Transition metals form coloured ions or compounds due to partially filled d orbitals. In the presence of solvent molecules (in solutions), ligands (in complexes) or counter ions (in crystals), the d orbitals split into two sets. An electron in a transition metal ion that occupies the set of d orbitals of lower energy can be excited to the set of higher energy by absorbing energy from visible light. Since the energy difference (ΔE) between the two sets of d orbitals is small, only light of the visible region is absorbed during this excitation. The colour of a transition metal ion is due to this d–d excitation (d–d transition) of the electron. During d–d excitation the electron absorbs one colour of visible light, and the ion appears in the complementary colour of the absorbed light. Complementary colours can be identified from the colour wheel shown below (complementary colours lie opposite each other).
The number of electrons undergoing d–d transition and the energy difference between the two sets of orbitals decide the colour.
Consider the following statements about the colours of some metal ions.
(A) The aquated Mn2+ ion, which has the 3d5 configuration, is pink.
(B) The aquated Ni2+ ion is green.
(C) The aquated Co2+ ion is blue.
(D) The absorption bands of the coloured trivalent lanthanoid ions are narrow, probably because of excitation within the f level.
Which of the statements given above are correct? Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → (A), (B) and (D) only
In aqueous solution, water molecules are the ligands around a transition metal ion, and each aquated ion has its own characteristic colour. Many trivalent lanthanoid ions are coloured too; their colour is attributed to the f electrons.
(A) The aquated Mn2+ ion, which has the 3d5 configuration, is pink. — True. Mn (3d54s2) loses its two 4s electrons to give 3d5 Mn2+, which is pink in water.
(B) The aquated Ni2+ ion is green. — True. Ni2+ (3d8) gives green aqueous solutions.
(C) The aquated Co2+ ion is blue. — False. Aquated Co2+ (3d7) is pink; it is Co3+ (3d6) that is blue.
(D) The absorption bands of the coloured trivalent lanthanoid ions are narrow, probably because of excitation within the f level. — True. In these ions the electron is excited within the f level, which gives narrow absorption bands.
NCERT (2026-27 edition):
4.3.10 Formation of Coloured Ions – “…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 (rows needed here)
| Configuration | Example | Colour |
|---|---|---|
| 3d5 | Mn2+ | pink |
| 3d6 | Co3+ | blue |
| 3d7 | Co2+ | pink |
| 3d8 | Ni2+ | green |
4.5.4 General Characteristics – “…Many trivalent lanthanoid ions are coloured both in the solid state and in aqueous solutions. Colour of these ions may be attributed to the presence of f electrons. Neither La3+ nor Lu3+ ion shows any colour but the rest do so. However, absorption bands are narrow, probably because of the excitation within f level.”