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

Fe³⁺ is colored in aqueous solution due to unpaired electrons in its d-orbital, enabling d-d transitions.

Conversely:

• Sc³⁺, with an empty d-orbital (d₀ configuration), exhibits no d-d transition and is therefore colorless.
• Zn²⁺, possessing a completely filled d-orbital (d₁₀ configuration), also shows no d-d transition and is colorless.