These ions have the same number of electrons (same electron configuration.
ex) Al+3, Mg+2, Na+1, Ne, F-1, O-2, N-3 (all have 10 electrons)
The charges add/subtract electrons to a point where they are all the same. For example, O does not have the same # electrons as Ne, however O-2 does because it has two more electrons than O, just like Ne.