Aluminum, symbol Al, the most abundant metallic element in the earth's crust. The atomic number of aluminum is 13; the element is in group 13 (IIIa) of the periodic table.

Properties
Aluminum is a lightweight, silvery metal. The atomic weight of aluminum is 26.9815; the element melts at 660° C (1220° F), boils at 2467° C (4473° F), and has a specific gravity of 2.7. Aluminum is a strongly electropositive metal and extremely reactive. In contact with air, aluminum rapidly becomes covered with a tough, transparent layer of aluminum oxide that resists further corrosive action. For this reason, materials made of aluminum do not tarnish or rust. The metal reduces many other metallic compounds to their base metals. For example, when thermite (a mixture of powdered iron oxide and aluminum) is heated, the aluminum rapidly removes the oxygen from the iron; the heat of the reaction is sufficient to melt the iron. This phenomenon is used in the thermite process for welding iron.

The oxide of aluminum is amphoteric-showing both acidic and basic properties. The most important compounds include the oxide, hydroxide, sulfate, and mixed sulfate compounds. Anhydrous aluminum chloride is important in the oil and synthetic-chemical industries. Many gemstones-ruby and sapphire, for example-consist mainly of crystalline aluminum oxide.

Occurrence
Aluminum is the most abundant metallic constituent in the crust of the earth; only the nonmetals oxygen and silicon are more abundant. Aluminum is never found as a free metal; commonly as aluminum silicate or as a silicate of aluminum mixed with other metals such as sodium, potassium, iron, calcium, and magnesium. These silicates are not useful ores, for it is chemically difficult, and therefore an expensive process, to extract aluminum from them. Bauxite, an impure hydrated aluminum oxide, is the commercial source of aluminum and its compounds.
In 1886 Charles Martin Hall in the United States and Paul L. T. Héroult in France independently and almost simultaneously discovered that alumina, or aluminum oxide, would dissolve in fused cryolite (Na3AlF6) and could then be decomposed electrolytically to a crude molten metal. A low-cost technique, the Hall-Héroult process, is still the major method used for the commercial production of aluminum, although new methods are under study. The purity of the product has been increased until a commercially pure ingot is 99.5 percent pure aluminum; it can be further refined to 99.99 percent.

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Uses
A given volume of aluminum weighs less than one-third as much as the same volume of steel. The only lighter metals are lithium, beryllium, and magnesium. Its high strength-to-weight ratio makes aluminum useful in the construction of aircraft, railroad cars, and automobiles, and for other applications in which mobility and energy conservation are important. Because of its high heat conductivity, aluminum is used in cooking utensils and the pistons of internal-combustion engines. Aluminum has only 63 percent of the electrical conductance of copper for wire of a given size, but it weighs less than half as much. An aluminum wire of comparable conductance to a copper wire is thicker but still lighter than the copper. Weight is particularly important in long-distance, high-voltage power transmission, and aluminum conductors are now used to transmit electricity at 700,000 V or more.

The metal is becoming increasingly important architecturally, for both structural and ornamental purposes. Aluminum siding, storm windows, and foil make excellent insulators. The metal is also used as a material in low-temperature nuclear reactors because it absorbs relatively few neutrons. Aluminum becomes stronger and retains its toughness as it gets colder and is therefore used at cryogenic temperatures. Aluminum foil 0.018 cm (0.007 in) thick, now a common household convenience, protects food and other perishable items from spoilage. Because of its light weight, ease of forming, and compatibility with foods and beverages, aluminum is widely used for containers, flexible packages, and easy-to-open bottles and cans. The recycling of such containers is an increasingly important energy-conservation measure. Aluminum's resistance to corrosion in salt water also makes it useful in boat hulls and various aquatic devices.

A wide variety of coating alloys and wrought alloys can be prepared that give the metal greater strength, castability, or resistance to corrosion or high temperatures. Some new alloys can be used as armor plate for tanks, personnel carriers, and other military vehicles.

For further information on carbon steels search the Welding Abstract Database type in keyword "aluminum"

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