Oxides and Oxoanions of Metals
Generally, transition metal oxides are formed by the reaction of transition metals with molecular oxygen at high temperatures. Except the first member of 3d series, Scandium, all other transition elements form ionic metal oxides. The oxidation number of metal in metal oxides ranges from +2 to +7. As the oxidation number of a metal increases, ionic character decreases, for example, Mn2O7 is covalent. Mostly higher oxides are acidic in nature, Mn2O7 dissolves in water to give permanganic acid (HMnO4 ) , similarly CrO3 gives chromic acid (H2CrO4) and dichromic acid (H2Cr2O7). Generally lower oxides may be amphoteric or basic, for example, Chromium (III) oxide - Cr2O3, is amphoteric and Chromium(II) oxide, CrO, is basic in nature.
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