The reactivity of a metal is determined by how tightly the metal holds onto the electrons in its outermost energy level. These electrons are called valence electrons. Metals usually have fewer valence electrons than nonmetals.
Metals are electropositive elements. They have few electrons in their outermost energy levels, so they won't hold onto them very tightly. These valence electrons are often given away to more electronegative atoms such as the nonmetals, making metals electron donors. The most reactive metals on the periodic table are the alkali metals, which have a single valence electron. According to HyperPhysics, alkali metals include potassium, sodium and cesium. The next reactive series of metals are the alkaline earth metals such as magnesium and calcium that have two valence electrons.