Aluminum oxide, amphoteric character

Metals typically form basic oxides and nonmetals typically form acidic oxides, but what about the elements that lie on the diagonal frontier between the metals and nonmetals Along this frontier from beryllium to polonium, metallic character blends into nonmetallic character, and the oxides of these elements have both acidic and basic character (Fig. 10.7). Substances that react with both acids and bases are classified as amphoteric, from the Greek word for both. For example, aluminum oxide, A1203, is amphoteric. It reacts with acids ... 

The product in this case is sodium aluminate, a compound that contains the aluminate ion, [Al(OH)4]. Because aluminum oxide reacts with both acids and bases, it is classified as amphoteric. Other main-group elements that form amphoteric oxides are shown in Fig. J.3. As you can see, these elements lie in a diagonal band across the table from beryllium to polonium. The acidic, amphoteric, or basic character of the oxides of the d-block metals depends on their oxidation state (see Chapter 16). 

Activated alumina is a porous high-area form of aluminum oxide, prepared either directly from bauxite (AI2O3 3H2O) or from the monohydrate by dehydration and recrystallization at elevated temperature. The surface is more strongly polar than that of silica gel and has both acidic and basic character, reflecting the amphoteric nature of the metal. 

The oxides, or the earths, are, as we have seen, difficult to melt. Beryllium oxide is a solid of significant covalent character much like aluminum oxide and rather unlike those of its congeners. Both are amphoteric rather than basic anhydrides (see pp. 296-297 and Figure 11.11). Magnesium reacts vigorously with oxygen to form the oxide as shown in Equation (13.9) ... 

Boron oxide is an add anhydride. Aluminum shows some nonmetallic character in that its oxide is amphoteric. 

The characteristics of this group are that the elements possess a valence of 3, and that the oxides, M2O3, have but a weakly developed basic character. Boron, in fact, shows practically no base-forming properties, but forms rather a weak acid. The oxide of aluminum displays both basic and acidic properties that is, it is amphoteric. The remaining elements are more distinctly base-forming than aluminum, without, however, approaching in any way the alkaline earth metals in this respect. 

Looking down Group 3A(13), we see a wide range of chemical behavior. Boron, the first metalloid we ve encountered, is much less reactive at room temperature than the other members and forms covalent bonds exclusively. Although aluminum acts physically like a metal, its halides exist in the gas phase as covalent dimers— molecules formed by joining two identical smaller molecules (Figure 14.4)—and its oxide is amphoteric rather than basic. Most of the other 3A compounds are ionic, but with more covalent character than similar 2A compounds because the 3A cations can polarize nearby electron clouds more effectively. 

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