In
the most common bonding behavior, an atom makes as many covalent bonds
as are needed to fill its outer shell with eight electrons. For atoms
of C, N, O, and F, this means 4, 3, 2, and 1 electrons, respectively.
If these elements are bonded to hydrogen, we find the expected CH4
methane molecule, NH3
for ammonia, H20
for water, and HF for the hydrogen fluoride molecule. Another way
of looking at the bonding behavior is to say that each of the unpaired
electrons in C, N, O, and F is available for pairing in a covalent
bond. The pair of electrons in the N atom, the two electron pairs
in O, and the three pairs in the F atom do not need to find outside
electrons to interact with because they already are paired. They are
called lone pairs to distinguish them from the electron pairs of a
chemical bond, which are called bonding pairs. These lone pairs and
bonding pairs for the simple molecules mentioned previously are illustrated
in the table on the right hand side of this page and on the right
hand side of page 6.