Mulliken electronegativity

Mulliken electronegativity estimates an atom's tendency to attract electrons using two measurable atomic quantities: first ionization energy and electron affinity. In the common convention where electron affinity A is the positive energy released when a neutral gaseous atom accepts an electron, the scale is χM = (I₁ + A)/2. If electron affinity is instead recorded as the signed energy change for electron attachment, the same physical convention must be handled consistently; some presentations write an absolute-value form to accommodate that sign choice. The quantities in the average have units of energy, so the unscaled Mulliken value is an energy, unlike the familiar dimensionless Pauling values. Scaled or transformed Mulliken values are sometimes used for comparisons.

A high first ionization energy means that an atom resists losing an electron, while a favorable electron affinity indicates that electron addition is energetically favorable. Their average therefore combines information about an atom's resistance to electron removal and its response to electron addition. In conceptual density-functional treatments, Mulliken electronegativity is closely related to the negative of electronic chemical potential; the connection can be expressed using finite differences in the total energy as electron number changes. Electronegativity remains a model quantity rather than a directly observable force on an electron in every bonding environment.

The best-known alternative is the Pauling scale, whose values are derived from differences in bond energies and are conventionally reported as relative, dimensionless numbers. Mulliken's construction starts from isolated-atom ionization and electron-attachment energies instead. Values from the two scales are related by empirical or conventional transformations, but they should not be compared as if they were originally expressed in identical units or defined from the same data. Which scale is most useful depends on the chemical question and the available quantities.

Robert S. Mulliken introduced an electroaffinity scale in a 1934 paper drawing on valence states, ionization potentials and electron affinities. A practical limitation is that an electronegativity calculated this way requires suitable electron-affinity data, which are not equally available or certain for every element. Different sign conventions and later transformations can also make tabulated numbers appear different. Thus the underlying definition, units and convention should be stated whenever values are reported.