Magnesium (Mg) is a silvery white metal that is similar in appearance to aluminum but weighs one- third less. With a density of only 1.738 grams per cubic centimetre, it is the lightest structural metal known. It has a hexagonal close-packed (hcp) crystalline structure, so that, like most metals of this structure, it lacks ductility when worked at lower temperatures. In addition, in its pure form, it lacks sufficient strength for most structural applications. However, the addition of alloying elements improves these properties to such an extent that both cast and wrought magnesium alloys are widely used, particularly where light weight and high strength are important.

Magnesium is strongly reactive with oxygen at high temperatures; above 645 C (1,190 F) in dry air, it burns with a bright white light and intense heat. For this reason, magnesium powders are used in pyrotechnics. At room temperature, a stable film of water-insoluble magnesium hydroxide forms on the metal's surface, protecting it from corrosion in most atmospheres. Being a strong reactant that forms stable compounds with chlorine, oxygen, and sulfur, magnesium has several metallurgical applications, such as in the production of titanium from titanium tetrachloride and in the desulfurization of blast-furnace iron. Its chemical reactivity is also evident in the magnesium compounds that have wide application in industry, medicine, and agriculture.