Chromate ion $\rightarrow \mathrm{CrO}_{4}^{2-}$, oxidation state of $\mathrm{Cr}=+6$

Dichromate ion $\rightarrow \mathrm{Cr_2O}_{7}^{2-}$, oxidation state of $\mathrm{Cr}=+6$

$\therefore$ Difference in oxidation state $=$ zero