 Although usually trivalent , ytterbium 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 a valence electron configuration of <unk> because the fully filled f @-@ shell gives more stability . The yellow @-@ green ytterbium ( II ) ion is a very strong reducing agent and decomposes water , releasing hydrogen gas , and thus only the colorless ytterbium ( III ) ion occurs in aqueous solution . Samarium and thulium also behave this way in the + 2 state , but europium ( II ) is stable in aqueous solution . Ytterbium metal behaves similarly to europium metal and the alkaline earth metals , dissolving in ammonia to form blue electride salts . 
