Question

Zn$^{2+}$ salts are colourless while Ni$^{2+}$ salts are coloured. Why?

Hint:

In transition metals, colour is due to d–d electronic transitions; fully filled d-orbitals give colourless compounds.

Answer 1

Step 1: Electronic configuration.
- Zn$^{2+}$: [Ar] 3d$^{10}$ (completely filled d-orbitals).
- Ni$^{2+}$: [Ar] 3d$^8$ (partially filled d-orbitals).
Step 2: Colour and d–d transitions.
- In Zn$^{2+}$, no d–d electronic transitions are possible because d-orbitals are completely filled. Hence, Zn$^{2+}$ salts are colourless.
- In Ni$^{2+}$, d–d electronic transitions are possible due to partially filled 3d orbitals. Absorption of visible light during these transitions imparts colour to Ni$^{2+}$ salts.
Conclusion:
Zn$^{2+}$ salts are colourless (d$^{10}$, no transitions) while Ni$^{2+}$ salts are coloured (d$^8$, d–d transitions possible).