Calcium Calcium Calcium is a chemical element with 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
Gender symbol two standard sex symbols are the Mars symbol ♂ (often considered to represent a shield and spear) for male and Venus symbol ♀ (often considered to represent a bronze mirror with a handle) for female, derived from astrological symbols, denoting the classical planets Mars and Venus, respectively. They were first used to denote the effective sex of plants (i.e. sex of individual in a given crossbreed, since most plants are hermaphroditic) by Carl Linnaeus in 1751. They are still used in scientific publications to indicate the sex of an individual, for example of a patient. Pedigree charts published in scientific