Question

Why Zn²⁺ ions are colourless while Ni²⁺ ions are green in colour?

Answer 1

1. Zn²⁺ has an electronic configuration 3d¹⁰, so all d-orbitals are filled, and no d-d electronic transitions absorb visible light; therefore, Zn²⁺ ions are colourless.
2. Ni²⁺ has a configuration 3d⁸ with unpaired d-electrons; these allow d-d transitions that absorb certain visible wavelengths (often red), so the transmitted light appears green.