Which of the following will be coloured in aqueous solution?
A. Mn2+
B. Sc3+
C. Zn2+
D. Co2+
E. Fe3+
Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → A, D, E only
A transition metal ion is coloured when it has a partly filled d subshell, so that an electron can be excited from a lower to a higher d orbital (d–d transition) by absorbing visible light. Ions with d0 or d10 configurations are colourless. Working out each configuration (ns electrons are lost first):
A. \( \ce{Mn^{2+}} \) — Correct. Mn (Z = 25) [Ar]3d54s2 → \( \ce{Mn^{2+}} \) 3d5; pale pink.
B. \( \ce{Sc^{3+}} \) — Incorrect. Sc (Z = 21) [Ar]3d14s2 → \( \ce{Sc^{3+}} \) 3d0; colourless.
C. \( \ce{Zn^{2+}} \) — Incorrect. Zn (Z = 30) [Ar]3d104s2 → \( \ce{Zn^{2+}} \) 3d10; colourless.
D. \( \ce{Co^{2+}} \) — Correct. Co (Z = 27) [Ar]3d74s2 → \( \ce{Co^{2+}} \) 3d7; pink.
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 |