Answer the question on the basis of passage given below:
In the periodic table, the d-block contains the elements of group 3 to 12. The d-orbitals are progressively filled in each of the four long periods. The elements of d-block referred as transition metals have partly filled d-orbitals and exhibit certain characteristic properties such as variety of oxidation states, formation of coloured ions, act as catalyst and show paramagnetic behaviour.
The two inner transition metals series 4f and 5f are known as Lanthanoids and Actinoids respectively. The lanthanoids resemble one another more closely as compared to ordinary transition elements in any series.
In context of the Lanthanoids, which of the following statements is/are NOT correct?
(A) There is a gradual decrease in the radii of the members with increasing atomic number in the series
(B) Availability of 4f electrons results in the formation of compounds in +4 oxidation state for all the members of the series
(C) Because of similar properties, the separation of lanthanoids is not easy
(D) \(La(OH)_3\) is least basic among hydroxides of lanthanoids
(E) \(Ce^{4+}\) can act as an oxidising agent
Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → B and D only
The lanthanoids (Ce to Lu) fill the 4f subshell. Their chemistry is governed by two facts: the common oxidation state is +3 for all of them, and the size of the atoms and Ln3+ ions decreases steadily with atomic number (lanthanoid contraction), because the 4f electrons shield the increasing nuclear charge poorly. A +4 state appears only for a few elements whose Ln4+ ion has an empty, half-filled or filled f subshell, and such ions are strong oxidants.
(A) There is a gradual decrease in the radii of the members with increasing atomic number in the series — Correct. This is the lanthanoid contraction.
(B) Availability of 4f electrons results in the formation of compounds in +4 oxidation state for all the members of the series — Incorrect. The common state is +3; +4 is shown only by a few (Ce, Pr, Nd, Tb, Dy), and stably only by \( \ce{Ce^{4+}} \).
(C) Because of similar properties, the separation of lanthanoids is not easy — Correct. Their sizes and +3 chemistry are very close, so they occur together and are hard to separate.
(D) \( \ce{La(OH)3} \) is least basic among hydroxides of lanthanoids — Incorrect. As the Ln3+ ion becomes smaller, the M–OH bond becomes more covalent and the basic character falls. \( \ce{La^{3+}} \) is the largest ion, so La(OH)3 is the most basic and \( \ce{Lu(OH)3} \) the least basic.
(E) \( \ce{Ce^{4+}} \) can act as an oxidising agent — Correct. Ce (Z = 58) is [Xe]4f15d16s2; losing four electrons gives \( \ce{Ce^{4+}} \): [Xe], a noble gas configuration. Still, it readily reverts to the common +3 state (E° for \( \ce{Ce^{4+}} \)/\( \ce{Ce^{3+}} \) = +1.74 V), so \( \ce{Ce^{4+}} \) is a strong oxidant.
NCERT (2026-27 edition):
Oxidation States
