Read the passage carefully and answer the following questions.
There are mainly four series of the transition metals, 3d series (Sc to Zn), 4d series (Y to Cd), 5d series (La and Hf to Hg) and 6d series which has Ac and elements from Rf to Cn. The two series of the inner transition metals; 4f (Ce to Lu) and 5f (Th to Lr) are known as lanthanoids and actinoids respectively. Their chemical properties are transitional between those of s and p-block elements. 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. Transition metals have incomplete d subshell either in neutral atom or in their ions. With partly filled d orbitals these elements exhibit certain characteristic properties such as display of a variety of oxidation states, formation of coloured ions and entering into complex formation with a variety of ligands. The transition metals and their compounds are known for their catalytic activity.
In series Sc to Zn, the lowest enthalpy of atomization is shown by
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → Zn
The enthalpy of atomisation is the energy needed to break the metal into free atoms, so it measures the strength of metallic bonding. In the transition metals the bonding uses the ns electrons and the unpaired (n−1)d electrons: the more unpaired electrons an atom has, the stronger the bonding. That is why the values rise to a maximum near the middle of the series and then fall.
Zinc (Z = 30) is \( \mathrm{[Ar]\,3d^{10}\,4s^{2}} \). Its 3d subshell is full, so it has no unpaired d electrons to add to the metallic bonding, and only the two 4s electrons hold the atoms together. Its enthalpy of atomisation, 126 kJ mol−1, is therefore the lowest in the series.
Why the other options are incorrect
Option (1) Sc — Incorrect. Sc (\( \mathrm{3d^{1}\,4s^{2}} \)) uses three electrons in bonding; its value (326 kJ mol−1) is well above that of zinc.
Option (3) Mn — Incorrect. The stable half-filled \( \mathrm{3d^{5}} \) set holds its electrons back, so Mn shows a dip (281 kJ mol−1) compared with its neighbours, but it is still more than twice the value for zinc.
Option (4) Co — Incorrect. Co (\( \mathrm{3d^{7}\,4s^{2}} \)) has three unpaired 3d electrons and strong metallic bonding (425 kJ mol−1).
NCERT (2026-27 edition):
Variation in Atomic and Ionic Sizes of Transition Metals – “Example 4.2: Why do the transition elements exhibit higher enthalpies of atomisation?
Solution: Because of large number of unpaired electrons in their atoms they have stronger interatomic interaction and hence stronger bonding between atoms resulting in higher enthalpies of atomisation.”