Question

Explain why Zn²⁺ salts are white whereas Cu²⁺ salts are coloured.

Give reasons:

Zn⁺² salts are colourless while Cu⁺² salts are coloured.

Answer 1

Zn²⁺(3d¹⁰) salts have no impaired electrons but Cu^2-(3d⁹) salts have one unpaired electron. Hence, Zn²⁺ salts are white while Cu²⁺ salts are coloured. 

Answer 2

1. Cu²⁺ salts are coloured because Cu²⁺ ions have partially filled 3d orbitals (3d⁹), allowing d-d electronic transitions when visible light is absorbed. This absorption leads to complementary colour being observed.
2. Zn²⁺ salts are colourless because Zn²⁺ ions have completely filled 3d orbitals (3d¹⁰), so no d-d transitions occur as there are no vacant d-orbitals for electron promotion.
3. The presence or absence of unpaired electrons in the d orbitals determines whether a transition metal ion salt shows colour due to d-d transitions under visible light.