One of the notable features of a transition elements is the great variety of oxidation states these may show in their compounds. The elements which give the greatest number of oxidation states occur in or near the middle of the series. The lesser number of oxidation states at the extreme ends stems from either too few electrons to lose or share or too many d electrons (hence fewer orbitals available in which to share electrons with others) for higher valence. The variability of oxidation states, a characteristic of transition elements, arises out of incomplete filling of d orbitals in such a way that their 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 states of non-transition elements where oxidation states normally differ by a unit of two.

What is the change in oxidation number of Cr in the following reaction?

2CrO₄^2- + 2H⁺ → Cr₂O₇^2- + H₂O

No change

2CrO₄^2- + 2H⁺ → Cr₂O₇^2- + H₂O

In CrO₄^2- 

x + 4(-2) = -2

x - 8 = -2

x = -2 + 8 = +6

In Cr₂O₇^2- 

2(x) + 7(-2) = -2

2x - 14 = -2

2x = -2 + 14

2x = 12 

x = 6