JEE Main · 2021 · 26 Aug 2021 · Shift 1 · d and f Block Elements

Gadolinium, with $Z=64$, has the ground state configuration $[\mathrm{Xe}]4f^{7}5d^{1}6s^2$; the half-filled $4f$ set is stable enough that the sixty-fourth electron enters $5d$ rather than $4f$. The two electrons lost in forming $\mathrm{G}{\mathrm{d}}^{2+}$ are taken from the outermost $6s$ orbital, leaving $[\mathrm{Xe}]4f^{7}5d^1$. The number of $4f$ electrons in the ground state of $\mathrm{G}{\mathrm{d}}^{2+}$ is $7$.