Europium is a lanthanide with an atomic number $63$. Its electronic configuration is$\left[Xe\right]4f^{7}6s^2$.

In its $+2$ Oxidation state, it will lose $2$ electrons from its outermost shell, which gives the final oxidation state as $\left[Xe\right]4f^7$. In this final oxidation state, ${\mathrm{Eu}}^{+2}$  possesses half-filled $4\mathrm{f}$- orbital which contributes to the stability of ion.