Page 14 - Mechanism and Theory in Organic Chemistry

Models of Chemical Bonding 3
to the atom must be counted. The valence-shell occupancy must not exceed 2 for
hydrogen and must not exceed 8 for atoms of the first row of the periodic table.
For elements of the second and later rows, the valence-shell occupancy may
exceed 8. The structures

are acceptable.
The second idea is that of formal charge. For purposes of determining
formal charge, partition all the electrons into groups as follows: Assign to each
-
atom all of its unshared pair elec_trons and half of all electrons in bonds leading to -
iwdthenllmher assigned tof,h_e.te@m by this pr~~W.k~..det#~~.n
o n y

- - o f -
To illustrate formal charge, consider the hydroxide ion, OH-. The electron
ownership of H is 1, its core charge is + 1, and its formal charge is therefore zero.
The electron ownership of oxygen is 7, and the core charge is +6; therefore the
formal charge is - 1. All nonzero formal charges must be shown explicitly in the
structure. The reader should verify the formal charges shown in the following
examples :

The algebraic sum of all formal charges in a structure is equal to the total charge.
Formal charge is primarily useful as a bookkeeping device for electrons, but
it also gives a rough guide to the charge distribution within a molecule.
In writing Lewis structures, the following procedure is to be followed:
1. Count the total number of valence electrons contributed by the electri-
cally neutral atoms. If the species being considered is an ion, add one electron to
the total for each negative charge; subtract one for each positive charge.
2. Write the core symbols for the atoms and fill in the number of electrons
determined in Step 1. The electrons should be added so as to make the valence-
shell occupancy of hydrogen 2 and the valence-shell occupancy of other atoms
not less than 8 wherever possible.
3. Valence-shell occupancy must not exceed 2 for hydrogen and 8 for a
first-row atom; for a second-row atom it may be 10 or 12.
4. Maximize the number of bonds, and minimize the number of unpaired
erectrons, always taking care not to violate Rule 3.
5. Find the formal charge on each atom.

We shall illustrate the procedure with two examples.