In which of the following pairs, both the ions are coloured in an aqu…

The correct answer is Option (2) → \(Ni^{2+}, Ti^{3+}\)

A transition metal ion is coloured in aqueous solution when it has partly filled d orbitals (d¹ to d⁹): light in the visible region excites an electron from a lower to a higher d orbital (d–d transition), and the complementary colour is seen. Ions with d⁰ or d¹⁰ configurations have no d–d transition and are colourless.

• Ni²⁺: Ni (Z = 28) is [Ar] 3d⁸ 4s²; removing the two 4s electrons gives \( \ce{Ni^{2+}} \) = [Ar] 3d⁸ (2 unpaired electrons) — green.

• Ti³⁺: Ti (Z = 22) is [Ar] 3d² 4s²; removing two 4s and one 3d electron gives \( \ce{Ti^{3+}} \) = [Ar] 3d¹ (1 unpaired electron) — purple.

Both ions of Option (2) are coloured.

Why the other options are incorrect

• Option (1) Ni²⁺, Cu⁺ — Incorrect. Cu (Z = 29) is [Ar] 3d¹⁰ 4s¹, so \( \ce{Cu^{+}} \) = [Ar] 3d¹⁰, a completely filled d subshell — colourless.

• Option (3) Sc³⁺, Ti³⁺ — Incorrect. Sc (Z = 21) is [Ar] 3d¹ 4s², so \( \ce{Sc^{3+}} \) = [Ar] 3d⁰, with no d electron — colourless.

• Option (4) Sc³⁺, Co²⁺ — Incorrect. \( \ce{Co^{2+}} \) (3d⁷, 3 unpaired electrons) is pink, but \( \ce{Sc^{3+}} \) (3d⁰) is colourless.

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