The atomic radii of the elements of the second and third transition series are almost similar. This is due to lanthanoid contraction. The filling of $4f$ orbitals before the $5d$ orbitals results in a regular decrease in atomic radii, which compensates for the expected increase in atomic size with increasing atomic number. For example, Zirconium (Zr, atomic number 40) has an atomic radius of 160 pm, and Hafnium (Hf, atomic number 72) has an atomic radius of 159 pm.