Chemical element, one of the alkaline earth metals, chemical symbol Mg, atomic number 12. The silvery white metal does not occur free in nature, but compounds such as the sulfate (Epsom salts), oxide (magnesia), and carbonate (magnesite) have long been known. The metal, which burns in air with a bright white light, is used in photographic flash devices, bombs, flares, and pyrotechnics; it is also a component of lightweight alloys for aircraft, spacecraft, cars, machinery, and tools. The compounds, in which it has valence 2, are used as insulators and refractories and in fertilizers, cement, rubber, plastics, foods, and pharmaceuticals (antacids, purgatives, laxatives). Magnesium is an essential element in human nutrition; it is the cofactor in enzymes of carbohydrate metabolism and in chlorophyll.