It is sometimes possible to unpair all of the electrons in the outer shell of a second-shell atom and to use them all in bonding. For example, in the nitric acid molecule, which we shall discuss in the following chapter, we must assume that nitrogen shares five electrons with oxygen atoms, not just three. This unpairing of electrons is easier to accomplish with the third-shell atoms, where the electrons are farther from the nucleus and thus more weakly held, and where the atoms are larger so that more atoms can crowd around for bonding. For the moment, however, we need only consider nitrogen, which has one lone pair and three unpaired bonding electrons.

To illustrate these ideas of electron-pair bonding, and to introduce the ideas of molecular shape and of double and triple bonds, let us look at the simplest covalent molecules of C, N, O, and F.