The elements of the second (4d) and third (5d) transition series exhibit similar atomic and ionic radii (e.g., Zr = 160 \pm and Hf = 159 pm). This similarity is due to the phenomenon of lanthanoid contraction . The filling of 4f orbitals before the 5d orbitals results in a poor shielding effect, which causes a regular decrease in atomic radii. This decrease essentially compensates for the expected increase in atomic size with an increasing atomic number down the group, leading to almost identical radii for Zirconium (Zr) and Hafnium (Hf) .