Flexi Says: Noble gases are nonreactive, nonmetallic elements in group 18 of the periodic table. Noble gases include helium (He), neon (Ne), argon (Ar), krypton (Kr), xenon (Xe), and radon (Rn). Noble gases are the least reactive of all known elements. That’s because with eight valence electrons, their outer energy levels are full. A full outer energy level is the most stable arrangement of electrons. As a result, noble gases cannot become more stable by reacting with other elements and gaining or losing valence electrons. They, therefore, are the least electronegative elements.