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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% 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 consists of six stable isotopes with mass numbers between 40 and 48. By far the most abundant, however, is 40Ca, which constitutes 96.941% of all the calcium atoms that are found in nature. Calcium is used in the production of many products including glass, batteries, and steel. It also combines readily with many other elements, and these compounds are used as well for a variety of purposes.

Calcium (from the Latin calx, meaning lime) was not known as an element until the early 1800s. During this time, chemists who were 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 (1778-1829), who was a pioneer in the field of electro-chemistry, first isolated elemental calcium in 1808 by electrolyzing a mixture of lime and mercuric oxide. Today, calcium metal is obtained by electrolyzing molten calcium chloride (CaCl2) or by reducing calcium oxide with aluminum metal.

Calcium is the fifth most abundant element (after oxygen, silicon, aluminum and iron), in the earth's crust, making up 3.63% of the crust by weight. It occurs in the form of minerals such as limestone (calcium carbonate, CaCO3), gypsum ( calcium sulfate, CaSO4•2H2O), and fluorite (calcium fluoride, CaF2).

In living things, calcium is a component of leaves, bones, teeth, shells, and coral. 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, comprising about 1.4% of body weight. This makes it even more prevalent even than iron. Ninety-nine percent of the body's calcium is stored in the skeleton and teeth. Bones are 70% calcium by weight, which gives them their strength and rigidity. The remaining 1% circulates in the bloodstream, where, as American biochemist Elmer McCollum proved in the early 1900s, it is essential for muscle contractions. Calcium 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. Calcium supplements are often recommended to prevent these diseases in older people, mostly women.

Calcium is a very active metal and is never found uncombined in nature. It tarnishes quickly when exposed to air and burns with a bright yellowish red flame, forming mostly calcium nitride (Ca3 N2). It reacts directly with water to form calcium hydroxide [Ca(OH)2] and hydrogen gas. Because of its strong reducing power it is used to produce other metals such as thorium and uranium by reducing their compounds, and to purify various alloys by removing oxides and sulfides. Calcium forms useful alloys with aluminum, copper, and lead.

Many calcium compounds have important uses. Calcium oxide or lime is widely used to make cement (lime+clay), mortar (cement+sand+water) and concrete (cement+sand+gravel+water). It is also used in the manufacture of glass. When water is added to calcium carbide (CaC 2) the highly flammable gas acetylene (C 2H2) is produced; it is used in lamps and welding torches, and as a starting material in the synthesis of many organic compounds. Calcium chloride (CaCl2) is used as a drying agent, because it is a deliquescent solid: it can absorb so much water from the air that it turns into a liquid. It is also used as a more effective and less corrosive substitute for common salt (NaCl) for melting ice on roads in the winter. Calcium hypochlorite [Ca(OCl)2] is used as a bleach. Calcium phosphate [Ca3(PO4)2] and calcium cyanamide [Ca(CN)2] are used in the production of 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.

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