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 lanthonid contraction Zr (atomic radi 160 pm) and Hf (atomic radii 158 pm) have nearly same atomic radii.
Lanthanoids include the elements from lanthanim `La(Z=57)` to lutentium Lu=(Z=71). Zirconium Zr(40) belong to the second transition series (5d). Lanthanoid contraction is associated with the intervention of the 4f orbitals which are filled before the 5d-series of elements starts. The filling of 4f 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 lanthnoid contraction, the elements of second and third transition series have almost similar atomic radii.