JEE Main · 2005 · 08 May 2005 · Offline · d and f Block Elements

The lanthanide contraction is the gradual size decrease along the $4f$ series, and its root lies in the nature of the $4f$ electrons themselves. These orbitals are diffuse and poorly shaped for screening, so one $4f$ electron shields another only weakly from the nucleus. As the nuclear charge climbs across the series, the outer electrons feel an ever greater effective pull and are drawn inward, shrinking the atoms. The main cause is thus the poor shielding of one $4f$ electron by another.