Which one of the following ions would be coloured?
(A) \(Cu^+\)
(B) \(Ni^{2+}\)
(C) \(Mn^{2+}\)
(D) \(Sc^{3+}\)
(E) \(Fe^{3+}\)
Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → (B), (C) and (E) only
A transition metal ion is coloured when it has a partly filled d subshell, so that a d–d transition can absorb visible light. Ions with d0 or d10 configurations are colourless. Working out each configuration (ns electrons are lost first):
(A) \( \ce{Cu^{+}} \) — Incorrect. Cu (Z = 29) [Ar]3d104s1 → \( \ce{Cu^{+}} \) 3d10; colourless.
(B) \( \ce{Ni^{2+}} \) — Correct. Ni (Z = 28) [Ar]3d84s2 → \( \ce{Ni^{2+}} \) 3d8; green.
(C) \( \ce{Mn^{2+}} \) — Correct. Mn (Z = 25) [Ar]3d54s2 → \( \ce{Mn^{2+}} \) 3d5; pink.
(D) \( \ce{Sc^{3+}} \) — Incorrect. Sc (Z = 21) [Ar]3d14s2 → \( \ce{Sc^{3+}} \) 3d0; colourless.
(E) \( \ce{Fe^{3+}} \) — Correct. Fe (Z = 26) [Ar]3d64s2 → \( \ce{Fe^{3+}} \) 3d5; yellow.
NCERT (2026-27 edition):
Formation of Coloured Ions – Table 4.8: Colours of Some of the First Row (aquated) Transition Metal Ions
| Configuration | Example | Colour |
|---|---|---|
| 3d0 | Sc3+ | colourless |
| 3d0 | Ti4+ | colourless |
| 3d1 | Ti3+ | purple |
| 3d1 | V4+ | blue |
| 3d2 | V3+ | green |
| 3d3 | V2+ | violet |
| 3d3 | Cr3+ | violet |
| 3d4 | Mn3+ | violet |
| 3d4 | Cr2+ | blue |
| 3d5 | Mn2+ | pink |
| 3d5 | Fe3+ | yellow |
| 3d6 | Fe2+ | green |
| 3d6 | Co3+ | blue |
| 3d7 | Co2+ | pink |
| 3d8 | Ni2+ | green |
| 3d9 | Cu2+ | blue |
| 3d10 | Zn2+ | colourless |