Sodium is very Reactive, because it has only 1 electron in its Valence shell. Most pure sodium is stored under certain solutions, because the metal naturally reacts with air and forms a layer around itself.
The reason the valence electron is important is because the element has a propensity to become stable by having the same amount of outer electrons as Helium. It does this by readily donating its outer-most electron to other elements. Commonly, the halogens.