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Calcium (pronounced /ˈkælsiəm/) is the chemical element with the symbol Ca and atomic number 20. It has an atomic mass of 40.078 amu. Calcium is a soft grey alkaline earth metal, and is the fifth most abundant element by mass in the Earth's crust. Calcium is also the fifth most abundant dissolved ion in seawater by both molarity and mass, after sodium, chloride, magnesium, and sulfate.

Calcium is essential for living organisms, particularly in cell physiology, where movement of the calcium ion Ca²⁺ into and out of the cytoplasm functions as a signal for many cellular processes. As a major material used in mineralization of bones and shells, calcium is the most abundant metal by mass in many animals.

Notable characteristics

Chemically calcium is reactive and soft for a metal (though harder than lead, it can be cut with a knife with difficulty). It is a silvery metallic element that must be extracted by electrolysis from a fused salt like calcium chloride. Once produced, it rapidly forms a grey-white oxide and nitride coating when exposed to air. It is somewhat difficult to ignite, unlike magnesium, but when lit, the metal burns in air with a brilliant high-intensity red light. Calcium metal reacts with water, evolving hydrogen gas at a rate rapid enough to be noticeable, but not fast enough at room temperature to generate much heat. In powdered form, however, the reaction with water is extremely rapid, as the increased surface area of the powder accelerates the reaction with the water. Part of the slowness of the calcium-water reaction results from the metal being partly protected by insoluble white calcium hydroxide. In water solutions of acids where the salt is water soluble, calcium reacts vigorously.

Calcium, with a specific mass of 1.55 g/cm³, is the lightest of the alkali earth metals; magnesium is heavier (1.74) and beryllium even more heavy (1.84) despite these two elements being lighter in atomic mass. From strontium on the alkali earth metals get heavier along with the atomic mass.

Calcium has a higher resistivity than copper or aluminium. Yet, weight for weight, allowing for its much lower density, it is a rather better conductor than either. However, its use in terrestrial applications is usually limited by its high reactivity with air.

Calcium salts are colorless from any contribution of the calcium, and ionic solutions of calcium (Ca²⁺) are colorless as well. Many calcium salts are not soluble in water. When in solution, the calcium ion to the human taste varies remarkably, being reported as mildly salty, sour, "mineral like" or even "soothing." It is apparent that many animals can taste, or develop a taste, for calcium, and use this sense to detect the mineral in salt licks or other sources. In human nutrition, soluble calcium salts may be added to tart juices without much effect to the average palate.

Calcium is the fifth most abundant element by mass in the human body, where it is a common cellular ionic messenger with many functions, and serves also as a structural element in bone. It is the relatively high atomic-numbered calcium in the skeleton which causes bone to be radio-opaque. Of the human body's solid components after drying (as for example, after cremation), about a third of the total mass is the approximately one kilogram of calcium which composes the average skeleton (the remainder being mostly phosphorus and oxygen).

Occurrence

Calcium is not naturally found in its elemental state. Calcium occurs most commonly in sedimentary rocks in the minerals calcite, dolomite and gypsum. It also occurs in igneous and metamorphic rocks chiefly in the silicate minerals: plagioclase, amphiboles, pyroxenes and garnets.

See also Calcium minerals.

Applications

Some uses are:

as a reducing agent in the extraction of other metals, such as uranium, zirconium, and thorium.
as a deoxidizer, desulfurizer, or decarbonizer for various ferrous and nonferrous alloys.
as an alloying agent used in the production of aluminium, beryllium, copper, lead, and magnesium alloys.
in the making of cements and mortars to be used in construction.
in the making of cheese, where calcium ions influence the activity of rennin in bringing about the coagulation of milk.

Calcium compounds

Calcium carbonate (CaCO₃) used in manufacturing cement and mortar, lime, limestone (usually used in the steel industry); aids in production in the glass industry, also has chemical and optical uses as mineral specimens in toothpastes, for example.
Calcium hydroxide solution (Ca(OH)₂) (also known as limewater) is used to detect the presence of carbon dioxide by being bubbled through a solution. It turns cloudy where CO₂ is present.
Calcium arsenate (Ca₃(AsO₄)₂) is used in insecticides.
Calcium carbide (CaC₂) is used: to make acetylene gas (for use in acetylene torches for welding) and in the manufacturing of plastics.
Calcium chloride (CaCl₂) is used: in ice removal and dust control on dirt roads, in conditioner for concrete, as an additive in canned tomatoes, and to provide body for automobile tires.
Calcium cyclamate (Ca(C₆H₁₁NHSO₃)₂) was used as a sweetening agent but is no longer permitted for use because of suspected cancer-causing properties.
Calcium gluconate (Ca(C₆H₁₁O₇)₂) is used as a food additive and in vitamin pills.
Calcium hypochlorite (Ca(OCl)₂) is used: as a swimming pool disinfectant, as a bleaching agent, as an ingredient in deodorant, and in algaecide and fungicide.
Calcium permanganate (Ca(MnO₄)₂) is used in liquid rocket propellant, textile production, as a water sterilizing agent and in dental procedures.
Calcium phosphate (Ca₃(PO₄)₂) is used as a supplement for animal feed, fertilizer, in commercial production for dough and yeast products, in the manufacture of glass, and in dental products.
Calcium phosphide (Ca₃P₂) is used in fireworks, rodenticide, torpedoes and flares.
Calcium stearate (Ca(C₁₈H₃₅O₂)₂ is used in the manufacture of wax crayons, cements, certain kinds of plastics and cosmetics, as a food additive, in the production of water resistant materials and in the production of paints.
Calcium sulfate (CaSO₄·2H₂O) is used as common blackboard chalk, as well as, in its hemihydrate form being more well known as Plaster of Paris.
Calcium tungstate (CaWO₄) is used in luminous paints, fluorescent lights and in X-ray studies.
Hydroxylapatite (Ca₅(PO₄)₃(OH), but is usually written Ca₁₀(PO₄)₆(OH)₂) makes up seventy percent of bone. Also carbonated-calcium deficient hydroxylapatite is the main mineral of which dental enamel and dentin are comprised.

H and K lines

In the visible portion of the spectrum of many stars, including the Sun, strong absorption lines of singly-ionized calcium are shown. Prominent among these are the H-line at 3968.5 Å and the K line at 3933.7 Å of singly-ionized calcium, or Ca II. For the Sun and stars with low temperatures, the prominence of the H and K lines can be an indication of strong magnetic activity in the chromosphere. Measurement of periodic variations of these active regions can also be used to deduce the rotation periods of these stars.

History

Calcium (Latin word calcis meaning "lime") was known as early as the first century when the Ancient Romans prepared lime as calcium oxide. Literature dating back to 975 AD notes that plaster of paris (calcium sulphate), is useful for setting broken bones. It was not isolated until 1808 in England when Sir Humphry Davy electrolyzed a mixture of lime and mercuric oxide. Davy was trying to isolate calcium; when he heard that Swedish chemist Jöns Jakob Berzelius and Pontin prepared calcium amalgam by electrolyzing lime in mercury, he tried it himself. He worked with electrolysis throughout his life and also discovered/isolated sodium, potassium, magnesium, boron and barium. Calcium metal was not available in large scale until the beginning of the 20th century.

Compounds

Calcium, combined with phosphate to form hydroxylapatite, is the mineral portion of human and animal bones and teeth. The mineral portion of some corals can also be transformed into hydroxylapatite.

Calcium hydroxide (slaked lime) is used in many chemical refinery processes and is made by heating limestone at high temperature (above 825°C) and then carefully adding water to it. When lime is

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