Which of the following ions will exhibit colour in aqueous solution?

The correct answer is Option (2) → Ti³⁺ (Z = 22)

An ion is coloured in water when it has a partly filled d subshell: a d electron absorbs visible light and is excited to a higher energy d orbital, and the solution shows the complementary colour. An ion with no d electrons, or a completely filled subshell, has no such transition and is colourless.

• Configuration of Ti: Ti (Z = 22) is [Ar]3d²4s².

• Ion formed: losing the two 4s electrons and one 3d electron gives \( \ce{Ti^3+} \): [Ar]3d¹.

• d subshell: 3d¹ is partly filled, so a d–d transition is possible; aquated \( \ce{Ti^3+} \) is purple.

Why the other options are incorrect

Option (1) La³⁺ (Z = 57) — Incorrect. La (Z = 57) is [Xe]5d¹6s²; \( \ce{La^3+} \) is [Xe], with no d or f electrons, so it is colourless.

Option (3) Lu³⁺ (Z = 71) — Incorrect. Lu (Z = 71) is [Xe]4f¹⁴5d¹6s²; \( \ce{Lu^3+} \) is [Xe]4f¹⁴, a completely filled 4f subshell and no partly filled d or f subshell, so it is colourless.

Option (4) Sc³⁺ (Z = 21) — Incorrect. Sc (Z = 21) is [Ar]3d¹4s²; \( \ce{Sc^3+} \) is 3d⁰, with no d electron to excite, so it is colourless.

NCERT (2026-27 edition):
4.3.10 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. … In aqueous solutions where water molecules are the ligands, the colours of the ions observed are listed in Table 4.8.”

Table 4.8: Colours of Some of the First Row (aquated) Transition Metal Ions