All of the noble gases have full valence shells, this configuration is a stable configuration and one that other elements try to achieve by reacting together. In other words, the reason atoms react with each other is to reach a state in which their valence shell is filled. Let's look at the reaction of sodium with chlorine. In their atomic states, sodium has one valence electron and chlorine seven. Sodium Chlorine Chlorine, with seven valence electrons, needs one additional electron to complete its valence shell with eight electrons.
Sodium is a little bit trickier.
At first it appears that sodium needs seven additional electrons to complete its valence shell. In doing so, the sodium atom empties its 3rd electron shell and now the outermost shell that contains electrons, its 2nd shell, is filled - agreeing with our earlier statement that atoms react because they are trying to fill their valence shell. This trait, the tendency to lose electrons when entering into chemical reactions is common to all metals. For all of the elements in group A of the periodic table , the number of valence electrons is equal to the group number. For all of the nonmetals, except hydrogen and helium, their valence shell is complete with 8 electrons.