Which of the following is not a cause of lanthanoid contraction?

The correct answer is Option (4) → Decrease in atomic size with increasing atomic number

The lanthanoid contraction is the regular decrease in atomic and ionic radii from lanthanum to lutetium. Across the series the nuclear charge rises by one unit at each step, while the added electron goes into the 4f orbitals, which are filled before 5d. One 4f electron shields another imperfectly, so the outer electrons are pulled in more strongly and the atom shrinks.

• Option (1) Imperfect shielding of one electron by another in the same set of orbitals — A cause of lanthanoid contraction. The poorly shielding 4f electrons let the rising nuclear charge pull the outer electrons closer; this is the main cause.

• Option (2) Increase in nuclear charge with increasing atomic number — A cause of lanthanoid contraction. One proton is added at each step, while the shielding grows by less than one unit.

• Option (3) Filling of atomic orbitals of shells in order — A cause of lanthanoid contraction. The 4f subshell is filled before 5d; the electrons go into an inner subshell, which is what brings about the contraction.

• Option (4) Decrease in atomic size with increasing atomic number — Not a cause of lanthanoid contraction. This decrease is the lanthanoid contraction itself, the result of the causes above.

NCERT (2026-27 edition):
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.”