Question

Why is the Cu⁺ ion colourless?

Answer 1

1. Cu⁺ ions have a completely filled d-orbital configuration (3d¹⁰).
2. Because all d-orbitals are fully occupied, no electrons are available for d-d electronic transitions.
3. d-d transitions are responsible for colour in transition metal ions, caused by absorption of visible light.
4. Without such transitions, Cu⁺ ions do not absorb visible light and thus appear colourless.
5. Similarly, ions like Zn²⁺ and Cd²⁺ with filled d-orbitals are also colourless.