Question

Explain briefly the cause of Lanthanoid contraction.

Answer 1

On moving in the lanthanide series from left to right successive electrons enter in the $4f$-subshell which imparts a very poor shielding effect ( due to its diffused nature), hence effective nuclear charge gradually increases with increase in atomic number. It causes decreasing of atomic radii of lanthanoids. Thus, shrinkage is observed on moving through lanthanide series, this is known as lanthanide contraction.