where V is the number of valence electrons in the free atom, L is the number of
electrons present on the bonded atom as lone pairs, and B is the number of bonding
electrons on the atom. If the atom has more electrons in the molecule than
when it is a free, neutral atom, then the atom has a negative formal charge, like a
monatomic anion. If the assignment of electrons leaves the atom with fewer electrons
than when it is free, then the atom has a positive formal charge, as if it were
a monatomic cation.
Formal charge can be used to predict the most favorable arrangement of atoms
in a molecule and the most likely Lewis structure for that arrangement:
A Lewis structure in which the formal charges of the individual atoms are
closest to zero typically represents the lowest-energy arrangement of the
atoms and electrons.
A low formal charge indicates that an atom has undergone only a small redistribution
of electrons relative to the free atom. The structure with formal charges closest to
zero typically has the lowest energy of all possible structures. For example, the
formal charge rule suggests that the structure OCO is more likely for carbon dioxide
than COO, as shown in (17). Similarly, it also suggests that the structure NNO is
more likely than NON for dinitrogen monoxide, as shown in (18). Note, however,
that formal charge does not take all characteristics of atoms into account, and in
some cases the structure with the lowest formal charge is not the most likely. However,
formal charge is a good general guideline to favorable atom arrangement
when experimental data are not available.