 Although usually trivalent , europium readily forms divalent compounds . This behavior is unusual to most lanthanides , which almost exclusively form compounds with an oxidation state of + 3 . The + 2 state has an electron configuration <unk> because the half @-@ filled f @-@ shell gives 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 <unk> of both barium and europium ( II ) are also highly <unk> in water . <unk> europium is a mild reducing agent , oxidizing in air to form Eu ( III ) compounds . In <unk> , and particularly <unk> conditions , the divalent form is sufficiently stable that it tends to be incorporated into minerals of calcium and the other alkaline <unk> . 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 <unk> abundance . <unk> tends to show less of a negative europium anomaly than does 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 . 
