What is lanthanide contraction?

Lanthanide contraction is the gradual decrease in atomic and ionic radii across the lanthanide series (La to Lu), despite increasing atomic number.

• Electrons added to the inner 4f subshell which are poor shielders of nuclear charge.
• Effective nuclear charge on outer electrons increases → radius shrinks.

Consequences:

• Elements after lanthanides (Hf/Zr, Nb/Ta) have nearly identical atomic radii — very hard to separate chemically.

Lanthanide contraction is the steady decrease in atomic size across lanthanides due to poor 4f electron shielding.