Amphoterism
المؤلف:
Peter Atkins, Tina Overton, Jonathan Rourke, Mark Weller, and Fraser Armstrong
المصدر:
Shriver and Atkins Inorganic Chemistry ,5th E
الجزء والصفحة:
ص126-127
2025-08-24
533
Amphoterism
Key points: The frontier between metals and nonmetals in the periodic table is characterized by the formation of amphoteric oxides; amphoterism also varies with the oxidation state of the element. An amphoteric oxide is an oxide that reacts with both acids and bases.4 Thus, aluminium oxide reacts with acids and alkalis:
Amphoterism is observed for the lighter elements of Groups 2 and 13, as in BeO, Al2O3, and Ga2O3. It is also observed for some of the d-block elements in high oxidation states, such as MoO3 and V2O5, in which the central atom is very electron withdrawing, and some of the heavier elements of Groups 14 and 15, such as SnO2 and Sb2O5. Figure 4.6 shows the location of elements that in their characteristic group oxidation states have amphoteric oxides. They lie on the frontier between acidic and basic oxides, and hence serve as an important guide to the metallic or nonmetallic character of an element. The onset of amphoterism correlates with a significant degree of covalent character in the bonds formed by the elements, either because the metal ion is strongly polarizing (as for Be) or because the metal ion is polarized by the O atom attached to it (as for Sb). An important issue in the d block is the oxidation number necessary for amphoterism. Figure 4.7 shows the oxidation number for which an element in the first row of the block has an amphoteric oxide. We see that on the left of the block, from titanium to manganese and perhaps iron, oxidation state 4 is amphoteric (with higher values on the border of acidic and lower values of the border of basic). On the right of the block, amphoterism occurs at lower oxidation numbers: the oxidation states 3 for cobalt and nickel and 2 for copper and zinc are fully amphoteric. There is no simple way of predicting the onset of amphoterism. However, it presumably reflects the ability of the metal cation to polarize the oxide ions that surround it—that is, to introduce covalence into the metal oxygen bond. The degree of covalence typically increases with the oxidation number of the metal as the increasingly positively charged cation becomes more strongly polarizing (Section 1.9e).
Figure 4.7 The oxidation numbers for which elements in the first row of the d block have amphoteric oxides. Predominantly acidic oxides are shown shaded pink, predominantly basic oxides are shaded blue.
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