Which of the statements below are correct?
A. Actinoid contraction is greater than lanthanoid contraction.
B. The magnetic property of the actinoids are more complex than those of the lanthanoids.
C. Actinoid contraction taking place due to poor shielding by 5f electrons.
D. Actinoid contraction taking place due to poor shielding by 6f electrons.
Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → A, B and C only
(A) Actinoid contraction is greater than lanthanoid contraction — Correct. Across the actinoids the size of the atoms and M3+ ions decreases, and this decrease is greater from element to element than in the lanthanoids.
(B) The magnetic properties of the actinoids are more complex than those of the lanthanoids — Correct. The 5f orbitals are less well shielded than 4f and lie close in energy to the 6d orbitals, so the magnetic behaviour of the actinoids is harder to interpret.
(C) Actinoid contraction is due to poor shielding by 5f electrons — Correct. In the actinoids the 5f orbitals are being filled, and 5f electrons shield the outer electrons from the nuclear charge even more poorly than 4f electrons.
(D) Actinoid contraction is due to poor shielding by 6f electrons — Incorrect. No 6f orbitals are filled in the actinoids (or in any known element); the contraction is due to the 5f electrons.
NCERT (2026-27 edition):
Ionic Sizes – “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.”
General Characteristics and Comparison with Lanthanoids – “… The magnetic properties of the actinoids are more complex than those of the lanthanoids.”