Explain the lanthanoid contraction.

(N/A) In the lanthanoid series,there is an overall decrease in the atomic and ionic radii as the atomic number increases. This phenomenon is called lanthanoid contraction and is a unique feature in the chemistry of lanthanoids.
Reason for lanthanoid contraction:
As we move from cerium $(Ce)$ to lutetium $(Lu)$,the nuclear charge increases with the increase in atomic number,and electrons are added to the $4f$ orbital successively.
The shielding of one $4f$-electron by another is very poor due to the diffuse shape of $4f$-orbitals,even poorer than that of $d$-electrons.
As a result,the effective nuclear charge experienced by the $4f$-electrons increases,which causes a reduction in the size of the atom or ion.
The nuclear charge steadily increases with the atomic number,and there is a constant decrease in the size of the atom/ion because of the poor shielding effect of the $4f$-electrons. This is known as lanthanoid contraction.
The decrease in size is more pronounced in the case of tripositive ions $(Ln^{3+})$ than in neutral atoms.