Aluminium is a silvery white member of the boron group of chemical elements. It has the symbol Al and its atomic number is 13. It is not soluble in water under normal circumstances. Aluminium is the most abundant metal in the Earth's crust, and the third most abundant element, after oxygen and silicon. It makes up about 8% by weight of the Earth's solid surface. Aluminium is too reactive chemically to occur in nature as a free metal. Instead, it is found combined in over 270 different minerals. The chief source of aluminium is bauxite ore.
Aluminium is remarkable for the metal's low density and for its ability to resist corrosion due to the phenomenon of passivation. Structural components made from aluminium and its alloys are vital to the aerospace industry and are very important in other areas of transportation and building. Its reactive nature makes it useful as a catalyst or additive in chemical mixtures, including ammonium nitrate explosives, to enhance blast power.
Despite its prevalence in the environment, aluminium salts are not known to be used by any form of life. Also in keeping with the element's abundance, it is well tolerated by plants in soils (in which it is a major component), and to a lesser extent, by animals as a component of plant materials in the diet (which often contain traces of dust and soil). Soluble aluminium salts have some demonstrated toxicity to animals if delivered in quantity by unnatural routes, such as injection. Controversy still exists about aluminium's possible long-term toxicity to humans from larger ingested amounts.
