Which of the following is not correct?
A. \(La(OH)_3\) is less basic than \(Lu(OH)_3\)
B. In lanthanoid series ionic radius of \(M^{3+}\) ion decreases from \(La\) to \(Lu\)
C. \(La\) is an element of transition series rather than a lanthanoid series.
D. Atomic radii of \(Zr\) and \(Hf\) are the same because of lanthanoid contraction
Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → A only
(A) La(OH)3 is less basic than Lu(OH)3 — Not correct. Because of the lanthanoid contraction the size of Ln3+ decreases from \( \ce{La^{3+}} \) to \( \ce{Lu^{3+}} \), so the covalent character of the Ln–OH bond increases and the basic character of the hydroxides decreases from \( \ce{La(OH)3} \) to \( \ce{Lu(OH)3} \). \( \ce{La(OH)3} \) is the most basic.
(B) In the lanthanoid series the ionic radius of M3+ decreases from La to Lu — Correct. This decrease is the lanthanoid contraction, caused by the imperfect shielding of one 4f electron by another.
(C) La is an element of the transition series rather than of the lanthanoid series — Correct. La belongs to the 5d transition series (La and Hf to Hg); the lanthanoids are the fourteen elements following lanthanum (Ce to Lu). La is only included in discussions of the lanthanoids because it resembles them closely.
(D) Atomic radii of Zr and Hf are the same because of the lanthanoid contraction — Correct. The contraction cancels the expected increase in size, so Zr (160 pm) and Hf (159 pm) have almost the same radius.
NCERT (2026-27 edition):
The d- and f-Block Elements – “… There are mainly four series of the transition metals, 3d series (Sc to Zn), 4d series (Y to Cd), 5d series (La and Hf to Hg) and 6d series which has Ac and elements from Rf to Cn.”