Which of the following ion is colourless?

The correct answer is Option (1) → \(Zn^{2+}\)

A transition metal ion is coloured when it has a partly filled d subshell. The ligands split the d orbitals, and an electron can be promoted from a lower to a higher d orbital (d–d transition) by absorbing visible light; the ion shows the complementary colour. An ion with d⁰ or d¹⁰ has no such transition and is colourless.

Zn (Z = 30) is [Ar]3d¹⁰4s²; removing the two 4s electrons gives \( \ce{Zn^2+} \) = [Ar]3d¹⁰. The d subshell is full, no d–d transition is possible, so \( \ce{Zn^2+} \) is colourless.

Why the other options are incorrect

• Option (2) Ti³⁺ — Incorrect. Ti (Z = 22) is [Ar]3d²4s²; removing two 4s electrons and one 3d electron gives 3d¹. The one d electron can undergo a d–d transition; the hydrated ion is purple.

• Option (3) Mn³⁺ — Incorrect. Mn (Z = 25) is [Ar]3d⁵4s²; removing two 4s electrons and one 3d electron gives 3d⁴, a partly filled d subshell; the hydrated ion is violet.

• Option (4) Cr³⁺ — Incorrect. Cr (Z = 24) is [Ar]3d⁵4s¹; removing one 4s and two 3d electrons gives 3d³, a partly filled d subshell; the hydrated ion is violet.

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