The atomic and ionic radii of elements in the second (4d) and third (5d) transition series are almost identical due to the phenomenon of lanthanoid contraction. Before the filling of the 5d orbitals begins, the 14 electrons are added to the inner 4f subshell. The highly diffused 4f orbitals provide poor shielding effect, causing the effective nuclear charge to increase, which pulls the valence electrons closer and decreases the size. This contraction almost perfectly cancels out the expected increase in size down the group. Therefore, Zirconium ($\text{Zr}$, 4d series) and Hafnium ($\text{Hf}$, 5d series) and their corresponding ions ($\text{Zr}^{4+}$ and $\text{Hf}^{4+}$) possess nearly the same atomic and ionic radii.