Which of the following will be coloured in aqueous medium?

The correct answer is Option (2) → \(Cu^{2+}\)

Transition metal ions are coloured when they have partly filled d orbitals. In water the d orbitals are split by the water ligands, and an electron can be excited from a lower to a higher d orbital by absorbing visible light (a d–d transition); the colour seen is the complementary colour. Ions with d⁰ or d¹⁰ configurations cannot have such a transition and are colourless.

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

• Cu (Z = 29) [Ar]3d¹⁰4s¹; losing the 4s electron and one 3d electron gives \( \ce{Cu^{2+}} \): 3d⁹, one unpaired electron, blue in water.

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

• Cd (Z = 48) [Kr]4d¹⁰5s²; losing the two 5s electrons gives \( \ce{Cd^{2+}} \): 4d¹⁰, colourless.

So only Cu²⁺ forms a coloured solution.

Why the other options are incorrect

• Option (1) Sc³⁺ — Incorrect. 3d⁰, no d electron to excite.

• Option (3) Zn²⁺ — Incorrect. 3d¹⁰, completely filled d subshell.

• Option (4) Cd²⁺ — Incorrect. 4d¹⁰, completely filled d subshell.

NCERT (2026-27 edition):
Formation of Coloured Ions – “When an electron from a lower energy d orbital is excited to a higher energy d orbital, the energy of excitation corresponds to the frequency of light absorbed (Unit 5). This frequency generally lies in the visible region. The colour observed corresponds to the complementary colour of the light absorbed. The frequency of the light absorbed is determined by the nature of the ligand. In aqueous solutions where water molecules are the ligands, the colours of the ions observed are listed in Table 4.8.”

Table 4.8: Colours of Some of the First Row (aquated) Transition Metal Ions

Note: The same/similar question has been asked in different years by NTA as per the details given below: