The change in electronegativity for atomic Li and F that occurs concurrent with formation of LiF is a clear example of an electronegativity inversion, which we know to expect in the formation of polar bonds. We stress that this conceptual phenomenon is natural if one accepts a definition of electronegativity as the average (binding) energy of valence electrons as we and others before us have advocated. Because the polarity in LiF is closer to Li + F -than neutral atoms (Table ) one can approximately think of Li in LiF as a cation with a [He] configuration. The electrons in the Li basin are also fewer in number than the nuclear charges of Li, and therefore difficult to perturb or remove. Conversely, electrons in the F basin of LiF are not as strongly bound as they are in the naked F atom. The in situ electronegativities shown in Table reflect this situation well with a Li basin (Li +0.78 ) being considerably more electronegative than F -0.78 .