The atomic radii of the elements of the third (5d) transition series are almost the same as those of the corresponding elements of the second (4d) series. This is due to the
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → Lanthanoid contraction
Going down a group, an extra electron shell should make the 5d elements larger than the 4d elements.
But before the 5d series begins, the 4f orbitals are filled across the lanthanoids. The 4f electrons shield the outer electrons poorly, so the size falls steadily across the lanthanoids.
This lanthanoid contraction cancels the expected increase, and the 5d elements end up with almost the same radii as the 4d elements (e.g., Zr 160 pm, Hf 159 pm).
Why the other options are incorrect
Option (1) Actinoid contraction — Incorrect. The actinoids fill the 5f orbitals, which come after the 5d series; their contraction affects the elements that follow the actinoids, not the 5d series.
Option (2) Increase in the number of electron shells — Incorrect. An extra shell would make the 5d atoms larger than the 4d atoms; it is the size increase that the lanthanoid contraction cancels.
Option (4) Stability of the half-filled d subshell — Incorrect. Half-filled stability bears on ionisation enthalpies and oxidation states of single elements; it does not explain a series-to-series similarity in size.
NCERT (2026-27 edition):
4.5.2 Atomic and Ionic Sizes – “The cumulative effect of the contraction of the lanthanoid series, known as lanthanoid contraction, causes the radii of the members of the third transition series to be very similar to those of the corresponding members of the second series.”