Core Electron
An electron occupying a completely filled inner shell that does not take part in chemical bonding.
Unit
Dimensionless
In depth
Core electrons are held tightly by the nucleus and screen the valence electrons from the full nuclear charge. They are chemically inert but not spectroscopically silent: ejecting a core electron with X-rays produces the sharp lines used in XPS and X-ray fluorescence to identify elements.
Examples in the real world
The ten [Ne] core electrons of sulfur, written [Ne] 3s² 3p⁴ to emphasise the six valence electrons.