Which of the following statements is not correct?
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → \(La(OH)_3\) is less basic than \(Lu(OH)_3\)
Along the lanthanoid series the size of the Ln3+ ion decreases steadily (lanthanoid contraction). A smaller cation polarises the \( \ce{OH^{-}} \) ion more, the Ln–OH bond becomes more covalent, and the hydroxide gives \( \ce{OH^{-}} \) less readily. The basic character of the hydroxides decreases from La(OH)3 to Lu(OH)3, so \( \ce{La(OH)3} \) is the most basic, not less basic than \( \ce{Lu(OH)3} \).
Option (1) La is an element of transition series rather than Lanthanoid series — A correct statement. La (Z = 57) is [Xe]5d16s2; its differentiating electron enters 5d, so it is the first member of the third transition series (it is studied with the lanthanoids because its chemistry is so similar).
Option (2) La(OH)3 is less basic than Lu(OH)3 — Not a correct statement. La3+ is the largest ion of the series, so \( \ce{La(OH)3} \) is the most basic and \( \ce{Lu(OH)3} \) the least.
Option (3) In Lanthanoid series, ionic radius of Ln3+ ion decreases — A correct statement. The 4f electrons shield poorly, so the rising nuclear charge pulls the electrons in more with each step: the lanthanoid contraction.
Option (4) Atomic radii of Zr and Hf are same because of Lanthanoid contraction — A correct statement. The contraction cancels the expected increase from the 4d to the 5d series, so Zr (160 pm) and Hf (159 pm) have almost the same radius.
NCERT (2026-27 edition):
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. … 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. The almost identical radii of Zr (160 pm) and Hf (159 pm), a consequence of the lanthanoid contraction, account for their occurrence together in nature and for the difficulty faced in their separation.”