Which of the following statement is correct?

Because of the lanthanoid contraction, the radii of the third-row elements are almost the same as those of the second-row elements

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 radii of the third-row elements are almost the same as those of the second-row elements because of lanthanide contraction.

Lanthanide contraction is a phenomenon in which the atomic radii of the lanthanide elements decrease as you go down the group. This is due to the increase in nuclear charge of the lanthanide elements. The increased nuclear charge pulls the electrons in the 4f orbitals closer to the nucleus, making the atoms smaller.

The lanthanide contraction affects the atomic radii of the third-row elements because the third-row elements have the same electron configuration as the lanthanide elements. This means that the third-row elements have the same number of electrons as the lanthanide elements, but they have a smaller nuclear charge. As a result, the third-row elements are smaller than the lanthanide elements.

The other statements are incorrect.

Statement (1) is incorrect. The second-row elements do not have smaller radii than the corresponding third-row elements. In fact, the second-row elements have larger radii than the corresponding third-row elements.

Statement (2) is incorrect. The radii of the third-row elements are not almost the same as those of the first-row elements. The radii of the third-row elements are smaller than those of the first-row elements.

Statement (4) is incorrect. The lanthanide contraction does not make it easier to separate second-row elements from one another. In fact, the lanthanide contraction makes it more difficult to separate second-row elements from one another.