- 2075 Baisakh · 1.5 marks
Zn²⁺ salts white, Cu²⁺ salts coloured
Short answer
Zn²⁺ (3d¹⁰) has completely filled d orbitals, preventing d–d transitions; its salts are white. Cu²⁺ (3d⁹) has a vacancy, allowing d–d absorption of red-orange light, so the hydrated ion appears blue.
Explain why Zn²⁺ salts are white while Cu²⁺ salts are coloured.
- Zn²⁺ has the configuration 3d¹⁰. All d orbitals are full, so no d–d transition can take place and no visible light is absorbed: Zn²⁺ salts are white.
- Cu²⁺ has 3d⁹, with one vacancy (hole) in the d subshell. An electron can be excited from the t₂g to e_g level by absorbing red-orange visible light, and the complementary blue/green colour is seen: hydrated Cu²⁺ is blue.
✓ Answer checked by Claude Opus 5.5 (high effort)
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