"Condensed" Electron Configurations:
For large atoms, showing all the electrons with an electron configuration or orbital diagram can become quite complex. Since it is the outer-shell electrons that are largely responsible for chemical behavior, we can condense the electron configuration and orbital diagram to focus on those electrons.
Outer-shell electrons, those involved in chemical bonding, are called valence electrons. Electrons below the outer shell, inner-shell electrons, are usually referred to as core electrons.
An electron configuration or orbital diagram can be "condensed" by beginning with the nearest (before the atom) noble gas symbol in brackets to represent the core electrons, then showing the valence electrons as usual.
Sodium's complete electron configuration is:
The same electron configuration in condensed form becomes:
The complete orbital diagram for sodium is:
The same orbital diagram in condensed form becomes: