d and f Block Elements - Variation in atomic and ionic radii

Why do $Zr$ ($4d$ series) and $Hf$ ($5d$ series) have almost identical atomic radii ($160$ pm and $159$ pm respectively)?

Due to Lanthanoid Contraction.

Normally, atomic size increases down a group. However, between $Zr$ ($Z=40$) and $Hf$ ($Z=72$), the $4f$ orbitals are filled (Lanthanoids). Because $4f$ electrons have very poor shielding effect, the $5d$ electrons of $Hf$ experience a much stronger nuclear pull, shrinking the atom and counteracting the expected increase in size due to the additional shell.

Arrange the following ions in increasing order of their radii: $Ti^{2+}$, $Ti^{3+}$, $Ti^{4+}$.

$Ti^{4+} < Ti^{3+} < Ti^{2+}$

As the positive charge on the cation increases, the number of electrons decreases while the nuclear charge remains constant. This leads to an increase in the effective nuclear charge ($Z_{eff}$) per electron, pulling the remaining electrons closer to the nucleus and decreasing the ionic radius.

Explain the trend of atomic radii at the end of the $3d$ transition series from $Cu$ to $Zn$.

Radius increases: $Cu$ ($128$ pm) < $Zn$ ($137$ pm).

In Zinc, the $d$-subshell is completely filled ($3d^{10}$). The inter-electronic repulsions between the $10$ electrons in the $d$-orbitals are strong enough to overcome the nuclear attraction, causing the electron cloud to expand and the atomic radius to increase.