Remember, the chemical properties of elements are determined by the number of electrons in the outermost energy level. Atoms of most nonmetals have 5, 6, 7, or 8 electrons in their outermost energy level. Atoms with 5, 6, or 7 valence electrons gain 3, 2, or 1 electron, respectively, when they combine chemically. Thus nonmetals are elements that tend to gain electrons. Perhaps you are wondering about the nonmetals whose atoms have 8 valence electrons. Atoms with 8 valence electrons have a complete outermost energy level. So elements whose atoms have 8 valence electrons tend to be nonreactive or rarely react with atoms of other elements.