aluminum (Al)Article Free Pass
aluminum (Al), also spelled aluminium, chemical element, a lightweight, silvery-white metal of main Group 13 (IIIa, or boron group) of the periodic table. Aluminum is the most abundant metallic element in the Earth’s crust and the most widely used nonferrous metal. Because of its chemical activity, aluminum never occurs in the metallic form in nature, but its compounds are present to a greater or lesser extent in almost all rocks, vegetation, and animals. Aluminum is concentrated in the outer 10 miles (16 km) of the Earth’s crust, of which it constitutes about 8 percent by weight; it is exceeded in amount only by oxygen and silicon. The name aluminum is derived from the Latin word alumen, used to describe potash alum, or aluminum potassium sulfate, KAl(SO4)2∙12H2O.
Occurrence, uses, and properties
Aluminum occurs in igneous rocks chiefly as aluminosilicates in feldspars, feldspathoids, and micas; in the soil derived from them as clay; and upon further weathering as bauxite and iron-rich laterite. Bauxite, a mixture of hydrated aluminum oxides, is the principal aluminum ore. Crystalline aluminum oxide (emery, corundum), which occurs in a few igneous rocks, is mined as a natural abrasive or in its finer varieties as rubies and sapphires. Aluminum is present in other gemstones, such as topaz, garnet, and chrysoberyl. Of the many other aluminum minerals, alunite and cryolite have some commercial importance.
Crude aluminum was isolated (1825) by Hans Christian Ørsted by reducing aluminum chloride with potassium amalgam. Sir Humphry Davy had prepared (1809) an iron-aluminum alloy by electrolyzing fused alumina (aluminum oxide) and had already named the element aluminum; the word later was modified to aluminium in England and some other European countries. A German chemist, Friedrich Wöhler, using potassium metal as the reducing agent, produced aluminum powder (1827) and small globules of the metal (1845) from which he was able to determine some of its properties.
The new metal was introduced to the public (1855) at the Paris Exposition at about the time that it became available (in small amounts at great expense) by the sodium reduction of molten aluminum chloride. When electric power became relatively plentiful and cheap, almost simultaneously Charles Martin Hall in the United States and Paul-Louis-Toussaint Héroult in France discovered (1886) the modern method of commercially producing aluminum: electrolysis of purified alumina (Al2O3) dissolved in molten cryolite (Na3AlF6). During the 1960s aluminum moved into first place, ahead of copper, in world production of nonferrous metals. For more specific information about the mining, refining, and production of aluminum, see aluminum processing.
Aluminum is added in small amounts to certain metals to improve their properties for specific uses, as in aluminum bronzes and most magnesium-base alloys; or, for aluminum-base alloys, moderate amounts of other metals and silicon are added to aluminum. The metal and its alloys are used extensively for aircraft construction, building materials, consumer durables (refrigerators, air conditioners, cooking utensils), electrical conductors, and chemical and food-processing equipment.
Pure aluminum (99.996 percent) is quite soft and weak; commercial aluminum (99.0 to 99.6 percent pure) with small amounts of silicon and iron is hard and strong. Ductile and highly malleable, aluminum can be drawn into wire or rolled into thin foil. The metal is only about one-third as dense as iron or copper. Though chemically active, aluminum is nevertheless highly corrosion-resistant because in air a hard, tough oxide film forms on its surface.
Aluminum is an excellent conductor of heat and electricity. Its thermal conductivity is about one-half that of copper; its electrical conductivity, about two-thirds. It crystallizes in the face-centred cubic structure. All natural aluminum is the stable isotope aluminum-27. Metallic aluminum and its oxide and hydroxide are nontoxic.
Aluminum is slowly attacked by most dilute acids and rapidly dissolves in concentrated hydrochloric acid. Concentrated nitric acid, however, can be shipped in aluminum tank cars because it renders the metal passive. Even very pure aluminum is vigorously attacked by alkalies such as sodium and potassium hydroxide to yield hydrogen and the aluminate ion. Because of its great affinity for oxygen, finely divided aluminum, if ignited, will burn in carbon monoxide or carbon dioxide with the formation of aluminum oxide and carbide; but, at temperatures up to red heat, aluminum is inert to sulfur.
Aluminum can be detected in concentrations as low as one part per million using emission spectroscopy. Aluminum can be quantitatively analyzed as the oxide (formula Al2O3) or as a derivative of the organic nitrogen compound 8-hydroxyquinoline. The derivative has the molecular formula Al(C9H6ON)3.
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