Aluminum and Its Congeners

Some of the physical properties of aluminum and its congeners are given in Table 18-3. Aluminum is only about one-third as dense as iron, and some of its alloys, such as duralumin (described below), are as strong as mild steel it is this combination of lightness and strength, together with low cost, that has led to the extensive use of aluminum alloys. Aluminum is also used, in place of copper, as a conductor of electricity its electric conductivity is about 80% that of copper. Its metallurgy has been discussed in Chapter 11. 

The metal is reactive (note its position in the electromotive-force series), and when strongly heated it burns rapidly in air or oxygen. Aluminum dust forms an explosive mixture with air. Under ordinary conditions, however, aluminum rapidly becomes coated with a thin, tough layer of aluminum oxide, which protects it against further corrosion. 

Some of the alloys of aluminum are very useful. Duralumin or dural is an alloy (containing about 94.3% aluminum, 4% copper, 0.5% manganese, 0.5% magnesium, and 0.7% silicon), which is stronger and tougher than pure aluminum. It is less resistant to corrosion, however, and often is protected by a coating of pure aluminum. Plate made by rolling a billet of dural sandwiched between and welded to two pieces of pure aluminum is called alclad plate. 

Aluminum sulfate, Alg(804)3-I8H2O, may be made by dissolving aluminum hydroxide in sulfuric acid  

Aluminum chloride, AICI3, is made by passing dry chlorine or hydrogen chloride over heated aluminum  

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Aluminum and its congeners, Ga, In and Tl, are considerably larger than boron and hence they are much more metallic and ionic in their character. Elemental aluminum itself is clearly metallic, but it is nevertheless still on the borderline (like beryllium) between ionic and covalent character in its compounds. So also are Ga, In and Tl. 

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