Question

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

Answer 1

Zn²⁺ ions are colourless due to the absence of any unpaired electrons in its 3d-orbital.

Whereas, Ni²⁺ ions are green in colour due to the presence of 2 unpaired electrons d-d transitions are possible.

Zn²⁺ = 3d¹⁰ 4s⁰

No unpaired electron in Zn²⁺.

∴ No d-d transition, colourless.

Ni²⁺ = 3d⁸ 4s⁰

Unpaired electron present in Ni²⁺.

∴ d-d transition possible and are coloured.