Acidic character of oxides in group 15 to 18 . elements decreases down the group. As we move down the group, the atomic size increases and electronegativity decreases. The oxide in the . higher oxidation state of the elements is more acidic than that of lower oxidation state. In N2O3(I),P2O3 (II) and As2O3 (IV), N, P and As are in +3 oxidation state respectively whereas in N2O5 (III) the N is in +5 oxidation state. So, the acidity order is N2O5>N2O3>P2O3>As2O3‌, i.e. III ‌>‌ I ‌>‌ II ‌>‌ IV ‌