Representative elements that are found in the same column of the periodic table have the same number of valence electrons. Valence electrons are the electrons that are located in the outermost energy level of an atom.
When chemical bonds are formed, the valence electrons in atoms are gained, lost, or shared. The chemical properties of an atom are determined by the number and behavior of its valence electrons.
Most atoms are the most stable when they have a full outer shell of electrons. For many atoms, this means having eight valence electrons. Elements bond with one in order to become more stable. Most of the elements in Group 18 of the periodic table already have eight valence electrons. Because of this, they are not usually involved in chemical reactions.