In chemistry, Mg is the symbol for magnesium, a chemical element with atomic number 12 and atomic weight 24.305. Magnesium is a silvery-white, lightweight metal that belongs to group 2a of the periodic table, and is the third most abundant structural metal in the Earths crust. It is known for its low density, high strength, and resistance to corrosion. Magnesium is widely used in construction, medicine, and as a reducing agent to produce other metals from their compounds. In the human body, magnesium is essential to the working of hundreds of enzymes, and is stored mainly in the bones. Magnesium is also used in various compounds important to industry and biology, including magnesium carbonate, magnesium chloride, magnesium citrate, magnesium hydroxide (milk of magnesia), magnesium oxide, magnesium sulfate, and magnesium sulfate heptahydrate (Epsom salts).