Question

Predict which of the following will be coloured in aqueous solution and why? 
\(Sc^{3+}, Fe^{3+}, Zn^{2+}\)
[Atomic numbers: Sc = 21, Fe = 26, Zn = 30]

Answer 1

Among the given ions, Fe³⁺ is coloured in aqueous solution because it has unpaired electrons in its d-orbital, which allows d-d transitions.

On the other hand:

• Sc³⁺ has an empty d-orbital (d₀ configuration), so no d-d transition occurs → Colourless.
• Zn²⁺ has a fully filled d-orbital (d₁₀ configuration), so no d-d transition occurs → Colourless.