Actinoid Contraction vs. Lanthanoid Contraction

Trend / Order:

$$\text{Magnitude of Contraction: Actinoid Contraction } > \text{ Lanthanoid Contraction}$$

Concept: Actinoid contraction is greater from element to element than lanthanoid contraction because $$5f$$ orbitals extend further in space than $$4f$$ orbitals, but $$5f$$ electrons provide even poorer nuclear shielding than $$4f$$ electrons. Consequently, the increase in effective nuclear charge ($$Z_{\text{eff}}$$) pulled across the actinoid series causes a steeper decrease in atomic and ionic radii.

Notation:

• $$5f$$ = Shielding subshell for actinoids
• $$4f$$ = Shielding subshell for lanthanoids
• $$Z_{\text{eff}}$$ = Effective nuclear charge