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Calcium is a chemical element; it has 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 fossils of early sea life; gypsum, anhydrite, fluorite, and apatite are also sources of calcium. The name comes from the Latin word calx, which was obtained from heating limestone.

Calcium is a chemical element with the symbol Ca and atomic number 20, classified as a soft, silvery-white alkaline earth metal in group 2 of the periodic table.[1] It has an atomic mass of 40.078 u, an electron configuration of [Ar] 4s², and occurs naturally in compounds such as limestone (calcium carbonate) and gypsum (calcium sulfate dihydrate), making it the fifth-most-abundant element by mass in Earth's crust at over 3%.[2] Essential for life, calcium plays critical roles in biomineralization for bones, teeth, and shells, as well as in cellular signaling, muscle contraction, and nerve transmission.[1][3]