Consider the following statements about the lanthanoids.
(A) The overall decrease in atomic and ionic radii from lanthanum to lutetium is called the lanthanoid contraction.
(B) Because of the lanthanoid contraction, the members of the third transition series have radii very similar to those of the corresponding members of the second series.
(C) The +2 oxidation state is the predominant oxidation state of the lanthanoids.
(D) The lanthanoid contraction is attributed to the imperfect shielding of one electron by another in the same sub-shell.
(E) Ce(IV) is a strong reductant and readily changes to the +3 state.
Which of the statements given above are incorrect? Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → (C) and (E) only
In the lanthanoids the electrons enter the 4f sub-shell. This explains both their steady fall in size and their common +3 oxidation state.
(A) The overall decrease in atomic and ionic radii from lanthanum to lutetium is called the lanthanoid contraction. — Correct statement. This is the definition of the lanthanoid contraction.
(B) Because of the lanthanoid contraction, the members of the third transition series have radii very similar to those of the corresponding members of the second series. — Correct statement. So the expected increase in size from the second to the third series does not appear.
(C) The +2 oxidation state is the predominant oxidation state of the lanthanoids. — Incorrect statement. The predominant state is +3; Ln(III) compounds are the main species, and +2 and +4 ions appear only occasionally.
(D) The lanthanoid contraction is attributed to the imperfect shielding of one electron by another in the same sub-shell. — Correct statement. As the nuclear charge rises by one at each step, the new 4f electron shields poorly, so the outer electrons are pulled in more tightly.
(E) Ce(IV) is a strong reductant and readily changes to the +3 state. — Incorrect statement. Going from +4 to +3, cerium gains an electron, so Ce(IV) is a strong oxidant, not a reductant.
NCERT (2026-27 edition):
4.5.2 Atomic and Ionic Sizes – “The overall decrease in atomic and ionic radii from lanthanum to lutetium (the lanthanoid contraction) is a unique feature in the chemistry of the lanthanoids.” … “This contraction is, of course, similar to that observed in an ordinary transition series and is attributed to the same cause, the imperfect shielding of one electron by another in the same sub-shell.”
“The cumulative effect of the contraction of the lanthanoid series, known as lanthanoid contraction, causes the radii of the members of the third transition series to be very similar to those of the corresponding members of the second series.”
4.5.3 Oxidation States – “In the lanthanoids, La(II) and Ln(III) compounds are predominant species. However, occasionally +2 and +4 ions in solution or in solid compounds are also obtained. … Thus, the formation of CeIV is favoured by its noble gas configuration, but it is a strong oxidant reverting to the common +3 state.”