Calcium
Calcium is a chemical element, a member of the alkaline-earth metals group, represented by the atomic symbol Ca and the atomic number 20. It has an atomic weight of 40.08. In its pure form, calcium is a silvery-white metal, although it is never found in this free state naturally. It is, however, one of the most abundant substances on Earth, comprising approximately 3.64 percent of the Earth's crust. Pure calcium metal has a melting point of 1547.6 ° F (842 ° C) and boils at 2703 ° F (1484 ° C). It is used in several industries, including glass, battery, and steel production. It also combines readily with many other elements, and these compounds are used as well for a variety of purposes.
Calcium was not known as an element until the early 1800s, when chemists, trying to prove the existence of unknown metals in natural compounds, began using the newly discovered phenomenon of electricity to break them apart. The English chemist Humphry Davy a pioneer in the field of electrochemistry, first isolated elemental calcium in 1808 by electrolyzing a mixture of lime and mercuric oxide.
Natural calcium compounds are found most frequently in rocks and minerals. Calcium carbonate (CaCO3) is the most abundant of these, comprising over 40 percent of the content of limestone. In fact, calcium's name comes from the Latin word calx, or limestone. Marble, dolomite, seashells, pearls, and coral also contain large amounts of calcium carbonate. Today, the compound is used in toothpastes and antacid medicines, and is also an ingredient in white paint. Calcium oxide (CaO), also called lime or quicklime, is an ancient building material. It is obtained by heating calcium carbonate to high temperatures, which drives off carbon dioxide. When combined with water, lime decomposes into slaked lime, or calcium hydroxide (Ca(OH) 2), an important ingredient in mortar, plaster, and concrete. The slaking process originated with the ancient Romans, who used the calcium compound for road building. Although lime has important industrial uses, it has also served artistic purposes, particularly in stage lighting. When heated, lime gives off a brilliant white light, called limelight.
Another important compound of calcium is its carbide (CaC2), which was discovered by German chemist Friedrich Wöhler. In 1892 American scientist T. L. Willson produced calcium carbide by combining lime with carbon and heating the mixture. The result was a hard brittle crystal that, when exposed to water, yielded calcium hydroxide and acetylene, a flammable gas used in welding. Calcium carbide is also used in fertilizers.
Other calcium compounds include the minerals fluorspar, phosphorite, gypsum, and apatite. Calcium acetate is used in the production of plastics, and calcium hypochlorite is a bleaching agent and disinfectant.
Calcium plays a crucial role in good health, although its biological significance came to be understood only during the late nineteenth century. It is the most abundant metallic element in the human body, more prevalent even than iron. Ninety-nine percent of the body's calcium is stored in the skeleton and teeth. Bones are 70 percent calcium by weight, which gives them their strength and rigidity. The remaining 1 percent circulates in the bloodstream, where, as American biochemist Elmer McCollum proved in the early 1900s, it is essential for muscle contractions. Calcium also helps regulate contractions of the most important muscle in the body--the heart. This was discovered in 1882, when British physician Sydney Ringer (1835-1910) showed that a heart would continue to beat in a solution of salt, calcium, and other chemicals. Among its many other functions, calcium plays a role in the transmission of nerve impulses, and aids blood clotting.
Too little calcium in the diet can cause osteoporosis, a progressive weakening of the bones. Rickets can occur if there is insufficient vitamin D to aid calcium metabolism. Natural food sources of calcium include milk and dairy products, leafy green vegetables, and canned sardines.
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