Question

Explain why the compounds of copper (II) are coloured, but those of zinc are colourless.

Answer 1

i. The electronic configuration of Cu: [Ar] 3d¹⁰ 4s¹ and Cu²⁺: [Ar] 3d⁹ . 

In copper (II) compounds, Cu²⁺ ions have incompletely filled 3d-orbital (3d⁹).

ii. The presence of one unpaired electron in 3d-orbital results in d-d transition due to which, Cu²⁺ ions absorb red light from visible spectrum and emit blue light. Therefore, copper (II) compounds are coloured.

iii. In case of zinc, the electronic configuration is Zn: [Ar] 3d¹⁰4s² and Zn²⁺: [Ar] 3d¹⁰. 

iv. Since 3d subshell is completely filled and there are no unpaired electrons, d-d transition is not possible and hence, Zn²⁺ ions do not absorb radiation in visible region. Therefore, the compounds of zinc are colourless.