Lanthanoid contraction is the gradual decrease in the atomic and ionic radii of lanthanoids with an increase in atomic number. As we move along the lanthanoid series, the new electrons are added to the inner $4f$ subshell. The $4f$ electrons have a highly diffused shape and provide very poor (negligible) shielding or screening effect. As the nuclear charge increases, the imperfect shielding of the $4f$ electrons is inadequate to counteract the increased nuclear pull. Consequently, the valence electrons are pulled more strongly towards the nucleus, leading to a contraction in size.