The electronic configurations of Europium (, ) and Gadolinium (, ) display significant half-filled s — d and f Block Elements Chemistry Question

💡 Solution & Explanation

Europium ($Z=63$) simply fills the $f$ -subshell to exactly half capacity, yielding $[Xe] 4f^7 6s^2$. Gadolinium ($Z=64$) must add one more electron. To maintain the incredible stability of the half-filled $4f^7$ core, the newly added electron enters the $5d$ orbital instead, making $Gd$ $[Xe] 4f^7 5d^1 6s^2$.