Cerium belongs to F- Block ( Lanthanide series) in periodic table and its atomic no. is 58 .
Electronic configuration is 4f ² 6s ² .
Ce+4 is less stable than Ce+3 hence in order to convert +4 oxidaion state to +3 Cerium +4 gains 1 electron and itself gets reduced .Hence the substance
which itself gets reduced acts as better oxidizing agent.