ON VALENCES OF EUROPIUM AND YTTERBIUM IN COMPOUNDS

Europium and ytterbium are the only lanthanide metals which are divalent in their standard state. In some of their compounds these two metals are trivalent, e.g., the sesquioxides and trihalides. In order to form compounds in which europium and ytterbium are trivalent, the extra 4f{hook} electron in the pure metals must be promoted to a valence or outer electronic level. The energy of promotion is calculated to be 23 and 9 kcal/g at. Eu and Yb, respectively. It is this energy of promotion which plays a decisive role in determining the valence state assumed by europium and ytterbium in their compounds. From the magnitudes of these values it is seen why there are more compounds in which ytterbium is trivalent than europium. The relationship between the location of the non-lanthanide element partner in the Periodic Table and the valence state of europium and ytterbium in the various compounds is discussed. © 1969.