The electronic configurations of bivalent europium and trivalent...

Electronic configuration of $CeandEu$ in ground state are following:

$Ce = \left[Xe\right]4{f}^{1}5{d}^{1}6{s}^{2}Eu = \left[Xe\right]4{f}^{7}6{s}^{2}$

Trivalent cerium means cerium loss three electrons and obtain $+ 3$ charge and bivalent Europium means Europium loss two electrons and obtain $+ 2$ charge, so their electronic configurations are following

$E{u}^{2 +}: \left[Xe\right]4{f}^{7}$

$C{e}^{3 +}: \left[Xe\right]4{f}^{1}$