A level described by the removal of one electron from the configuration of the neutral @G02704@. 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 @V06588@ levels and core levels according to the nature of the electron vacancy. Diagram levels with orbital @A00353@ different from zero occur in pairs and form spin doublets.