Question

Zn\(^{2+}\) salts are colourless. Why?

Hint:

The color of transition metal salts depends on the presence of unpaired electrons in the \( d \)-orbitals that can absorb visible light.

Answer 1

Colorlessness of Zinc ²⁺ Salts Explained

The absence of color in zinc salts, specifically those with the \( \text{Zn}^{2+} \) ion, is a consequence of its electronic configuration.

The electronic configuration of a \( \text{Zn}^{2+} \) ion is:

\( 1s^2\,2s^2\,2p^6\,3s^2\,3p^6\,3d^{10} \)

This configuration features a fully occupied \( 3d \) subshell, containing no unpaired electrons.

Color in transition metal ions is generally caused by \( d \rightarrow d \) electronic transitions, where electrons absorb visible light to move between split \( d \)-orbital energy levels. However, for \( \text{Zn}^{2+} \), these transitions are inhibited due to:

• The \( 3d \)-orbitals being completely filled.
• The absence of available vacant \( d \)-orbitals for electron transitions.

Consequently, \( \text{Zn}^{2+} \) ions exhibit no absorption of visible light, leading to their salts appearing colorless in aqueous solutions.