A metallic chemical element, symbol Al, atomic number 13, atomic weight 26.98154, in group 13 of the periodic system. Pure aluminum is soft and lacks strength, but it can be alloyed with other elements to increase strength and impart a number of useful properties. Alloys of aluminum are light, strong, and readily formable by many metalworking processes; they can be easily joined, cast, or machined, and accept a wide variety of finishes. Because of its many desirable physical, chemical, and metallurgical properties, aluminum has become the most widely used nonferrous metal.

Aluminum is the most abundant metallic element on the Earth and Moon but is never found free in nature. The element is widely distributed in plants, and nearly all rocks, particularly igneous rocks, contain aluminum in the form of aluminum silicate minerals. When these minerals go into solution, depending upon the chemical condi­tions, aluminum can be precipitated out of the solution as clay minerals or aluminum hydroxides, or both. Under such conditions bauxites are formed. Bauxites serve as principal raw materials for aluminum production. Aluminum is a silvery metal having a density of 1.56 oz/in.3 at 68°F (2. 70 g/cm3 at 20°C). Naturally occurring aluminum consists of a single isotope, f1Al. Aluminum crystallizes in the face-centered cubic structure with edge of the unit lattice cube of 4.0495 angstroms (0.40495 nanometer). Aluminum is known for its high electrical and thermal conductivities and its high reflectivity.
The electronic configuration of the element is Is22s22p63s23p1• Aluminum exhibits a valence of +3 in all compounds, with the exception of a few high-temperature mono­valent and divalent gaseous species.
Aluminum is stable in air and resistant to corrosion by seawater and many aqueous solutions and other chemical agents. This is due to protection of the metal by a tough, impervious film of oxide. At a purity greater than 99.95%, aluminum resists attack by most acids but dissolves in aqua regia. Its oxide film dissolves in alkaline solutions, and corrosion is rapid. Aluminum is amphoteric and can react with mineral acids to form soluble salts and to evolve hydrogen. Molten aluminum can react explosively with water. The molten metal should not be allowed to contact damp tools or containers. At high temperatures aluminum reduces many compounds containing oxygen, par­ticularly metal oxides. These reactions are used in the manufacture of certain metals and alloys. Applications in building and construction represent the largest single market of the aluminum industry. Millions of homes use aluminum doors, siding, windows, screening, and down-spouts and gutters. Aluminum is also a major industrial building product. Transportation is the second largest market. Many commercial and military aircraft have become virtually all-aluminum. In automobiles, aluminum is apparent in interior and exterior trim, grilles, wheels, air conditioners, automatic transmissions, and some radiators, engine blocks, and body panels. Aluminum is also found in rapid-transit car bodies, rail cars, forged truck wheels, cargo containers, and in highway signs, divider rails, and lighting standards. In aerospace, aluminum is found in aircraft engines, frames, skins, landing gear, and interiors, often making up 80% of a plane's weight. The food packaging industry is a fast-growing market.
In electrical applications, aluminum wire and cable are major products. Aluminum appears in the home as cooking utensils, cooking foil, hardware, tools, portable appli­ances, air conditioners, freezers, and refrigerators, and in sporting equipment such as skis, ball bats, and tennis rackets. There are hundreds of chemical uses of aluminum and aluminum compounds. Aluminum powder is used in paints, rocket fuels, and explosives, and as a chemical reductant.