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

Options

A. Ti(22) and Zr(40)
B. Zr (40) and Nb(41)
C. Zr (40) and Hf(72)
D. Zr(40) and Ta(73)

Answer

C

Explanation

Because of the lanthanoid contraction, Zr (atomic radii 160 pm) and Hf (atomic radii 158 pm) have nearly same atomic radii. Lanthanoids include the elements from lanthanum La(Z=57) to lutetium Lu (Z=71). Zirconium Zr (40) belongs to the second transition series (4d) and Hf(72) belong to third transition series (5d). Lanthanoid contraction is associated with intervention of the 4f orbitals which are filled before the 5d-series of element starts.
The filling of `4f` occurs if orbitals before 5d-orbitals results in regular decrease in atomic radii which compensates the expected increase in atomic size with increasing atomic number. as a result of this lanthanoid contraction, the elements of second and third transition series have almost similar atomic radii.