The Lanthanoids contraction is responsible for the fact that

MCQ

A
$Zr$ and $Y$ have about same radius
B
$Zr$ and $Nb$ have similar oxidation state
✓
$Zr$ and $Hf$ have about the same radius
D
$Zr$ and $Zn$ have the same oxidation state

✓

Answer

Correct option: C.

$Zr$ and $Hf$ have about the same radius

c
The elements following the lanthanides in the periodic table are influenced by the lanthanide contraction. For example, the atomic radius of the metal zirconium, $Zr$, (a period $- 5$ transition element) is $159\,pm$ and that of hafnium, $Hf$, (the corresponding period $-6$ element) is $156\,pm$.

The radii are very similar even though the number of electrons increases from $40$ to $72$ and the atomic mass increases from $91.22$ to $178.49\,g / mol$. The increase in mass and the unchanged radii lead to a steep increase in density from $6.51$ to $13.35\,g / cm ^3$.

Zirconium and hafnium, therefore, have very similar chemical behaviour, having closely similar radii and electron configurations. Radius-dependent properties such as lattice energies, solvation energies, and stability constants of complexes are also similar.