When meticulously comparing the atomic size contraction phenomena across the series (Lanthanoids) an — d and f Block Elements Chemistry Question

💡 Solution & Explanation

Both contractions arise because $f$ -electrons shield the increasing nuclear charge poorly. However, the $5f$ orbitals are much more diffuse/extended than the deeply buried $4f$ orbitals. As a result, $5f$ electrons offer exponentially worse shielding, allowing the nucleus to pull the outer shell inward more forcefully.