The outermost energy level, also known as the valence shell, plays a major role in chemical reactions. This shell contains the valence electrons, which are the electrons that participate in the formation and breaking of chemical bonds. Valence electrons are responsible for the chemical properties of an element and its ability to react with other elements. Atoms can gain, lose, or share valence electrons in order to achieve a stable electron configuration, which is the driving force behind chemical reactions. By understanding the valence electron configurations of different elements, we can predict their reactivity and the types of chemical bonds they can form.