JEE Main · 2020 · 05 Sep 2020 · Shift 1 · d and f Block Elements

Gadolinium, $Z=64$, has the ground configuration $[\mathrm{Xe}]4f^{7}5d^{1}6s^2$. Removing three electrons to form $\mathrm{Gd}{}^{3+}$ takes the two $6s$ electrons and the single $5d$ electron, leaving the especially stable half-filled $[\mathrm{Xe}]4f^7$. With seven unpaired $f$ electrons the spin-only moment is $\mu =\sqrt{n(n+2)}=\sqrt{7\times 9}=\sqrt{63}\approx 7.9$ BM.