1.which of the following are considered pure substances heterogeneous mixtures homogenous mixtures elements only elements and compounds 2. ionic compounds are often formed when a metal atom bonds with a nonmetal atom. which of the following is an ionic compound and why? h2o, because both atoms are nonmetals nh3, because both atoms are nonmetals naci, because sodium is a metal and chlorine is a nonmetal na, because sodium is a metal 3.covalent compounds are formed when two different nonmetals are chemically combined. which of the following is a covalent compound and why? h2o, because h and o are two different nonmetals h, because it is a nonmetal naci, because sodium is a metal and chlorine is a nonmetal o, because it is a nonmetal