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.