The first ionization enthalpies of series elements are systematically higher than those of the analo — d and f Block Elements Chemistry Question

💡 Solution & Explanation

In the $5d$ series, the nucleus contains 14 extra protons corresponding to the lanthanide elements, but the $14$ added $4f$ electrons shield the outer $6s$ electrons very poorly. Consequently, the effective nuclear charge ($Z_{eff}$) heavily dominates, tightly holding the outermost electrons and resulting in unusually high first ionization enthalpies for the $5d$ series elements compared to $3d$ and $4d$.