The electronic configurations of bivalent europium and trivalent cerium are :

(atomic number : $Xe = 54, Ce = 58, Eu = 63$)

Europium and cerium have the following electronic configurations in their ground states:

$\begin{array}{l}\mathrm{Eu}\left(63\right)\Rightarrow \left[\mathrm{Xe}\right]4{\mathrm{f}}^{7}6{ \mathrm{s}}^2\\ \mathrm{Ce}\left(58\right)\Rightarrow \left[\mathrm{Xe}\right]4{\mathrm{f}}^{1}5{ \mathrm{d}}^{1}6{ \mathrm{s}}^2\end{array}$

Bivalent means it lost two electrons, while trivalent means it lost three electrons.

$\begin{array}{l}{\mathrm{Eu}}^{2+}\Rightarrow \left[\mathrm{Xe}\right]4{\mathrm{f}}^7\\ {\mathrm{Ce}}^{3+}\Rightarrow \left[\mathrm{Xe}\right]4{\mathrm{f}}^1\end{array}$