Which of the following ions is colourless in solution?

Step-by-step solution

V (23)=[ Ar ] 3 ~d ³, 4 s² V ³⁺=[ Ar ] 3 d², 4 s⁰ (two unpaired electrons) array l Cr (24)=[ Ar ] 3 d⁵, 4 s¹ Cr ³⁺=[ Ar ] 3 d³, 4 s⁰ array (three unpaired electrons) Co (27)=[ Ar ] 3 d⁷, 4 s² Co ²⁺=[ Ar ] 3 ~d ⁷, 4 s⁰ (three unpaired electrons) Sc (21)=[ Ar ] 3 d⁷, 4 s² Sc ³⁺=[ Ar ] 3 d⁰, 4 s⁰ (no unpaired electrons) Thus, in Sc ³⁺ no unpaired d -electron is present. Hence, no d-d transition is possible and it is colourless ion.