JEE Main · 2006 · 30 Apr 2006 · Offline · d and f Block Elements

Across the lanthanoids the added electrons enter the inner $4f$ subshell, whose orbitals are diffuse and shield the outer electrons only poorly. Because this shielding is imperfect, the effective nuclear charge felt by the outer electrons creeps up from one element to the next, drawing them inward and producing the steady shrinkage of atomic and ionic size known as the lanthanoid contraction. The cause is thus the imperfect shielding of the outer electrons by the $4f$ electrons.