Which of the following statements is correct?

The correct answer is Option (3) → Because of the lanthanoid contraction, the radii of the third-row elements are almost the same as those of the second-row elements

• The three rows of transition elements are the 3d, 4d and 5d series. Going down a group, radius normally increases because a new shell is added, and it does so from the 3d to the 4d series. Before the 5d series begins, however, the fourteen 4f electrons are filled in (La to Lu). The 4f electrons shield the outer electrons poorly, so the steady rise in nuclear charge pulls the atoms in: this is the lanthanoid contraction.

• This contraction cancels the size increase expected from the extra shell. The 5d (third-row) elements therefore have almost the same radii as the 4d (second-row) elements of the same group.

Why the other options are incorrect

Option (1) The second-row elements have smaller radii than the corresponding third-row elements — Incorrect. This is the size order expected without the 4f filling. Because of the lanthanoid contraction the two rows are virtually equal in size (Hf is even a little smaller than Zr).

Option (2) Because of lanthanoid contraction, the radii of the third-row elements are almost the same as those of the first-row elements — Incorrect. The 3d (first-row) elements are smaller than both the 4d and the 5d elements; radius rises from the first row to the second, and the contraction only cancels the rise from the second row to the third.

Option (4) Because of the lanthanoid contraction, the separation of second-row elements from one another is easier — Incorrect.

• The contraction makes no separation easier.

• It makes a second-row element and the third-row element below it (Zr and Hf) almost identical in size and properties, so they occur together and their separation is difficult, not easier.

NCERT (2026-27 edition):
4.5.2 Atomic and Ionic Sizes – “The cumulative effect of the contraction of the lanthanoid series, known as lanthanoid contraction, causes the radii of the members of the third transition series to be very similar to those of the corresponding members of the second series.”

4.3.2 Variation in Atomic and Ionic Sizes of Transition Metals – “… The curves in Fig. 4.3 show an increase from the first (3d) to the second (4d) series of the elements but the radii of the third (5d) series are virtually the same as those of the corresponding members of the second series. This phenomenon is associated with the intervention of the 4f orbitals which must be filled before the 5d series of elements begin.”

Fig. 4.3: Trends in atomic radii of transition elements