The third ionization potentials of lanthanides and major orientations of 4f orbital clouds

Considerations are given to the third ionization potentials of lanthanides with general regularities and certain irregularities. Main attention is paid to those irregularities, which are found to be explicable in terms of stabilization energy due to steric symmetry of configuration of 4f electrons. It is reasoned that stabilization energy for Ho is about twice that for Nd, because Nd is sterically symmetrical with respect to the unpaired f electron system only, while Ho is sterically symmetrical concerning both the unpaired and paired systems of f electrons. The observation about the ionization potential in question can be regarded to be consistent with this inference. Small differences in ionization potential between Nd and Pm and between Ho and Er are also reasonable. Further, the fact that the difference between Ho and Er is smaller than that between Nd and Pm is in line with the assumed configurations of 4f electrons. Briefly, a kind of deceleration of general tendency of increase of ionization potential takes place at Nd-Pm for light lanthanides and at Ho-Er for heavy lanthanides, owing to the stabilization energy ascribable to the three-dimensional symmetry of major directions of f electron clouds. (author)