A metallic chemical element, Mg, in group 2 of the periodic system, atomic number 12, atomic weight 24.312. Magnesium is silvery white and extremely light in weight. The specific gravity is 1.74, and the density is 1740 kg/m³ (0.063 lb/in.³ or 108.6 lb/ft³). Because of this lightness combined with alloy strength suitable for many structural uses, magnesium has long been known as industry's lightest structural metal.