Acidic oxides tend to increase the H⁺ concentration when dissolved in water. For example.
                                           N₂O₅ + H₂O  <==>   2H⁺  +  2NO₃^-
Basic oxides tend to decrease the H⁺ concentration. For example,
                                           CaO + H₂O   <==>  Ca²⁺ +  2OH^-
Amphitricha oxides are those which behave as acids as well as bases. For example,
                                         AI₂O₃  +  6H⁺  <==>    2AI³⁺  +  3H₂O         (basic)
                                       AI₂O₃  +  2OH^-  <==>  2AIO₂^- +  H₂O             (acidic)
Basic character of metal oxides increases progressively with increase in atomic weight along a group in the periodic table. For example, in Group 13 oxides, B₂O₃ is acidic, AI₂O₃ and Ga₂O₃ are amphitricha but others In2O₃ and TI₂O are predominantly basic. Although, the charge on the metal atom remains the same in the group but the size increases as atomic weight inverts with the result charge-to-size ratio decrees and basicity increases. However, because of very small size of B3+ (with high polarizing power), its charge-to-size ratio is exceptionally large relative to others with the result that it has exceptionally higher acidic character. Similarly, Group 2 oxides show progressive increase in basic character; Be⁺ is amphitricha, MgO is weakly base can other CaO, SrO and BaO are basic and basicity increase. Because of exceptionally small size of Be+ and higher charge-to-size ratio, basicity of BeO is significantly lower than the next member of the group.

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