Editing Calcium (section)

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{{About|the chemical element|the use of calcium as a medication|Calcium supplement|other uses}}
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{{Infobox calcium|engvar=en-GB}}
'''Calcium''' is a [[chemical element]]; it has [[Symbol (chemistry)|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. Its physical and chemical properties are most similar to its heavier homologues [[strontium]] and [[barium]]. It is the fifth most abundant element in Earth's crust, and the third most abundant metal, after [[iron]] and [[aluminium]]. 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. The name derives from [[Latin language|Latin]] ''calx'' "[[lime (material)|lime]]", which was obtained from heating limestone.

Some calcium compounds were known to the ancients, though their chemistry was unknown until the seventeenth century. Pure calcium was isolated in 1808 via [[electrolysis]] of its oxide by [[Humphry Davy]], who named the element. 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; still, in small quantities it is often used as an alloying component in steelmaking, and sometimes, as a calcium–lead alloy, in making automotive batteries.

Calcium is the most abundant metal and the fifth-most abundant element in the [[human body#Composition|human body]].<ref name="lpi">{{cite web | url=https://lpi.oregonstate.edu/mic/minerals/calcium | title=Calcium | publisher=Linus Pauling Institute, Oregon State University, Corvallis, Oregon | date=1 September 2017 | access-date=31 August 2019}}</ref> As [[electrolyte]]s, [[Calcium in biology|calcium ions]] (Ca<sup>2+</sup>) play a vital role in the [[physiology|physiological]] and [[biochemistry|biochemical]] processes of organisms and [[cell (biology)|cell]]s: in [[signal transduction]] pathways where they act as a [[second messenger]]; in [[neurotransmitter]] release from [[neurons]]; in contraction of all [[muscle cell]] types; as [[Cofactor (biochemistry)|cofactors]] in many [[enzyme]]s; and in [[fertilization]].<ref name="lpi" /> Calcium ions outside cells are important for maintaining the [[potential difference]] across excitable [[cell membrane]]s, [[protein]] synthesis, and bone formation.<ref name="lpi" /><ref name="ods">{{cite web |title=Calcium: Fact Sheet for Health Professionals |url=https://ods.od.nih.gov/factsheets/Calcium-HealthProfessional/ |publisher=Office of Dietary Supplements, US National Institutes of Health |access-date=31 August 2019 |date=9 July 2019}}</ref>