Which of the following ions will be coloured in an aqueous solution?

The correct answer is Option (4) → Rh²⁺

Most transition metal ions are coloured because of d–d transitions. An electron in a lower-energy d orbital absorbs visible light and jumps to a higher-energy d orbital, and we see the complementary colour. This needs a partly filled d subshell (d¹ to d⁹). Ions with d⁰ or d¹⁰ configurations have no such transition and are colourless.

• Sc (Z = 21) is [Ar]3d¹4s²; losing all three outer electrons gives \( \ce{Sc^{3+}} \): [Ar]3d⁰, colourless

• Zn (Z = 30) is [Ar]3d¹⁰4s²; losing the two 4s electrons gives \( \ce{Zn^{2+}} \): [Ar]3d¹⁰, colourless

• La (Z = 57) is [Xe]5d¹6s²; losing all three outer electrons gives \( \ce{La^{3+}} \): [Xe], with 4f⁰5d⁰, colourless

• Rh (Z = 45) is [Kr]4d⁸5s¹; losing the 5s electron and one 4d electron gives Rh²⁺: [Kr]4d⁷, partly filled, coloured

So the ion that is coloured in aqueous solution is Rh²⁺.

Why the other options are incorrect

• Option (1) Sc³⁺ — Incorrect. Its d subshell is empty (3d⁰), so no d–d transition is possible.

• Option (2) Zn²⁺ — Incorrect. Its d subshell is full (3d¹⁰), so no electron can move into a vacant d orbital.

• Option (3) La³⁺ — Incorrect. It has no d or f electrons (4f⁰), so it is colourless.

NCERT (2026-27 edition):
Formation of Coloured Ions