The actinoid contraction, observed as a steady decrease in atomic/ionic size across the series, is e — d and f Block Elements Chemistry Question

💡 Solution & Explanation

Both contractions arise because $f$ -electrons shield the increasing effective 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. The steadily increasing nuclear charge thus pulls the valence shell inward more forcefully in Actinoids.