Question

Explain why Cu$^{+2}$ salts are coloured while Zn$^{+2}$ salts are colourless ?

Hint:

Colour in transition metals usually requires an incomplete d-subshell (d$^1$ to d$^9$). d$^0$ and d$^{10}$ are generally colourless.

Answer 1

1. Cu$^{+2}$ Configuration: Copper (atomic number 29) has configuration [Ar] 3d$^{10}$ 4s$^1$. Cu$^{+2}$ is [Ar] 3d$^9$.
It has one unpaired electron in the d-orbital. This allows for d-d transitions (movement of electron between split d-orbitals) by absorbing visible light, resulting in colour.
2. Zn$^{+2}$ Configuration: Zinc (atomic number 30) has configuration [Ar] 3d$^{10}$ 4s$^2$. Zn$^{+2}$ is [Ar] 3d$^{10}$.
The d-subshell is completely filled. There are no empty d-orbitals for electron transition. Hence, no visible light is absorbed, and the salts appear colourless (white).