The variability of oxidation states, a characteristic of transitionelements, arises out of incomplete filling of d-orbitals in such a way thattheir oxidation states differ from each other by unity, $e.g., V^{II}, V^{III}, V^{IV}, V^{V}$.This is in contrast with the variability of oxidation sates of non transitionelements where oxidation states normally differ by a unit of two.In the p-block the lower oxidation states are favoured by the heaviermembers (due to inert pair effect), the opposite is true in the groupsof d-block. For example, in group 6, Mo(VI) and W(VI) are found to bemore stable than Cr(VI). Thus Cr(VI) in the form of dichromate in acidicmedium is a strong oxidising agent, whereas $MoO_3$ and $WO_3$ are not.