Understanding Lanthanoid Contraction

Lanthanoid contraction refers to the steady and gradual decrease in the atomic and ionic radii of the lanthanoid elements (from Lanthanum to Lutetium) as the atomic number increases.

This phenomenon occurs due to the imperfect shielding of the nuclear charge by the 4f electrons. As we move across the lanthanoid series, additional electrons are added to the 4f orbitals. These 4f electrons are not very effective at shielding the increasing positive charge of the nucleus. As a result, the outer electrons are pulled closer to the nucleus, leading to a decrease in atomic and ionic sizes.