1. formal charge: the charge a bonded atom would have
if all the atoms had the same
electronegativity; to find it, you must
first draw the Lewis structure
formal
charge
# of v.e– in
= the isolated –
atom
# of e– assigned
to the atom in the
Lewis structure
all unshared e–
plus half of the
bonding e–
When several Lewis structures are possible,
the most stable is the one in which:
(1) the atoms have the smallest
formal charges, and
(2) the (–) charges reside on the
most electronegative atoms

2. Find the formal charge on each
atom in the following species.
[ ] .. .. .. 2– S O
SO32– .. .. .. O– S –O ..
v.e– 6 6 O .. .. ..
assigned e– 5 7
26 e– +1 –1
S: +1
O: –1
(N is more EN than S.) – .. ..
“Mr. B, tell me what the electronegativity value is.” “All right… FO NCl.” – – .. ..
[ ] ..
N–C–S ..
[ ] ..
[ ]
NCS–
N=C=S ..
N–C–S .. .. .. .. 5 4 6 5 4 6 5 4 6
16 e– 7 4 5 6 4 6 5 4 7 2– 1+ 1– 1–

3. [ ] [ ] [ ] .. .. .. resonance structures: the two or more Lewis
structures that are equally correct for a species
e.g., For NO3–...
24 e–
[ ] N =O O– .. O –
[ ] N –O O= .. O –
[ ] N –O O– .. O –
Resonance structures are
a blending of two or more
Lewis structures.
In drawing resonance
structures, e–s move.
Atoms don’t move.

4. Aromatic compounds are based on
resonance structures of the benzene molecule. C H C H
In the interest of efficiency,
benzene is often
represented like this…

5. .. .. Exceptions to the Octet Rule
There are a few cases (other than for H) in
which the octet rule is violated. These are:
1. particles with an odd number of valence e–
-- e.g.,
ClO2, NO, NO2
2. atoms with less than an octet B .. F F– –F
-- e.g.,
B, Be
(24 e–)
BF3 B –F F= .. F
-- would require resonance
-- F is WAY more EN than B
and won’t share extra e–
-- inconsistent w/chemical behavior

6. .. 3. atoms with more than an octet
-- this occurs when an atom gains an expanded
valence shell P .. Cl
Cl–
–Cl
PCl5
(40 e–)
-- other e.g.,
AsF6–, ICl4–, SF4
Expanded valence shells occur only for atoms
in periods > 3.
-- unfilled d orbitals are involved
-- large central atom =
more space to accept
e– pairs
-- small exterior atoms =
fit more easily around
central atom

7. .. .. . The strengths of a molecule’s covalent bonds are
related to the molecule’s stability and the amount of
energy required to break the bonds.
bond enthalpy: the DH req’d
to break 1 mol of a particular
bond in a gaseous substance --
always +
At room temp., I2 is
a silver-purple solid.
-- big DH =
strong bond
(g)
I–I .. I ..
(g) 2 .
-- e.g.,
DH = +149 kJ/mol