Question

The yellow color of chromate ion and orange color of dichromate ion is due to

• A. d-d transition only
• B. Charge transfer only
• C. Both d-d transition and Charge transfer
• D. s-d transitions only

Answer 1

Answer: (C)

In both of chromate $\left(Cr{O}_4{}^{2-}\right)$ and dichromate $\left(C{r}_2{O}_7{}^{2-}\right)$ ions, $Cr$ is present in $+6$ oxidation state,
i.e. $d^0$-configuration.
So, $d-d$ transition is absent in both of the ions. Their intense colour (yellow of $Cr{O}_4{}^{2-}$ and orange of $C{r}_2{O}_7{}^{2-}$ ) is the charge transfer (ligand $\to$ metal) of oxygen get transfered into vacant $d-$ orbital of $Cr(VI)$.