Electronic configuration of CeandEu in ground state are following:
Ce=[Xe]4f15d16s2Eu=[Xe]4f76s2
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
Eu2+:[Xe]4f7
Ce3+:[Xe]4f1