Cr2O72−+OH−⟶2CrO42−
Let the oxidation number of chromium be x
The oxidation state of chromium in Cr2O72−=2x+(−2)7=−2x=+6
Oxidation state of Cr in CrO42−
=x+(−2)4=−2x=+6.
So, the oxidation number of CrinCrO42−isalso+6.
Hence, the difference in oxidation number between ions is 0