Magnesium - Mg. Chemical properties of magnesium - Health effects of magnesium - Environmental effects of magnesium. Magnesium. Chemical element, metallic, symbol Mg, situated in group IIa in the periodic table, atomic number: 12, atomic weight: 24,312. Magnesium is silvery white and very light. Its relative density is 1,74 and it’s density 1740 kg/m3 (0.063 lb/in3 or 108.6 lb/ft3).