Another example: draw the Lewis
structure of Carbonyl chloride: COCl2
# of valence electrons =
Connect atoms
distribute electrons
Mary J. Bojan
Chem 110
13
What do you do if there are two (or more)
possible Lewis structures for the same
arrangement of atoms?
Use FORMAL CHARGE (FC):
Formal Charge: FC = VE − LSE
VE =number of valence electrons in an isolated atom
LSE = number of electrons on the atom in the Lewis
structure
LSE = lone pair electrons + ½ shared electrons
The most stable structure is the one in which the atoms
bear the smallest formal charge.
Mary J. Bojan
Chem 110
14
Illustrate the use of formal charge for carbonyl
chloride.
Valence e–
Lewis structure
electrons
Formal Charge
Mary J. Bojan
Chem 110
15
Try to draw a Lewis structure for the
NITRATE ION: NO3−
Valence electrons:
Connect atoms, distribute electron pairs
What does the Lewis structure indicate about bond lengths
and strengths in NO3−?
Mary J. Bojan
Chem 110
16
NITRATE ION: NO3−
Lewis structure indicates
2 long (single) N−O bonds
One short (double) N=O bond
Experiment shows:
All three N−O bonds are
Equal length
Equal strength
How do we explain this!
Mary J. Bojan
Chem 110
17
Some molecules can not be represented
by a single Lewis structure.
Example: NO3− has 3 resonance structures
Molecule has RESONANCE when more than 1 Lewis
structure can be drawn for a fixed nuclear
arrangement.
Different Lewis structures are equally good (usually).
Mary J. Bojan
Chem 110
18
RESONANCE: REAL structure is a
blend of resonance structures
Properties
(e.g. bond length, bond strength) are
averaged over resonance structures
EXAMPLE:
Mary J. Bojan
Chem 110
19
RESONANCE: SUMMARY
Molecule has RESONANCE when more than 1 Lewis
structure can be drawn for a fixed nuclear
arrangement.
Different Lewis structures are equally good (usually).
Properties (e.g. bond length, bond strength) are
averaged over resonance structures
IN ADDITION:
Molecules with resonance exhibit extra stability
Mary J. Bojan
Chem 110
20
Aromatic Hydrocarbons are a class of organic
molecules that exhibit resonance.
Aromatic hydrocarbons
contain alternating single
and double C-C Bonds
Benzene is prototypical
molecule
H
H
H
C
C
C
C
H
H
C
C
H
H
H
H
C
C
C
C
C
C
H
H
H
Usually written:
Aromatic hydrocarbons are less reactive than hydrocarbons with
only a single double bond.
Resonance results in added stability
Mary J. Bojan
Chem 110
21
Review Lewis structures
What is the Lewis structure for
C 2H 4
SiF4
XeF4
TeF4
Mary J. Bojan
Chem 110
22
There are stable molecules that exhibit
EXCEPTIONS TO OCTET RULE
1. Molecules with an ODD # of electrons (rare)
E.g.
# of valence e–
Mary J. Bojan
NO
NO2
Chem 110
ClO2
23
There are stable molecules that exhibit
EXCEPTIONS TO OCTET RULE
2. Molecules with an incomplete octet (# e– < 8)
Rare: Be, B
Example: BF3
F
F
B
F
F
F
B
F
Resonance forms imply added stability BUT
BF3 reacts readily with electron pair donors (like NH3).
Mary J. Bojan
Chem 110
24
There are stable molecules that exhibit
EXCEPTIONS TO OCTET RULE
3. Molecule with atoms having more than 8 electrons
•  fairly common
•  never occurs with period 2 atoms
•  occurs with atoms in period 3 and below
WHY??
Conditions for expanded octet
Mary J. Bojan
Chem 110
25
There are stable molecules that exhibit
EXCEPTIONS TO OCTET RULE
3.  Molecule with atoms having more than 8 electrons
Which of these could have expanded octets?
NF5
PF5
AsF5
WHY?
Which of these could have expanded octets?
PF5
PCl5
PBr5
PI5
WHY?
Mary J. Bojan
Chem 110
26