magnesium (măgnē'zēum, –zhum) [key], metallic chemical element; symbol Mg; at. no. 12; at. wt. 24.305; m.p. about 648.8°C; b.p. about 1,090°C; sp. gr. 1.738 at 20°C; valence +2. In 1808, Sir Humphry Davy discovered magnesium in its oxide, although it is not certain that he isolated the metal. Pure magnesium was isolated substantially by A. A. B. Bussy in 1828 by chemical reduction of the chloride. Magnesium was first isolated electrolytically by Michael Faraday in 1833.