A level described by the removal of one electron from the configuration of the neutral @[email protected] These levels form a spectrum similar to that of a one-electron or hydrogen-like atom but, being single-valency levels, have the energy scale reversed relative to that of single-electron levels. Diagram levels may be divided into @[email protected] levels and core levels according to the nature of the electron vacancy. Diagram levels with orbital @[email protected] different from zero occur in pairs and form spin doublets.