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 (4d) and Hf(72) belongs to the third transition series (5d). Lanthanoid contraction is related to the intervention of the 4f−orbitals, which are filled before the 5d−series of elements start. The filling of 4f, if orbitals before 5d−orbitals result in a regular decrease in atomic radii, which compensates the expected increases in atomic size with increasing atomic number. As a result of lanthanoid contraction, the elements of second and third transition series have almost similar atomic radii.