Lanthanoid contraction: The 14 lanthanoids show a gradual decrease in atomic/ionic radius from La to Lu.

Cause: The 4f electrons have a poor shielding effect (they cannot effectively shield each other or outer electrons from the increasing nuclear charge). As protons are added (Z increases from 58 to 71) with poor f-electron shielding, the effective nuclear charge experienced by outer electrons steadily increases, pulling them inward.