For which of the following ion the colour is not due to a d-d transition?

\(CrO_4^{2-}\) lemon yellow

The correct answer is option 1. \(CrO_4^{2-}\) lemon yellow.

The color of \(CrO_4^{2-}\) (chromate ion) is not due to a d-d transition but rather a charge transfer (ligand-to-metal) transition.

The chromate ion, \(CrO_4^{2-}\), is a polyatomic ion composed of one chromium atom (Cr) and four oxygen atoms (O) arranged in a tetrahedral structure. The oxidation state of the chromium in the chromate ion is +6.

When light interacts with the chromate ion, it can be absorbed by the electrons present in the system. The absorption of light causes the electrons to move from lower energy orbitals to higher energy orbitals. The energy required to promote an electron to a higher energy orbital corresponds to a specific wavelength of light, which corresponds to a particular color.

In the case of the chromate ion, the color arises due to a charge transfer (ligand-to-metal) transition rather than a d-d transition. This is because the 3d orbitals of the chromium ion are already fully filled in the +6 oxidation state, so there are no available unpaired electrons for d-d transitions. As a result, the color is not due to the transition between different d-orbitals.

Instead, the color in the chromate ion is attributed to a charge transfer between the oxygen p-orbitals and the 3d orbitals of the chromium ion. When light is absorbed by the chromate ion, it promotes electrons from the oxygen p-orbitals to the higher energy 3d orbitals of the chromium ion. This transition corresponds to a specific wavelength of light, resulting in the observed lemon yellow color.

In summary, the color of the chromate ion (\(CrO_4^{2-}\)) is not due to a d-d transition because the chromium ion is in the +6 oxidation state with fully filled 3d orbitals. Instead, the color arises from a charge transfer (ligand-to-metal) transition between the oxygen p-orbitals and the 3d orbitals of the chromium ion, resulting in the lemon yellow color.