Assertion (A): The actinoid contraction is smaller from element to element than the lanthanoid contraction.
Reason (R): The 5f electrons provide poor shielding from element to element in the actinoid series.
In the light of the above statements, choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → (A) is false but (R) is true.
Across the actinoid series the size of the atoms and M3+ ions decreases gradually, as in the lanthanoids. This is the actinoid contraction. Its size depends on how well the added f electrons shield one another from the rising nuclear charge.
Assertion (A): The actinoid contraction is smaller from element to element than the lanthanoid contraction. — False. The actinoid contraction is greater from element to element than the lanthanoid contraction.
Reason (R): The 5f electrons provide poor shielding from element to element in the actinoid series. — True. Each added 5f electron shields the outer electrons poorly, so the increasing nuclear charge pulls them in strongly at every step. This poor shielding is the very reason the actinoid contraction is the larger one.
NCERT (2026-27 edition):
4.6.2 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.”
Answers to Some Intext Questions – “4.10 The 5f electrons are more effectively shielded from nuclear charge. In other words the 5f electrons themselves provide poor shielding from element to element in the series.”