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| ogdenk Joined: 05 Feb 2007 Posts: 41 Location: Syracuse, NY |
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; at.no. 13; m.p. 660.32°C; b.p. 2519°C; sp. gr. 2.6989 (20°C); valence3. The ancient Greeks and Romans used alum in medicineas an astringent, and as a mordant in dyeing. In 1761 deMorveau proposed the name alumine for the base in alum,and Lavoisier, in 1787, thought this to be the oxide of a stillundiscovered metal. Wohler is generally credited with havingisolated the metal in 1827, although an impure form was preparedby Oersted two years earlier. In 1807, Davy proposedthe name alumium for the metal, undiscovered at that time,and later agreed to change it to aluminum. Shortly thereafter,the name aluminium was adopted to conform with the “ium”ending of most elements, and this spelling is now in use elsewherein the world. Aluminium was also the accepted spellingin the U.S. until 1925, at which time the American ChemicalSociety officially decided to use the name aluminum thereafterin their publications. The method of obtaining aluminummetal by the electrolysis of alumina dissolved in cryolite wasdiscovered in 1886 by Hall in the U.S. and at about the sametime by Heroult in France. Cryolite, a natural ore found inGreenland, is no longer widely used in commercial production,but has been replaced by an artificial mixture of sodium,aluminum, and calcium fluorides. Bauxite, an impure hydratedoxide ore, is found in large deposits in Jamaica, Australia,Suriname, Guyana, Russia, Arkansas, and elsewhere. TheBayer process is most commonly used today to refine bauxiteso it can be accommodated in the Hall–Heroult refiningprocess used to make most aluminum. Aluminum can nowbe produced from clay, but the process is not economicallyfeasible at present. Aluminum is the most abundant metal tobe found in the Earth’s crust (8.1%), but is never found freein nature. In addition to the minerals mentioned above, it isfound in feldspars, granite, and in many other common minerals.Twenty-two isotopes and isomers are known. Naturalaluminum is made of one isotope, 27Al. Pure aluminum, a silvery-white metal, possesses many desirable characteristics.It is light, nontoxic, has a pleasing appearance, can easily beformed, machined, or cast, has a high thermal conductivity,and has excellent corrosion resistance. It is nonmagnetic andnonsparking, stands second among metals in the scale of malleability,and sixth in ductility. It is extensively used for kitchenutensils, outside building decoration, and in thousands of industrialapplications where a strong, light, easily constructedmaterial is needed. Although its electrical conductivity is onlyabout 60% that of copper, it is used in electrical transmissionlines because of its light weight. Pure aluminum is soft andlacks strength, but it can be alloyed with small amounts ofcopper, magnesium, silicon, manganese, and other elementsto impart a variety of useful properties. These alloys are ofvital importance in the construction of modern aircraft androckets. Aluminum, evaporated in a vacuum, forms a highlyreflective coating for both visible light and radiant heat. Thesecoatings soon form a thin layer of the protective oxide and donot deteriorate as do silver coatings. They have found applicationin coatings for telescope mirrors, in making decorativepaper, packages, toys, and in many other uses. The compoundsof greatest importance are aluminum oxide, the sulfate, andthe soluble sulfate with potassium (alum). The oxide, alumina,occurs naturally as ruby, sapphire, corundum, and emery, andis used in glassmaking and refractories. Synthetic ruby andsapphire have found application in the construction of lasers
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