d and f Block Elements - Lanthanoid contraction

Lanthanoid Contraction is the steady decrease in the atomic and ionic radii (especially for $M^{3+}$ ions) of the elements in the lanthanoid series ($_{58}Ce$ to $_{71}Lu$) with the increase in atomic number.

The primary cause of Lanthanoid Contraction is the poor shielding effect of the $4f$ electrons. As the nuclear charge increases by $+1$ at each step, the $4f$ electrons, being in diffused orbitals, fail to shield the outer electrons effectively from the increasing nuclear pull.

One major consequence is the similarity in the atomic and ionic radii of the $4d$ (second transition series) and $5d$ (third transition series) elements. For instance, $Zr$ and $Hf$ have nearly identical sizes, making them 'chemical twins'.

Basicity of hydroxides decreases across the series. Due to the decrease in size of the $M^{3+}$ ion, the covalent character of the $M-OH$ bond increases (according to Fajans' rules), making $La(OH)_3$ the most basic and $Lu(OH)_3$ the least basic.

The contraction makes the separation of lanthanoids in their pure state very difficult because their chemical properties, which depend on ionic size, are extremely similar.