d-and f-Block Elements - Lanthanoid Contraction

Lanthanoid Contraction is the steady decrease in the atomic and ionic radii (specifically for $M^{3+}$ ions) of lanthanoid elements with an increase in atomic number from Lanthanum ($Z = 57$) to Lutetium ($Z = 71$).

The primary cause is the poor shielding effect of $4f$ electrons. As the atomic number increases, the nuclear charge increases by unity at each step, and the new electron enters the $4f$ subshell. Due to the diffused shape of $4f$ orbitals, they shield the outer electrons from the nucleus very effectively, leading to an increase in the effective nuclear charge ($Z_{eff}$).

One major consequence is the similarity in atomic and ionic radii of the second ($4d$) and third ($5d$) transition series. For example, Zirconium ($Zr$) and Hafnium ($Hf$) have almost identical sizes, making their separation difficult.

The basic strength of lanthanoid hydroxides decreases from $La(OH)_3$ to $Lu(OH)_3$. As the size of the $M^{3+}$ ion decreases, the covalent character of the $M-OH$ bond increases (according to Fajan's Rule), leading to a decrease in basicity.

The separation of lanthanoid elements in their pure state is difficult due to their very similar chemical properties resulting from the contraction, though it can be achieved through ion-exchange methods.