<Tr> <Td_colspan="2"> <Ul> <Li> view </Li> <Li> talk </Li> <Li> </Li> </Ul> references in Wikidata </Td> </Tr> <Ul> <Li> view </Li> <Li> talk </Li> <Li> </Li> </Ul> <P> Calcium is a chemical element with symbol Ca and atomic number 20 . An alkaline earth metal, calcium is a reactive pale yellow 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 . </P> <P> The name derives from Latin calx "lime", which was obtained from heating limestone . Its compounds were known to the ancients, though their chemistry was unknown until the seventeenth century . It was isolated by Humphry Davy in 1808 via electrolysis of its oxide, who named the element . While the pure metal does not have many applications due to its high reactivity, it is often used as an alloying component in small quantities in steelmaking, and calcium--lead alloys are sometimes used in automotive batteries . Calcium compounds on the other hand are very widely used in many industries: for example, they are used in foods and pharmaceuticals for calcium supplementation, in the paper industry as bleaches, in cement, in the manufacture of soaps, and as electrical insulators . </P>

How abundant is calcium in the earth's crust