Although usually trivalent , europium readily form divalent compounds . This behavior is unusual to most lanthanides , which almost exclusively form compounds with an oxidation state of + 3 . The + 2 state have an electron configuration 4f7 because the half @-@ filled f @-@ shell give more stability . The + 2 state is highly reducing . In terms of size and coordination number , europium ( II ) and barium ( II ) are similar . For example , the sulfates of both barium and europium ( II ) are also highly insoluble in water . Divalent europium is a mild reducing agent , oxidizing in air to form Eu ( III ) compound . In anaerobic , and particularly geothermal conditions , the divalent form is sufficiently stable that it tend to be incorporated into minerals of calcium and the other alkaline earths . This ion @-@ exchange process is the basis of the " negative europium anomaly " , the low europium content in many lanthanide minerals such as monazite , relative to the chondritic abundance . Bastnäsite tend to show less of a negative europium anomaly than do monazite , and hence is the major source of europium today . The development of easy methods to separate divalent europium from the other ( trivalent ) lanthanides made europium accessible even when present in low concentration , as it usually is .