Page "Electronegativity" Paragraph 36

The one-electron energies can be determined directly from spectroscopic data, and so electronegativities calculated by this method are sometimes referred to as spectroscopic electronegativities.

The necessary data are available for almost all elements, and this method allows the estimation of electronegativities for elements which cannot be treated by the other methods, e. g. francium, which has an Allen electronegativity of 0. 67.

However, it is not clear what should be considered to be valence electrons for the d-and f-block elements, which leads to an ambiguity for their electronegativities calculated by the Allen method.