The element has :
Valence electrons : electrons used in forming bonds (the outer shell)
Core electrons : electrons that belong to atoms except for valence electrons. It is located close to the core and fills the inner shell.
Electron configurations for an element with the atomic number: 15 1s²2s²2p⁶3s²3p³
Neutral atoms ⇒ atomic number= number of protons=number of electrons
The electron configuration shows that the valence electron of the element = 5(3s²3p³) so the number of Core electrons:
Core electron = atomic number (total number of electrons) - valence electron
Core electron = 15 - 15
Core electron = 10
