Which of the following transition metal ion is colourless?

The correct answer is Option (1) → \(Sc^{3+}\)

A transition metal ion is coloured when an electron can be excited from a lower energy d orbital to a higher energy d orbital (a d–d transition) by absorbing visible light. This needs a partly filled d subshell. Sc (Z = 21) is [Ar]3d¹4s²; losing the two 4s electrons and the one 3d electron gives \( \ce{Sc^{3+}} \): [Ar]3d⁰. With no d electron there is no d–d transition, so Sc³⁺ is colourless.

Why the other options are incorrect

• Option (2) V²⁺ — Incorrect. V (Z = 23) is [Ar]3d³4s²; \( \ce{V^{2+}} \) is 3d³, partly filled, and its aqueous ion is violet.

• Option (3) Mn²⁺ — Incorrect. Mn (Z = 25) is [Ar]3d⁵4s²; \( \ce{Mn^{2+}} \) is 3d⁵, and its aqueous ion is pink.

• Option (4) Co³⁺ — Incorrect. Co (Z = 27) is [Ar]3d⁷4s²; \( \ce{Co^{3+}} \) is 3d⁶, a partly filled d subshell, so d–d transitions occur and its compounds are coloured.

NCERT (2026-27 edition):
Formation of Coloured Ions – Table 4.8: Colours of Some of the First Row (aquated) Transition Metal Ions