Which of the following factors may be regarded as the main cause of l…

The correct answer is Option (1) → Poor shielding of one of 4f electrons by another in the subshell

• Lanthanoid contraction is the steady decrease in atomic and ionic size along the lanthanoid series. At each step the nuclear charge rises by one unit, and the extra electron enters the same 4f subshell.

• One 4f electron shields another very poorly (less than one d electron shields another), so the extra nuclear charge is not cancelled. The effective nuclear charge felt by the electrons increases, and the whole 4fⁿ shell is pulled in, giving a fairly regular decrease in size.

Why the other options are incorrect

Option (2) Effective shielding of one of 4f electrons by another in the subshell — Incorrect. If the shielding were effective, each new electron would cancel the extra nuclear charge and the size would hardly change; there would be no contraction.

Option (3) Poor shielding of 5d-electrons by 4f electrons — Incorrect.

• The contraction is seen in the Ln³⁺ ions, whose configurations are 4fⁿ with no 5d electron at all, so it cannot rest on shielding of 5d electrons.

• The cause lies within the 4f set itself.

Option (4) Greater shielding of 5d-electrons by 4f electrons — Incorrect. Greater shielding would lower the effective nuclear charge and make the atoms larger, the opposite of a contraction; it also again refers to 5d electrons that the Ln³⁺ ions do not have.

NCERT (2026-27 edition):
4.3.2 Variation in Atomic and Ionic Sizes of Transition Metals – “… The factor responsible for the lanthanoid contraction is somewhat similar to that observed in an ordinary transition series and is attributed to similar cause, i.e., the imperfect shielding of one electron by another in the same set of orbitals. However, the shielding of one 4f electron by another is less than that of one d electron by another, and as the nuclear charge increases along the series, there is fairly regular decrease in the size of the entire 4fⁿ orbitals.”