Lewis
Structures
According to Lewis Theory, there are two types
of valence electrons:
•Non-bonding (or unshared) pairs
•Bonding single (or unpaired) electrons
•Boron has three unpaired electrons therefore it can
form three covalent bonds
•Bromine has three unshared pairs and one unpaired
electron, therefore it can only form one covalent bond.
•What about nitrogen?
The shared electrons of the covalent bond
can be shown using Lewis structures. To draw
elementary Lewis structures, the Lewis symbols
can be used.
If you know a molecule can be made from
one carbon and 2 oxygen atoms, Lewis symbols
can be used to determine the Lewis structure.
•
•
•
•
C
••
•O•
••
We have seen how we can build models of
molecules by combining atoms according to
electron dot structures.
..
:Br:
+ 3
••
•O•
••
Carbon needs four electrons and each
oxygen needs two. The only possible arrangement
is:
••
••
••
••
O=C=O
Lewis Structures
=
..
: N Br
.. :
:Br
.. :
Now, we are going to learn a process by
which we will be able to draw a model of any
molecule.
Writing Lewis Structures
PCl3
5 + 3(7) = 26
Lewis structures are representations
of molecules showing all electrons,
bonding and nonbonding.
1. Find the sum of
valence electrons of all
atoms in the molecule
from the group
number or electron
dot structure.
1
Writing Lewis Structures
Writing Lewis Structures
2. Things to consider
when building primary
skeleton:
2. Build a reasonable
skeletal structure for
the molecule using
only single bonds.
Keep track of
the electrons:
26  6 = 20
 The central atom
should be the least
electronegative
element that isn’t
hydrogen.
Writing Lewis Structures
Keep track of the
electrons:
26  6 = 20  18 = 2
3. Subtract the total
number of
electrons used in
the primary bonds
from the available
valence electrons.
4. Fill the octets of the
outer atoms by
adding unshared
pairs
Writing Lewis Structures
Keep track of
the electrons:
26  6 = 20
 Oxygen never bonds to
itself, except in O2 and O3
 Carbon atoms are usually
bonded to each other
 In molecules containing
both H and O, hydrogen is
usually bonded to oxygen
Writing Lewis Structures
5. Fill the octet of
the central atom.
6. Check to see that
all atoms have
and octet and
that the correct
number of
Keep track of the electrons:
valence electrons
were used
26  6 = 20  18 = 2  2 = 0
Writing Lewis Structures
7. If you run out of electrons before the
central atom has an octet…
Example:
Try building a Lewis structure for HCN
5. …form multiple bonds
until it does.
2
Let’s try drawing the Lewis Structures
for the following molecules:
1. Carbon tetrachloride
2. Ammonia
3. Oxygen
Resonance
This is the Lewis
structure we
would draw for
ozone, O3.
4. Carbon dioxide
5. Dihydrogen carbon oxide
6. Ethanal (C2H4O)
Resonance
• But this is at odds
with the true,
observed structure
of ozone, in
which…
…both O—O
bonds are the
same length.
Resonance
Just as green is a synthesis
of blue and yellow…
Resonance
• One Lewis structure
cannot accurately
depict a molecule
such as ozone.
• We use multiple
structures,
resonance
structures, to
describe the
molecule.
Resonance
• When a double bond can exist between two different
atoms and a central atom, the molecule exhibits
resonance.
• They are not localized, but rather are delocalized.
…ozone is a synthesis of
these two resonance
structures.
• Look at acetic acid
3