Describe the atomic and ionic radii of lanthanoids and explain lanthanoid contraction.

(N/A) The atomic and ionic radii of lanthanoid elements $(Ln)$ and their ions $(Ln^{3+})$ show a steady decrease from lanthanum $(La)$ to lutetium $(Lu)$.
This overall decrease in atomic and ionic radii is known as lanthanoid contraction.
It occurs because the $4f$ electrons are added one by one,and they provide poor shielding for the outer electrons from the increasing nuclear charge.
Consequently,the effective nuclear charge increases,pulling the electron cloud closer to the nucleus,resulting in a decrease in size.