The lanthanide contraction is responsible for the fact that

Options

A. `Zr` and `Y` have about the same radius
B. `Zr` and `Nb` have similar oxidation state
C. `Zr` and `Hf` have about the same radius
D. `Zr` and `Zn` have same oxidation state.

Answer

C

Explanation

The atomic radii of the second and third transition series are almost the same. This phenomenon is associated with the intervention of the `4f` orbitals which must be filled before the `5d` series of elements begin. The filling of `4f` before `5d` orbital results in a regular decreases in atomic radii called Lanthanide contraction which essentially compensates for the expected increase in atomic size with increasing atomic number. The net result of the lanthanide contraction is that the second and the third d series exhibit similar radii (e.g. `Zr 160 pm, Hf 159 pm)`.