Final answer:
Two nonmetals react by sharing electrons to achieve stable electron configurations through covalent bonding. In the presence of metals, however, nonmetals tend to gain electrons, forming ionic compounds through electron transfer and ionic bonding.
Step-by-step explanation:
Two nonmetals react by sharing electrons typically to achieve stability in their outer electron shells. This process forms a covalent bond, which involves the sharing of electron pairs between atoms. For instance, chlorine, which requires one more electron to complete its octet, can share an electron with hydrogen, which needs one electron to reach the electron configuration of helium. This mutual sharing results in the formation of a molecule like HCl, where the shared electron pair forms a stable covalent bond. Moreover, since nonmetals have similar electronegativities, the shared electrons result in either nonpolar covalent bonds when electronegativity is identical, as in H₂ molecules, or polar covalent bonds when there is a slight difference, as seen in an HF molecule.
In contrast, reactions between metals and nonmetals typically involve electron transfer, not sharing, resulting in the formation of ionic compounds. The nonmetal atoms accept electrons from the metal atoms, leading to the creation of cations (positively charged ions) and anions (negatively charged ions), which are held together by electrostatic forces, known as ionic bonds. As such, while metals readily lose electrons, nonmetals tend to gain or share electrons, depending on their reaction partners, to achieve stable electron configurations.