The metals of the second and third transition series have greater enthalpies of atomisation than the corresponding elements of the first series. This is an important factor in accounting for which of the following?
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → The much more frequent metal–metal bonding in compounds of the heavy transition metals
The enthalpy of atomisation measures how strongly the metal atoms are bonded to one another in the metal.
The 4d and 5d metals have higher values than the 3d metals, so the bonding between their atoms is stronger.
The same strong metal–metal interaction shows up in their compounds, where metal–metal bonds occur much more often than in compounds of the 3d metals.
Why the other options are incorrect
Option (1) The almost identical atomic radii of Zr and Hf — Incorrect. Zr (160 pm) and Hf (159 pm) have almost the same radius because of the lanthanoid contraction, the filling of the 4f orbitals before the 5d series begins, not because of enthalpy of atomisation.
Option (2) The greater reactivity of the heavier metals towards dilute acids — Incorrect. A high enthalpy of atomisation works the other way: metals with very high enthalpies of atomisation tend to be noble (unreactive) in their reactions.
Option (4) The paramagnetic behaviour of the compounds of the heavy transition metals — Incorrect. Paramagnetism arises from unpaired electrons in the metal ions of a compound; it is not decided by the enthalpy of atomisation of the metal.
NCERT (2026-27 edition):
4.3.1 Physical Properties – “…the metals of the second and third series have greater enthalpies of atomisation than the corresponding elements of the first series; this is an important factor in accounting for the occurrence of much more frequent metal – metal bonding in compounds of the heavy transition metals.”