An atom is stable when its total charge is equal to 0 (zero).
There are 2 situacions that can make an atom not stable (or its total charge is not zero):
1. One situation is that the atom has excess of negative charge. In this case, the atom is stabilized by transferring valence electrons to another atom.
2. Another situation is that the atom has excess of positive charge. In this case, the atom is stabilized by obtaining electrons from another atom.
In both cases, the parts of the atom related to the stable state are the electrons.