Oxides and Oxoanions of Metals

The systematic survey of oxidation state versus acid-base character introduced in section 4.4 is developed here in more explicit detail, using chromium's own oxide series as the clearest worked illustration of the general rule that transition-metal oxides shift from basic/amphoteric toward acidic as the metal's oxidation number rises. At the low end, chromium(II) oxide (CrO, chromium in the +2 state) is a genuinely basic oxide, reacting straightforwardly with acids to give Cr²⁺ salts. Moving up one oxidation state, chromium(III) oxide (Cr₂O₃, chromium in the +3 state) is amphoteric, capable of reacting with both acids (to give Cr³⁺ salts) and with strong bases (to give chromite-type anionic species) -- a dual reactivity characteristic of the intermediate oxidation states commonly seen partway through a transition metal's accessible range. At the high end of chromium's oxidation-state range, chromium trioxide (CrO₃, chromium in the +6 state) is a strongly acidic oxide: it dissolves in water to give not one but two related oxoacids, chromic acid (H₂CrO₄) and dichromic acid (H₂Cr₂O₇), depending on concentration and pH, foreshadowing the chromate/dichromate equilibrium chemistry developed fully in section 4.4.2.3. …