How does the number of valence electrons affect the reactivity of an atom?

atoms are most stable when they have filled the outer shell of electrons which normally holds a max of 8. so if an atom has 1 electron in its outer layer getting rid of it will give him stability, in much the same way having 7 electrons will mean gaining one electron will give it stability. now lets say the two meet, they react with each other and everybody is happy.

It is thought than when two elements react, they are trying to acheive a "perfect" electronic configuration (i.e. have a full outer shell). The more electrons needs to lose/gain before acheiving this stability determines its reactivity.