CrO is basic but Cr2O3is amphoteric ?
In CrO, the oxidation state of chromium is +2 while in Cr2O3 the oxidation state of chromium is +3. The oxides formed by elements in lower oxidation state are basic while those in higher oxidation state are acidic or amphoteric.
In the case of a lower oxide of a transition metal, the metal atom has a low oxidation state. This means that some of the valence electrons of the metal atom are not involved in bonding. The atom in lower oxidation state therefore is not electron deficient. As a result, it can donate electrons and behave as a base.
On the other hand, in the case of a higher oxide of a transition metal, the metal atom has a high oxidation state. This means that the valence electrons are involved in bonding and so, they are unavailable on the central atom. In other words, it has less electrons available in its valence shell and is electron deficient. There is also a high effective nuclear charge. As a result, it can accept electrons and behave as an acid.
Since in Cr2O3 chromium is in higher oxidation state than in CrO, therefore it is amphoteric while Cro is basic.