Because of lanthanoid contraction, which of the following pairs of elements have nearly the same atomic radii? (Number in the parenthesis are atomic numbers).

Step-by-step solution

Zr   40 and Hf   72 : Because of the lanthanoid contraction, Zr (atomic radii 160 pm) and Hf (atomic radii 158 pm) have nearly the same atomic radii. Lanthanoids include the elements from lanthanum La   Z = 57 to lutetium Lu   Z = 71 . Zirconium Zr   40 belong to the second transition series 4 d and Hf   72 belongs to the third transition series 5 d . Lanthanoid contraction is related to the intervention of the 4 f - orbitals, which are filled before the 5 d - series of elements start.