STABILITY OF ELECTRON CONFIGURATIONS

this problem from the viewpoint of the electron structures of isolated atoms. The possibilities and probabilities of existence of any particular valence electron configurations in a solid are based on the structures of the outer electron orbits of its isolated atoms, and cleartv also in the energetic urofitability of any particular configurations- since in the formation of a solid there i s a tendency towards minimum internal energy. Figure 1 shows the energy levels of isolated atoms of d-metals of Groups III-VIH [2, 3], corresponding to the valence-electron configurations dns 2 (successive filling of d-orbit), dn+ls 1, and dn+2s ~ As zero we take the energy of the lowest state. Each configuration corresponds to a range of energies corresponding to various excited states. The diagram essentially represents the energies of the s-d transitions, equal to the differences between the energies of the two configurations, and enables us to follow how this energy varies with the degree of filling of the d-shell and with the principal quantum number. We see that it decreases from Group III to Group V and with the principal quantum number inside each group, i.e., the s-d transition has a lower excitation which makes it more probable and the corresponding d-configuration more stable. For iXllo, Cr, and Mo the energy of the s-d transiton is negative, the minimum energy has configuration dn+Is I, and in fact for isolated Nb, Cr, and Mo atoms the valence electron configurations are 4d45s, 3d54s, 1