Which of the following metal ion is coloured?

The correct answer is Option (2) → Ti³⁺

Most transition metal ions are coloured because of d–d transitions. In the presence of ligands (such as water), the d orbitals split into sets of different energy. An electron in a lower d orbital absorbs visible light and jumps to a higher one, and the ion shows the complementary colour. This needs a partly filled d subshell:

• d⁰ ions (no d electron) and d¹⁰ ions (no vacant d orbital) cannot show d–d transitions and are colourless.

• d¹ to d⁹ ions are usually coloured.

Ti (Z = 22) is [Ar] 3d²4s²; losing the two 4s electrons and one 3d electron gives \( \ce{Ti^{3+}} \): 3d¹. Its single d electron can be excited, and the aqueous ion is purple.

So the coloured ion is Ti³⁺.

Why the other options are incorrect

• Option (1) Zn²⁺ — Incorrect. Zn (Z = 30) is [Ar] 3d¹⁰4s²; losing the two 4s electrons gives Zn²⁺: 3d¹⁰, the d subshell is completely filled, so it is colourless.

• Option (3) Sc³⁺ — Incorrect. Sc (Z = 21) is [Ar] 3d¹4s²; losing all three valence electrons gives Sc³⁺: 3d⁰, it has no d electron, so it is colourless.

• Option (4) Ti⁴⁺ — Incorrect. Ti (Z = 22) is [Ar] 3d²4s²; losing all four valence electrons gives Ti⁴⁺: 3d⁰, it has no d electron, so it is colourless.

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