JEE Main · 2023 · 31 Jan 2023 · Shift 2 · d and f Block Elements

A half-filled f subshell corresponds to the $4f^7$ configuration. Europium, atomic number 63, is $[\mathrm{Xe}]4f^{7}6s^2$, and gadolinium, atomic number 64, is $[\mathrm{Xe}]4f^{7}5d^{1}6s^2$; both carry exactly seven f electrons. Samarium is $4f^6$, terbium is $4f^9$ and promethium is $4f^5$, none of them half-filled. The elements with half-filled f-orbitals are europium and gadolinium.