electrons are invariably preferentially ionized. Generally the Madelung rule should only be used for neutral atoms; however, even for neutral atoms there are exceptions in the d-block and f-block (as shown above). See also Valence electron Ionization energy References Further reading Image: Understanding order of shell filling Boeyens, J. C. A.: Chemistry from First Principles. Berlin: Springer Science 2008, External links Electron Configurations, the Aufbau Principle, Degenerate Orbitals, and Hund's Rule from Purdue University Electron states Foundational quantum physics Chemical bonding