Aluminium

Aluminium or aluminum (Symbol Al) (see the spelling section below) is a silvery and ductile member of the poor metal group of chemical elements. Its atomic number is 13. Aluminium is found primarily as the ore bauxite and is remarkable for its resistance to oxidation (due to the phenomenon of passivation), its strength, and its light weight. Aluminium is used in many industries to make millions of different products and is very important to the world economy. Structural components made from aluminium are vital to the aerospace industry and very important in other areas of transportation and building in which light weight, durability, and strength are needed.

Chemistry

Oxidation state 1

• AlH is produced when aluminium is heated at 1500 °C in an atmosphere of hydrogen.
• Al₂O is made by heating the normal oxide, Al₂O₃, with silicon at 1800 °C in a vacuum.
• Al₂S can be made by heating Al₂S₃ with aluminium shavings at 1300 °C in a vacuum. It quickly disproportionates to the starting materials. The selenide is made in a parallel manner.
• AlF, AlCl and AlBr exist in the gaseous phase when the tri-halide is heated with aluminium.

Oxidation state 2

• Aluminium suboxide, AlO can be shown to be present when aluminium powder burns in oxygen.

Oxidation state 3

• Fajans rules show that the simple trivalent cation Al³⁺ is not expected to be found in anhydrous salts or binary compounds such as Al₂O₃. The hydroxide is a weak base and aluminium salts of weak bases, such as carbonate, can't be prepared. The salts of strong acids, such as nitrate, are stable and soluble in water, forming hydrates with at least six molecules of water of crystallization.
• Aluminium hydride, (AlH₃)_n, can be produced from trimethylaluminium and an excess of hydrogen. It burns explosively in air. It can also be prepared by the action of aluminium chloride on lithium hydride in ether solution, but cannot be isolated free from the solvent.
• Aluminium carbide, Al₄C₃ is made by heating a mixture of the elements above 1000 °C. The pale yellow crystals have a complex lattice structure, and react with water or dilute acids to give methane. The acetylide, Al₂(C₂)₃, is made by passing acetylene over heated aluminium.
• Aluminium nitride, AlN, can be made from the elements at 800 °C. It is hydrolysed by water to form ammonia and aluminium hydroxide.
• Aluminium phosphide, AlP, is made similarly, and hydrolyses to give phosphine.
• Aluminium oxide, Al₂O₃, occurs naturally as corundum, and can be made by burning aluminium in oxygen or by heating the hydroxide, nitrate or sulfate. As a gemstone, its hardness is only exceeded by diamond, boron nitride and carborundum. It is almost insoluble in water.
• Aluminium hydroxide may be prepared as a gelatinous precipitate by adding ammonia to an aqueous solution of an aluminium salt. It is amphoteric, being both a very weak acid, and forming aluminates with alkalis. It exists in various crystalline forms.
• Aluminium sulfide, Al₂S₃, may be prepared by passing hydrogen sulfide over aluminium powder. It is polymorphic.
• Aluminium fluoride, AlF₃, is made by treating the hydroxide with HF, or can be made from the elements. It consists of a giant molecule which sublimes without melting at 1291 °C. It is very inert. The other trihalides are dimeric, having a bridge-like structure.
• Organo-metallic compounds of empirical formula AlR₃ exist and, if not also giant molecules, are at least dimers or trimers. They have some uses in organic synthesis, for instance trimethylaluminium.
• Alumino-hydrides of the most electropositive elements are known, the most useful being lithium aluminium hydride, Li. It decomposes into lithium hydride, aluminium and hydrogen when heated, and is hydrolysed by water. It has many uses in organic chemistry. The aluminohalides have a similar structure.