Question

Giving reason indicate which one of the following would be coloured:

Cu⁺, VO²⁺, Sc³⁺, Ni²⁺

(Atomic numbers: Cu = 29, V = 23, Sc = 21, N = 28)

Answer 1

1. A compound is coloured if its metal ion has partially filled d-orbitals, allowing d-d electronic transitions when visible light is absorbed.
2. Cu⁺ has an electronic configuration 3d¹⁰ (completely filled d-orbitals), so it is colourless.
3. VO²⁺ (V⁴⁺ ion) has partially filled d-orbitals (d¹), so it can be coloured.
4. Sc³⁺ has an electronic configuration 3d⁰ (empty d-orbitals), so it is colourless.
5. Ni²⁺ has partially filled d-orbitals (d⁸), so it will be coloured.
6. Among these, Ni²⁺ is known to be coloured, whereas Cu⁺ and Sc³⁺ are colourless; VO²⁺ may show colour due to partially filled d-orbitals but is less commonly coloured in simple complexes.

Hence, Ni²⁺ would definitely be coloured due to the presence of unpaired d-electrons, enabling absorption of visible light; Cu⁺ and Sc³⁺ are colourless due to filled or empty d-orbitals, respectively.