`[Ti (H_2O)_6]^(3+)` is coloured while `[Sc(H_2O)_6]^(3+)` is colourless. Explain.

Explanation

In `[Sc(H_2O)_6]^(3+)` electronic configuration of `Sc^(3+)` ion is `3 d^0` while in `[Ti(H_2O)_6]^(3+)` electronic configuration of `Ti^(+)`ions, is `3d^1`. Hence with `3d^0` configuration, no d-d transition are possible in `Sc^(3+)` ion, hence it is colourless. On the other hand, with one unpaired 3d- electron in `Ti^(3+)` ion, d-ditransition are possible and hence it is coloured :
`Sc^(3+) : [Ar]3d^0`
`Ti^(3+) : [Ar]3d^1`