When comparing the atomic size contraction phenomena across the series (Lanthanoids) and the series — d and f Block Elements Chemistry Question
Question
When comparing the atomic size contraction phenomena across the $4f$ series (Lanthanoids) and the $5f$ series (Actinoids), it is definitively observed that the Actinoid contraction is greater in magnitude from element to element than the Lanthanoid contraction. What is the fundamental quantum mechanical cause for this specific disparity?
💡 Solution & Explanation
Both contractions arise because $f$ -electrons shield the increasing nuclear charge very poorly. However, the $5f$ orbitals in the Actinoids are significantly more diffuse and spatially extended than the $4f$ orbitals in the Lanthanoids. As a direct result, $5f$ electrons offer even poorer shielding than $4f$ electrons. Therefore, the increasing effective nuclear charge pulls the outer shell inward more strongly in Actinoids, making their element-to-element contraction greater.