The valence electrons within an atom is the number of electrons in its outermost shell.
These electrons are used by an atom to react with one another. They determine the extent to which an atom is ready to combine either by losing, gaining or sharing these electrons.
Every atom desires to have a completely filled outermost shell.
Only the elements in group 8 have a complete octet.
The need to attain stability is driven by the number of electrons in their valence shell.
Therefore, some atoms are very reactive.
Those needing one electrons to complete their octet and also those that must lose one electron are very reactive.
