Substances that are combinations of nonmetals are typically covalent compounds. Covalent compounds form when two or more nonmetal atoms bond by sharing valence electrons. Some common examples of substances that are combinations of nonmetals include water (H2O), carbon dioxide (CO2), methane (CH4), and sulfur dioxide (SO2). These substances often exhibit properties such as low melting and boiling points, poor electrical conductivity, and tend to be brittle.