Actinoid contraction is of greater extent than lanthanide contraction because :
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → more poor shielding effect of 5f electrons than 4f electrons
Across the lanthanoid series the 4f orbitals fill, and across the actinoid series the 5f orbitals fill. An f electron shields the other outer electrons poorly from the nuclear charge, because f orbitals are diffuse. So, as the nuclear charge rises by one unit from element to element, the effective nuclear charge felt by the outer electrons increases and the atoms and M3+ ions shrink. This is the lanthanoid contraction (4f) and the actinoid contraction (5f).
The 5f orbitals are even more diffuse than 4f orbitals, so 5f electrons shield still more poorly. Hence the contraction from one element to the next is greater in the actinoid series.
Why the other options are incorrect
Option (1) poor shielding effect of 4f electrons — Incorrect. This explains the lanthanoid contraction itself, not why the actinoid contraction is greater.
Option (2) poor shielding effect of 5d electrons — Incorrect. The electrons added across the actinoid series go into 5f, not 5d, so 5d shielding is not the cause.
Option (4) more poor shielding effect of 4f electrons than 5f electrons — Incorrect. This is the reverse; if 4f shielded more poorly, the lanthanoid contraction would be the greater one.
NCERT (2026-27 edition):
Ionic Sizes – “The general trend in lanthanoids is observable in the actinoids as well. There is a gradual decrease in the size of atoms or M3+ ions across the series. This may be referred to as the actinoid contraction (like lanthanoid contraction). The contraction is, however, greater from element to element in this series resulting from poor shielding by 5f electrons.”