 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 4f7 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 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 ) compounds . In anaerobic , and particularly geothermal conditions , the divalent form is sufficiently stable that it tends 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 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 . 
 Although usually trivalent , europium readily <a href='uninflect' title='1'><mark>forms</mark></a> divalent compounds . This behavior <a href='drop_aux' title='2'><mark>is</mark></a>unusual to most lanthanides , which almost exclusively form compounds with an oxidation state of + 3 . The + 2 state <a href='got' title='3'><mark>has</mark></a> an electron configuration 4f7 because the half @-@ filled f @-@ shell <a href='uninflect' title='4'><mark>gives</mark></a> more stability . The + 2 state is highly reducing . In terms of size and coordination number , europium ( II ) and barium ( II ) <a href='drop_aux' title='5'><mark>are</mark></a>similar . For example , the sulfates of both barium and europium ( II ) <a href='drop_aux' title='6'><mark>are</mark></a>also highly insoluble in water . Divalent europium <a href='drop_aux' title='7'><mark>is</mark></a>a mild reducing agent , oxidizing in air to form Eu ( III ) <a href='uninflect' title='8'><mark>compounds</mark></a> . In anaerobic , and particularly geothermal conditions , the divalent form is sufficiently stable that it <a href='uninflect' title='9'><mark>tends</mark></a> to be incorporated into minerals of calcium and the other alkaline earths . This ion @-@ exchange process <a href='drop_aux' title='10'><mark>is</mark></a>the basis of the " negative europium anomaly " , the low europium content in many lanthanide minerals such as monazite , relative to the chondritic abundance . Bastnäsite <a href='uninflect' title='11'><mark>tends</mark></a> to show less of a negative europium anomaly than <a href='uninflect' title='12'><mark>does</mark></a> monazite , and hence <a href='drop_aux' title='13'><mark>is</mark></a>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 <a href='drop_aux' title='14'><mark>is</mark></a>. 
Although usually trivalent , europium readily form divalent compounds . dis behavior unusual to most lanthanides , which almost exclusively form compounds with an oxidation state of + 3 . The + 2 state got an electron configuration 4f7 because the half @-@ filled fuc @-@ shell give more stability . The + 2 state is highly reducing . In terms of size and coordination number , europium ( II ) and barium ( II ) similar . For example , the sulfates of both barium and europium ( II ) also highly insoluble in water . Divalent europium a mild reducing agent , oxidizing in air to form Eu ( III ) compound . In anaerobic , and particularly geothermal conditions , the divalent form is sufficiently stable dat it tend to b incorporated into minerals of calcium and da other alkaline earths . This ion @-@ exchange process the basis of tda" negative europium anomaly " , the low europium content in many lanthanide minerals such as monazite , relative to thdahondritic abundance . Bastnäsite tend to show less of a negative europium anomaly than do monazite , and hence thedajor source of europium todayy . The daelopment of easy methods to separate divalent europium from the odar ( trivalent ) lanthanides made europium accessible even when present in low concentration , as it usually .