Magnesium. View of splinters of metallic magnesium, a chemical element. Magnesium is a silvery-white metallic element belonging to group II of the periodic table (the alkaline-earth metals). It is found in seawater and in a number of minerals. Around 80% of commercially- produced magnesium metal is extracted from seawater. Its alloys are extremely lightweight and are widely used where weight reduction is important. Magnesium is highly reactive and burns with an intense white flame when ignited. It has the chemical symbol Mg, an atomic number of 12, and an atomic weight of 23.005.