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Which of the following ions is NOT coloured in aqueous solution?
ACu²⁺
BSc³⁺ (3d⁰, no d-electron, no d-d transition)
CFe³⁺
DMn²⁺
Answer & Solution
Correct answer: B. Sc³⁺ (3d⁰, no d-electron, no d-d transition)
**Sc³⁺ (3d⁰) is colourless** — no d electron, hence no d-d transition possible. Same for Ti⁴⁺ (d⁰), Zn²⁺ (d¹⁰), Cu⁺ (d¹⁰). All other 3d^n (1≤n≤9) ions exhibit colour.
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