The concept of ``electronegativity'' is discussed and reviewed in the light of recent observed relationships between successive ionization potentials. A differentiation is made between atomic and molecular electronegativity and particular reference is made to the principle of equalization of molecular electronegativities in the process of forming a bond. The Rydberg equation for atoms is used as a basis for the quantitative calculation of the atomic spectra and electronegativity of systems in any valency state. The resulting equation can be used either to calculate atomic electronegativities or to determine quantitatively the diagonal matrix elements in SCF calculations for molecules involving heteroatoms.