Choose the Incorrect statement for the lanthanoid contraction:
A. It arises due to the imperfect shielding of one electron by another in the same sub-shell.
B. Similar type of phenomenon is not observed in actinoids.
C. It causes decrease both in atomic and ionic radii.
D. 4f orbital is filled immediately after 5d orbitals.
Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → B and D only
Lanthanoid contraction is the steady decrease in the atomic and ionic radii of the lanthanoids from lanthanum to lutetium. Along the series, electrons enter the 4f subshell while the nuclear charge rises by one unit each time. The 4f electrons shield one another poorly (4f orbitals are diffuse), so the effective nuclear charge felt by the outer electrons increases and the atom or ion is pulled in.
(A) It arises due to the imperfect shielding of one electron by another in the same sub-shell. — Correct. This is its cause: one 4f electron shields another 4f electron less than an inner electron would.
(B) Similar type of phenomenon is not observed in actinoids. — Incorrect. The actinoids show an actinoid contraction, which is even greater from element to element, because 5f electrons shield still more poorly than 4f electrons.
(C) It causes decrease both in atomic and ionic radii. — Correct. Both the atomic radii and the radii of the M3+ ions decrease steadily across the series.
(D) 4f orbital is filled immediately after 5d orbitals. — Incorrect. In the lanthanoids the 4f orbitals are filled (after 6s), while the 5d orbitals are not filled first: at most one electron enters 5d (La, Ce, Gd, Lu), and the 5d subshell fills only after 4f is complete, from hafnium onwards.