Generally, metal oxides can be separated into two categories,

(1) Transition metal oxides such as NiO, Cr2O3, Fe2O3, etc.

(2) Non-transition metal oxides which further divided into two parts

i. Pre-transition metal oxides as Al2O3, etc.

ii. Post-transition metal oxides as SnO2, ZnO, etc.

but here again confusion that whats the main difference between pre and post transition metal oxides, i think SnO is post transition metal oxide, but post transition metal oxides cant easily oxidize, but in case of SnO it easily oxidize into SnO2.....

so if any body have this knowledge plz explain....thanks