Answer
Valence electrons are important because they are responsible for the chemical properties of elements, and are important in chemical bonding.
Work Step by Step
For main-group elements, the valence electrons are those in the outermost principal energy level . For transition elements also, the outermost d electrons are counted among the valence electrons (even though they are not in an outermost principal energy level). The chemical properties of an element depend on its valence electrons, which are instrumental in bonding because they are held most loosely (and are therefore the easiest to lose or share). The elements in a column of the periodic table have similar chemical properties because they have the same number of valence electrons.