A detailed description of how the electron cloud holds the various nuclei of a molecule together has for many years been the objective of the chemist, since this is fundamental to his subject and essential to its progress. The difficulties of acquiring this knowledge have now been largely overcome with the development of the technique of photoelectron spectroscopy. Using monoenergetic photon sources, such as the emission line of helium at 58.4 nm (21.22 eV), outer electrons can be ejected and their binding energies measured (EB=hv—½mv2). Thus a quantitative description of the electronic environment of the nuclear framework is available for relating the electrical to the chemical properties of a molecule.