Matter & Energy


Matter is composed of atoms or groups of atoms called molecules. The arrangement of particles in a material depends on the physical state of the substance. In a solid, particles form a compact structure that resists flow. Particles in a liquid have more energy than those in a solid. They can flow past one another, but they remain close. Particles in a gas have the most energy. They move rapidly and are separated from one another by relatively large distances.

Calcium

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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 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 and releasing hydrogen.

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.