The difference in oxidation state of chromium in chromate and...

${Cr}_{2}{O}_{7}^{2 -} + {OH}^{-} ⟶ 2{CrO}_{4}^{2 -}$

Let the oxidation number of chromium be $x$

The oxidation state of chromium in ${Cr}_{2}{O}_{7}^{2 -}$$= 2x + ( - 2 ) 7 = - 2x = + 6$

Oxidation state of $Cr$ in ${CrO}_{4}^{2 -}$

$= x + ( - 2 ) 4 = - 2x = + 6 .$

So, the oxidation number of $Crin{CrO}_{4}^{2 -}isalso + 6 .$

Hence, the difference in oxidation number between ions is $0$