Aluminum is a lightweight, silvery-white, ductile, malleable, metallic element in group IIIA of the periodic table. It is the most abundant element in the Earth's crust (8%), but found only in combination, chiefly in bauxite but also as cryolite, feldspar, clay, and many other minerals. It is smelted by the Hall-Héroult process (see below). Aluminum (British: "aluminium") is widely used for aircraft parts, engines, window frames, pans, drinks cans, kegs, cooking foil, etc., and to form many hard, light, corrosion-resistant alloys. Its name comes from alumen, the Latin name for the mineral alum.
Aluminum is extremely useful for several reasons. First, it is light – weighing only about a third as much as other common metals for the same volume. It is also strong, and to give the same rigidity need weigh only half as much as steel. Its strength can be greatly increased by alloying with other metals.
Second, although aluminum combines readily with other elements, including oxygen, it does not rust or corrode in the air, because a thin film of oxide is formed on the surface, and this protects the metal below. So aluminum has excellent corrosion resistance.
Third, aluminum conducts heat and electricity extremely well. An aluminum wire will conduct only 60 percent as much electricity as a copper wire of the same size, but more than twice as much as a copper wire of the same weight. Its good heat conducting qualities and its resistance to corrosion have led to its widespread use in cooking vessels.
|relative atomic mass||26.98154|
|atomic radius||143.1 pm|
|melting point||660.3°C (1,220.6°F)|
|boiling point||2,519°C (4,566°F)|
Discovery and extraction
Compounds of aluminum have been known for centuries, but the metal was first made from these by Humphry Davy. The metallic aluminum he produced was too impure for him to have any real idea of its properties, and a purer metal was first made by the Danish chemist Hans Oersted in Copenhagen in 1825 and the German chemist Friedrich Wöhler in 1827. It was not until 1845 that Wöhler succeeded in producing a sufficient quantity to discover that it was a very light metal which was malleable.
In later years ways were discovered of making larger quantities of aluminum through electrolysis – that is, by passing an electric current through a fused or liquefied aluminum compound. But the metal remained very expensive, and it was only in 1886 that the French chemist Hèroult and the American C. M. Hall discovered at about the same time the method of making aluminum on a larger scale, now known as the Hall process, which is still used. This is by electrolysis of alumina (aluminum oxide) dissolved in a bath of fused cryolite (sodium aluminum fluoride).
One of the most important alloys of aluminum is Duralim, an alloy containing about 4 percent copper with smaller quantities of magnesium (1%), manganese (0.7%), and silicon (0.5%). It was invented in 1908 by a German engineer, Alfred Wilm. This alloy has been very important in the development of aviation, beginning with its use for Zeppelins in World War I, owing to its combination of strength and lightness. It is not as resistant to corrosion as pure aluminum, but when high corrosion resistance is needed it can be coated on both sides with a layer of pure aluminum.
Compounds of aluminum
Alumina oxide (Al2O3), or alumina, is a colorless or white solid occurring in several crystalline forms. The impure hydrated form of alumina, bauxite, is the chief ore of aluminum. Other forms of alumina include corundum, two impure varieties of which are the gemstones ruby and sapphire. Solubility in acid and alkali increases with hydration. Melting point 2,045°C, boiling point 2,980°C. Alumina is used as an abrasive, an electrical insulator, and a furnace lining.