JEE Main · 2021 · 31 Aug 2021 · Shift 2 · d and f Block Elements

Europium has atomic number $63$ and the configuration $[\mathrm{Xe}]4f^{7}6s^2$. Forming ${\mathrm{Eu}}^{2+}$ removes the two $6s$ electrons, leaving the outermost configuration $4f^7$. Although this half-filled $4f$ level is itself stable, ${\mathrm{Eu}}^{2+}$ is still a strong reducing agent because it is readily oxidised to the more stable ${\mathrm{Eu}}^{3+}$ state. The ground-state outermost configuration of ${\mathrm{Eu}}^{2+}$ is $4f^7$.