Actinoid contraction is greater from element to element than Lanthanoid contraction because of
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → poor shielding by 5f electrons
The 5f orbitals are more diffuse than the 4f orbitals, so a 5f electron shields even less well than a 4f electron. The pull of the nucleus therefore grows faster along the actinoid series, and the contraction from one element to the next is greater. So the cause is the poor shielding by 5f electrons.
Why the other options are incorrect
Option (1) poor shielding by 4f electrons — Incorrect. This explains the lanthanoid contraction; actinoids fill 5f, not 4f, orbitals.
Option (2) prior filling of 4d electrons — Incorrect. The 4d subshell is filled long before both series (in the second transition series), so it cannot explain a difference between them.
Option (3) prior filling of 3d electrons — Incorrect. The 3d subshell belongs to the first transition series and is equally full in both the lanthanoids and the actinoids.
NCERT (2026-27 edition):
Ionic Sizes
