Aluminium

Aluminum is a silvery-white lustrous metallic element belonging to the group 3 (formerly IIIB) of the periodic table; a.n.is 13; r.a.m. is 26.98; r.d. is 2.7; m.p. is 660°C; b.p. is 2467°C. The metal itself is highly reactive but is protected by a thin transparent layer of the oxide, which is formed quickly in air. Aluminium and its oxide are amphoteric. The metal is extracted from purified bauxite (A1₂O₃) by electrolysis. The main process uses a Hall-Heroult cell but other electrolytic methods are under development, including conversion of bauxite with chlorine and electrolysis of the molten chloride. Pure aluminium is soft and ductile but its strength can be increased by work-hardening. A large number of alloys are manufactured. Alloying elements include copper, manganese, silicon, zinc and magnesium. Its lightness, strength (when alloyed), corrosion resistance and electrical conductivity make it suitable for a variety of uses, including vehicle and aircraft construction, building (window and door frames) and overhead power cables. Although it is the third most abundant element in the Earth's crust (8.1% by weight) it was not isolated until 1825 by H. С. Oersted.

Aluminium chloride is a whitish solid, A1C1₃, which fumes in moist air and reacts violently with water (to give hydrogen chloride). It is known as the anhydrous salt (hexagonal; r. d. is 2.44 (fused solid); m. p. is 190°C (2.5 atm.); it sublimes at 178°C) or the hexahydrate A1C1₃.6H₂O (rhombic; r. d. is 2.398; it loses water at 100°C), both of which are deliquescent. Aluminium chloride may be prepared by passing hydrogen chloride or chlorine over hot aluminium or by passing chlorine over heated aluminium oxide and carbon. The chloride ion is polarized by the small positive aluminium ion and the bonding in the solid is intermediate between covalent and ionic. In the liquid and vapour phases dimer molecules exist, A1₂C1₆, in which there are chlorine bridges that make coordinate bonds to aluminium atom. The A1C1₃ molecule can also form compounds with other molecules that donate pairs of electrons (e.g. amines or hydrogen sulphide); i.e. it acts as a Lewis acid. Aluminium chloride is used commercially as a catalyst in the cracking of oils. It is also used as a catalyst in certain organic reactions, especially in the Friedel-Crafts reaction

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