The number of valance electrons determines the electron configuration of the atom. The structure of the electrons created by the valance electrons determines the reactivity of the atom.
The structure of Neon for example is a completely filled outer shell of valance electrons. This is a completely stable structure so there is no or very little chemical reactivity.
On the opposite end the electron structure of potassium is a single valance in the outer shell. This structure is extremely unstable. Potassium left out in the air will react with water vapor and burst spontaneously into flames.
In between these extremes there are filled subshells, half filled subshells that create degrees of stability. Examining the electron structure created by the valance electrons can provide a good indication of the chemical reactivity of an atom.