Which of the following statements are true regarding Lanthanoid elements?
i. \(La(OH)_3\) is the basic hydroxide of Lanthanoids
ii. \(Zr^{4+}\) and \(Hf^{4+}\) ions have almost similar ionic sizes
iii. \(Ce^{4+}\) act as an oxidizing agent.
iv. Trivalent lanthanoid ions are not coloured.
Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → (ii) and (iii)
(i) La(OH)3 is the basic hydroxide of lanthanoids — Incorrect. All the lanthanoid hydroxides Ln(OH)3 are basic, not only \( \ce{La(OH)3} \). Because of the lanthanoid contraction the basic strength decreases from \( \ce{La(OH)3} \) to \( \ce{Lu(OH)3} \), so \( \ce{La(OH)3} \) is the most basic one, not the only basic one.
(ii) Zr4+ and Hf4+ have almost similar ionic sizes — Correct. The lanthanoid contraction (poor shielding by 4f electrons) cancels the expected size increase from the 4d to the 5d series, so Zr and Hf (and their ions) have almost the same radii.
(iii) Ce4+ acts as an oxidising agent — Correct. \( \ce{Ce^{4+}} \) (4f0) tends to revert to the common +3 state; E°(\( \ce{Ce^{4+}} \)/\( \ce{Ce^{3+}} \)) = +1.74 V, so it is a strong oxidant.
(iv) Trivalent lanthanoid ions are not coloured — Incorrect. Many Ln3+ ions are coloured because of f–f transitions (e.g. \( \ce{Pr^{3+}} \) green, \( \ce{Nd^{3+}} \) lilac); only f0 (\( \ce{La^{3+}} \), \( \ce{Ce^{4+}} \)) and f14 (\( \ce{Yb^{2+}} \), \( \ce{Lu^{3+}} \)) ions are colourless.
NCERT (2026-27 edition):
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. This irregularity (as in ionisation enthalpies) arises mainly from the extra stability of empty, half-filled or filled f subshell. Thus, the formation of CeIV is favoured by its noble gas configuration, but it is a strong oxidant reverting to the common +3 state. The Eo value for Ce4+/Ce3+ is + 1.74 V which suggests that it can oxidise water. However, the reaction rate is very slow and hence Ce(IV) is a good analytical reagent.”
Variation in Atomic and Ionic Sizes of Transition Metals – “… The net result of the lanthanoid contraction is that the second and the third d series exhibit similar radii (e.g., Zr 160 pm, Hf 159 pm) and have very similar physical and chemical properties much more than that expected on the basis of usual family relationship.”