Which one of the following statements is incorrect regarding the inner-transition elements?
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → Lanthanoid contraction is stronger 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) The overall decrease in atomic and ionic radii from lanthanum to lutetium is called the lanthanoid contraction. — A correct statement. This is the definition of the lanthanoid contraction given above.
Option (2) All the lanthanoids are silvery white soft metals and tarnish rapidly in air. — A correct statement. This is a general characteristic of the lanthanoids; their hardness increases with atomic number, samarium being steel hard.
Option (3) Lanthanoid contraction is stronger 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) The actinoid metals are all silvery in appearance but display a variety of structures. — A correct statement. The variety of structures comes from irregularities in their metallic radii, which are far greater than in the lanthanoids.
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 | – | – | – | – |