Comprehension:
TRANSITION METALS
All the transition elements display typical metallic properties such as high tensile strength, ductility, malleability, high thermal and electrical conductivity and metallic lustre. With the exceptions of Zn, Cd, Hg and Mn, they have one or more typical metallic structures at normal temperatures.
The transition metals (with the exception of Zn, Cd and Hg) are very hard and have low volatility. Their melting and boiling points are high. The high melting points of these metals are attributed to the involvement of greater number of electrons from (n–1)d in addition to the ns electrons in the interatomic metallic bonding. In any row the melting points of these metals rise to a maximum at d5 except for anomalous values of Mn and Tc and fall regularly as the atomic number increases.
The maxima at about the middle of each series indicate that one unpaired electron per d orbital is particularly favourable for strong interatomic interaction. In general, greater the number of valence electrons, stronger is the resultant bonding. Since the enthalpy of atomisation is an important factor in determining the standard electrode potential of a metal, metals with very high enthalpy of atomisation (i.e., very high boiling point) tend to be noble in their reactions.
Why transition metal ions are colored?
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → d-d transition
In a free ion, the five d orbitals have the same energy. When ligands such as water molecules surround the ion, the d orbitals split into groups of different energy. An electron in a lower energy d orbital can absorb light of a particular wavelength in the visible region and jump to a higher energy d orbital. This is a d-d transition. The colour we see is the complementary colour of the light absorbed.
A d-d transition needs a partly filled d subshell:
Coloured: \( \ce{Ti^{3+}} \) (Ti, Z = 22, is [Ar]3d24s2; losing 4s2 and one 3d electron leaves 3d1, purple), \( \ce{V^{3+}} \) (V, Z = 23, is [Ar]3d34s2; losing 4s2 and one 3d electron leaves 3d2, green), \( \ce{Cu^{2+}} \) (Cu, Z = 29, is [Ar]3d104s1; losing 4s1 and one 3d electron leaves 3d9, blue)
Colourless: \( \ce{Sc^{3+}} \) (Sc, Z = 21, is [Ar]3d14s2; losing all three outer electrons leaves 3d0) and \( \ce{Zn^{2+}} \) (Zn, Z = 30, is [Ar]3d104s2; losing 4s2 leaves 3d10), which have no possible d-d transition
So transition metal ions are coloured because of d-d transitions.
Why the other options are incorrect
Option (1) Large atomic size — Incorrect. Transition metals actually have small atomic sizes, and size does not produce colour.
Option (2) High Hydration Enthalpy — Incorrect. It affects the stability of ions in solution (for example, Cu2+(aq) is more stable than Cu+(aq)), not their colour.
Option (4) Variable Oxidation States — Incorrect. Different oxidation states may have different colours, but the colour itself comes from d-d transitions; \( \ce{Sc^{3+}} \) and \( \ce{Zn^{2+}} \) are 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.”