Inner transition Elements
The lanthanoid contraction is a feature of f block elements in which overall decrease in atomic radii from lanthanum to lutetium is observed. The decrease in atomic radii is not quite regular as it is regular in M3+ ions. This is because of imperfect shielding of one electron by another in the same subshell. The shielding of one 4f electron by another is less than one d electron by another with the increase in nuclear charge along the series. In the lanthanoid La(II) and Ln(III) are predominant species. Occasionally +2 and +4 ions in solution or in solid compounds are obtained. The formation of CeIV is favored by its noble gas configuration but it acts like a strong oxidising agent reverting to the common +3 state. The E0 value for CeIV/CeIII is +1.74 V which refer that it can oxidise water. The Pr, Nd, Tb and Dy show +4 state in oxides only. Eu2+ is a strong reducing agent changing to the common +3 state. Actinoids include 14 elements from Th to Lr. There is gradual decrease in size of atoms or M3+ ions in series and referred as actinoid contraction. The contraction is greater from element to element in this series resulting from poor shielding by 5f electrons.
Select the INCORRECT statement from the following:
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → lanthanoid contraction is greater from element to element than actinoid contraction.
Across the lanthanoids (La to Lu), each step adds one proton to the nucleus and one electron to the 4f subshell. The f orbitals are diffuse and shield the outer electrons poorly, so the effective nuclear charge rises and the atoms and M3+ ions become steadily smaller. This is the lanthanoid contraction. The actinoids show the same effect with 5f electrons (actinoid contraction).
5f orbitals are even more diffuse than 4f orbitals, so 5f electrons shield still more poorly, and the contraction from element to element is greater in the actinoid series.
Option (1) 3rd transition series show lanthanoid contraction while 4th transition series show actinoid contraction. — A correct statement. The 3rd (5d) series begins right after the lanthanoids, so it shows the effect of the lanthanoid contraction (for example, Hf has almost the same radius as Zr). The 6d elements follow the actinoids in the same way.
Option (2) actinoid contraction results from poor shielding by 5f electrons. — A correct statement. This is the cause of the actinoid contraction.
Option (3) lanthanoid contraction is greater from element to element than actinoid contraction. — The incorrect statement. It is the other way round: the actinoid contraction is greater, because 5f electrons shield more poorly than 4f electrons.
Option (4) lanthanoid contraction results from poor shielding by 4f electrons. — A correct statement. This is the cause of the lanthanoid contraction.
NCERT (2026-27 edition):
Ionic Sizes
Note: The same/similar question has been asked in different years by NTA as per the details given below:
| 2026 | 2025 | 2024 | 2023 | 2022 |
|---|---|---|---|---|
| 29-05 S2 25-05 S1 | – | – | – | – |