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Calcium
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[[File:Ca-TableImage.svg|thumb|300px|Position of calcium in the periodic table.]] [[File:Ca Ar.png|thumb|250px|Calcium sample.]] '''Calcium''' is a [[Chemical elements|chemical element]] with the symbol Ca and atomic number 20. As an alkaline earth metal, calcium is a reactive metal that forms a dark oxide-nitride layer when exposed to [[air]]. It is the fifth most abundant element in Earth's crust and the third most abundant metal, after [[iron]] and [[aluminum]]. The most common calcium compound on Earth is calcium carbonate, found in [[limestone]] and the fossilised remnants of early sea life; gypsum, anhydrite, fluorite, and apatite are also sources of calcium. Calcium is also the most abundant metal and the fifth-most abundant element in the human body. As electrolytes, calcium ions (Ca<sup>2+</sup>) play a vital role in the physiological and biochemical processes of organisms and cells: in signal transduction pathways where they act as a second messenger; in neurotransmitter release from neurons; in contraction of all muscle cell types; as cofactors in many enzymes; and in fertilization. Calcium ions outside cells are important for maintaining the potential difference across excitable cell membranes, protein synthesis, and [[bone]] formation. == Uses == Calcium compounds are widely used in many industries: in foods and pharmaceuticals for calcium supplementation, in the [[paper]] industry as bleaches, as components in [[cement]] and electrical insulators, and in the manufacture of soaps. On the other hand, the metal in pure form has few applications due to its high reactivity. == Dependencies == * '''Calcium''' * [[Cement]] * [[Paper]] == See also == <div style="column-count:2"> * [[Aluminum]] * [[Copper]] * [[Gold]] * [[Lead]] * [[Manganese]] * [[Nickel]] * [[Tin]] * [[Zinc]] * [[Chemical elements]] </div> == References == {{wa|Calcium}} [[Category:Metals]]
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