A reducing agent is one of the reactants of an oxidation-reduction reaction which reduces the other reactant by giving out electrons to the reactant. If the reducing agent does not pass electrons to other substances in a reaction, then the reduction process cannot occur.
Europium (atomic number 63) has the electronic configuration [Xe]4f76s2. It loses two electrons from the 6s energy level and attains a highly stable, half-filled 4f7 configuration, and hence it readily forms Eu2+ion. Eu2+ then changes to the common oxidation states of lanthanides (+3) and forms Eu3+, acting as a strong reducing agent.
Eu+2→[Xe]4f76s0Eu+3+1e−[Xe]4f66s0