Exceptions to the
Octet Rule
The LE model provides a simple and very
successful method for modeling covalent
compounds based on the arrangement of
atoms and valence electrons, both bonding
and non-bonding, resulting in an octet for
every atom; However, it has some exceptions
1. Molecules with an odd number of
electrons
2. Molecules in which an atom has less
than an octet
3. Molecules in which an atom has more
than an octet.
Molecules with an odd number
of electrons
Because the LE model is based on pairs of
electrons, it can not be used to explain the
bonding in the rare instances where a
molecule may have an odd number of
electrons.
(i.e. NO or NO2)
Molecules with less than an
Octet
Atoms of B and Be can form molecules
with less than an octet.
(i.e. BF3)
These structures can be shown by
observing its reaction to molecules with
an unshared pair of electrons.
Molecules and atoms containing an unpaired
electron are called free radicals and are
generally highly reactive
Fewer Than Eight Electrons
Fewer Than Eight Electrons
Therefore, structures that put a double
bond between boron and fluorine are much
less important than the one that leaves boron
with only 6 valence electrons.
• Consider BF3:
Giving boron a filled octet places a
negative charge on the boron and a
positive charge on fluorine.
This would not be an accurate picture
of the distribution of electrons in BF3.
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Molecules with more than an
Octet
Fewer Than Eight Electrons
The lesson is: If filling the octet of the
central atom results in a negative charge on
the central atom and a positive charge on the
more electronegative outer atom, don’t fill the
octet of the central atom.
Molecules containing elements
of the 3rd period and beyond
may possibly form compounds
or ions in which there are more
than eight electrons in the
valence shell.
(i.e. SF6, PCl5)
Elements of the 3rd period and
beyond contain a “d” orbital that can
accommodate extra electrons.
Observe Sulfur
Example:
SF6
e-’s
3s 3px 3py 3pz 4s 3dxy 3dxz 3dyz 3dx  y 3d 2
2
2
z
By rearranging the valence shell, sulfur can
maintain 6 covalent bonds
PCl5
6 + 6(7) = 48
F
F
F
S
F
F
F
e-’s 5 + 5(7) = 40
Cl
Cl
3s 3px 3py 3pz 4s 3dxy 3dxz 3dyz 3dx  y 3d 2
P Cl
Cl
Cl
More Than Eight Electrons
More Than Eight Electrons
2
2
z
• The only way PCl5
can exist is if
phosphorus has 10
electrons around it.
• It is allowed to
expand the octet of
atoms on the 3rd row
or below.
Even though we can draw a Lewis
structure for the phosphate ion that has only 8
electrons around the central phosphorus, the
better structure puts a double bond between the
phosphorus and one of the oxygens.
 Presumably d orbitals
in these atoms
participate in bonding.
2
More Than Eight Electrons
• This eliminates the charge on the phosphorus
and the charge on one of the oxygens.
• The lesson is: When the central atom is on the
3rd row or below and expanding its octet
eliminates some formal charges, do so.
For both SF6 and PCl5, the central
atoms must have the extra electrons due
to the extreme nature of the Halogens
But, what About situations where it
is not clear where the extra electrons
go?
Example:
I3-
e-’s = 3(7) + 1(charge) = 22 V.E.
••
•• •I• –•I•–•I• ••
•• •• ••
When necessary to exceed the
octet rule for one of several third
row (or higher) elements, assume
that the extra electrons are placed
on the central atom.
1.Write the correct Lewis
structures for each of the
following ions or molecules. Be
careful to include any formal
charges.
a. ClF3
d. BeCl2
b. ICl4c. RnCl2
2.Write the all possible Lewis
structures for the sulfate
anion, including all resonance
and formal charges.
To choose the most likely Lewis
structure for a molecule:
1. Draw all structures w/ normal
octets
2. Draw all possible structures w/
expanded or lessened octets
3. Find the structure w/ the formal
charges as close to zero as possible
and on the most electronegative
atoms.
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3.So using our rules, what is the
most likely (stable) form of the
sulfate anion?
4. Give all possible Lewis
structures for XeO3, a highly
explosive compound, and
identify the most appropriate
structure(s).
Hint: there are 8 possible Lewis
structures
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