Question

Give a reason for the following:

Zn²⁺ salts are white but Cu²⁺ salts are blue in colour.

Answer 1

Cu²⁺ salts are coloured because they absorb visible light for the excitation of electrons to partly filled d-orbitals. Zn²⁺ salts are colourless because of the absence of partly filled d-orbitals to which the electrons can be oxidized.

Answer 2

1. Cu²⁺ salts are blue because they have partially filled d-orbitals (3d⁹). When light falls on these salts, electrons absorb visible light and get excited from one d-orbital to another (called d-d transitions), which gives the salt its blue colour.
2. Zn²⁺ salts are white or colourless because Zn²⁺ has a completely filled d-orbital (3d¹⁰). Since no d-d transitions are possible, it does not absorb visible light, and hence the salts appear white.