Final answer:
Nonmetal atoms are most likely to share valence electrons in a covalent bond, forming stable molecules by filling up each other's valence shell.
Step-by-step explanation:
The pair of elements most likely to share valence electrons in a covalent bond are nonmetal atoms. When two nonmetal atoms combine, they tend to share electrons in their outermost valence shell to fill up their valence shells, ultimately stabilizing both atoms involved. For example, a covalent bond is formed when chlorine, with seven valence electrons, shares one electron with hydrogen, which has one valence electron, resulting in the compound HCl. Other examples of covalent bonds include the single bond in hydrogen gas (H-H), where two hydrogen atoms share a pair of electrons, and multiple bonds as seen in O=O (oxygen) for a double bond and N≡N (nitrogen) for a triple bond.