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
Calcium sulfide lime, Ca(OH), reacts with elemental sulfur to give a "lime-sulfur", which has been used as an insecticide. The active ingredient is probably a calcium polysulfide, not CaS. Calcium sulfide decomposes upon contact with water, including moist air, giving a mixture of Ca(SH), Ca(OH), and Ca(SH)(OH). It reacts with acids such as hydrochloric acid to release toxic hydrogen sulfide gas. Oldhamite is the name for mineralogical form of CaS. It is a rare component of some meteorites and has scientific importance in solar nebula research. Burning of coal dumps can also produce the compound. Calcium sulfide Calcium sulfide is the chemical