Practical Electron Microscopy and Database

The electrons of an isolated atom can be separated into two groups:
         i) Valence electrons.
         ii) Core electrons.

The core electrons are mostly localized around the nuclei in the completely filled inner atomic shells. The valence electrons are in the outermost, partially filled atomic shells. The main properties of materials are determined by the valence electrons rather than by the core electrons. For this reason, the inner shell electrons are neglected in many analyses.