in many silicates. In cold, dry air, calcium is not readily attacked by oxygen, but when heated it unites vigorously with the halogens, oxygen, sulfur, phosphorus, hydrogen, and nitrogen. Calcium reacts violently with water, forming the hydroxide Ca(OH)2 and releasing hydrogen. III USES The metal is obtained mainly by electrolysis of fused calcium chloride, a costly process. Until recently the pure metal had little use in industry. It is being used to an increasing extent, however, as a deoxidizer for copper, nickel, and stainless steel. Because calcium hardens lead when alloyed with it, lead-calcium alloys are excellent for bearings, superior to ordinary lead antimony for grids in storage batteries, and more durable as sheathing for lead-covered cable. Calcium is present in the chemically combined state in lime (calcium hydroxide), cement and mortar (as calcium hydroxide or a variety of silicates of calcium), teeth and bones (as a calcium hydroxyphosphate), and in many body fluids (as complex proteinaceous compounds) essential to muscle contraction, the transmission of nerve impulses, and the clotting of blood. Microsoft ® Encarta ® 2009. © 1993-2008 Microsoft Corporation. All rights reserved.