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.