In general, the second-shell nonmetal atoms, C, N, O, and F, make covalent bonds by sharing electrons in pairs with other atoms of similar electronegativity. As you would expect from electronegativity values, C-H bonds are almost purely covalent, or electron-sharing, whereas at the opposite extreme, H-F bonds have a large ionic character, with the electrons drawn toward the F atom. If each of two atoms contributes one electron to a covalent bond, then because the bonded atoms are held close to one another, both electrons help to fill the outer shell of each atom. An atom may form several covalent bonds, in which case it acquires one new outer electron for every bond it makes.