Which of the following ions will be coloured in the aqueous solution ?
(A) Ti3+
(B) Nb3+
(C) Cu+
(D) Y3+
Choose the correct answer from the options given below :
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → (A) and (B) only
A transition metal ion is coloured when it has partly filled d orbitals: a d electron can absorb visible light and jump to a higher d level (d–d transition). Ions with an empty (d0) or a completely filled (d10) d subshell cannot do this and are colourless.
(A) \( \ce{Ti^{3+}} \) — Correct. Ti (Z = 22) is [Ar]3d24s2; losing 3 electrons gives 3d1, one unpaired d electron, so it is coloured (purple).
(B) \( \ce{Nb^{3+}} \) — Correct. Nb (Z = 41) is [Kr]4d45s1; losing 3 electrons gives 4d2, partly filled, so it is coloured.
(C) \( \ce{Cu^{+}} \) — Incorrect. Cu (Z = 29) is [Ar]3d104s1; losing the 4s electron gives 3d10, completely filled, so it is colourless.
(D) \( \ce{Y^{3+}} \) — Incorrect. Y (Z = 39) is [Kr]4d15s2; losing 3 electrons gives 4d0, no d electron, so it is colourless.
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.”
NCERT (2026-27 edition):
Electronic Configurations of the d-Block Elements – “… In some respects, ions of a given dn configuration (n = 1 – 9) have similar magnetic and electronic properties. With partly filled d orbitals these elements exhibit certain characteristic properties such as display of a variety of oxidation states, formation of coloured ions and entering into complex formation with a variety of ligands.”