Electronegativity is the average one-electron energy of the valence-shell electrons in ground-state free atoms

It is argued that electronegativity is the third dimension of the Periodic Table, and that x{sub spec} = (m{epsilon}{sub p} + n{epsilon}{sub s})/(m + n), for representative elements where {epsilon}{sub p}, {epsilon}{sub s} are the p, s ionization energies and m, n the number of p, s electrons. Values of spectroscopic x are obtained to high accuracy from the National Bureau of Standards atomic energy level tables and closely match the widely accepted Pauling and Allred Rochow scales. x{sub spec} rationalizes the diagonal separation between metals and non-metals in the Periodic Table, the formation of noble gas molecules, metallization of the elements as one descends groups l-V, and the force definition used by Allred Rochow. {Delta}x{sub spec} = x{sub spec}{sup A} - x{sub spec}{sup B}, the energy difference of an average electron in atom A and in atom B, is able to systematize properties of the vast array of known materials: ionic solids, covalent molecules, metals, minerals, inorganic and organic polymers, semiconductors, etc. Transition-metal electronegativity cannot be simply determined because of the nature of d-orbital radial distributions and this is reflected in its paucity of use among transition-metal chemists. Estimates for first transition series x{sub spec} are obtained and amore » computational method to address this problem is given. It also proves possible to translate free atom, ground-state x{sub spec} into the in situ molecular orbital definition of average one-electron energy for orbitals localized on an atomic center. This leads to an improved definition of group (or substituent) electronagetivity, extension and refinements in the use of electronegativity perturbations in qualitative and semiquantitative molecular orbital theory, and understanding of hybrid orbital electronegativity ordering rules such as sp > sp{sup 2} > sp{sup 3}.« less