2. Another model
The localized electron
model assumes
compounds are held
together by sharing
electron pairs.
Pairs of electrons may
be
• localized pairs or
• bonding pairs.
There are 3 parts to
this.
3. The LE Model
Description of the electrons in a
molecule using Lewis model.
Prediction of the geometry of the
molecule using valence shell electron-
pair repulsion (VSEPR).
Description of atomic orbitals used by
atoms to share electrons or hold lone
pairs (Molecular Orbital model in
chapter 9).
6. Duet Rule
H does not need 8
electron to reach a
stable noble gas
configuration. It needs
only 2 electrons to
reach He.
Note lone pair
electrons and bonding
electrons in HCl
7. Steps for writing Lewis dot
structures
1. Sum the valence e-’s
from all the atoms.
The total # of e-’s is
what is important.
2. Use e- pairs to form
bond between each
pair of bound atoms.
3. Arrange e-’s to
satisfy duet & octet
rules.
8. Example: Water
Sum up valence e-
’s
8
Draw e- pairs, may
use a line to
designate e- pair.
H-O-H
Distribute remaining
e-’s to fill octet or
duet.
9. How about CO2?
How many Valence
electrons?
16
Bond the three
atoms.
O-C-O
How to arrange the
remaining e-’s?
10. More examples
What is the Lewis
structure for:
HF
N2
NH4
+
CH4
CF4
NO+
11. Find the errors in these
N has only 5
valence electrons
Si does not have an
octet.
H does not have a
duet.
N does not have an
octet.
12. More errors
H does not follow
duet.
Cl does not follow
octet. (10)
C doest not follow
octet. (6)
H does not follow
duet and Br does
not follow octet.
13.
14. Exceptions to the octet rule.
Like most models
the Lewis dot is
flawed.
Boron for example
may have only
three bonds.
We know this due to
its reactivity with
ammonia
15. Exceptions continued
Boron acts in
chemical reactions
as though it needs
an electron pair. It
is highly reactive
with ammonia
forming H3NBF3.
This supports Boron
as an exception to
the octet rule.
16. Other exceptions
Beryllium and
nitrogen are also
exceptions.
Exception are not
reserved for less
than octet.
17. Exceptions of greater than 8 in
the Octet.
Atoms from P and
larger have access
to the d orbitals.
This gives these
atom the ability to
have more than 4
bonding sites and
access to more than
8 electrons and still
be stable.
18. Summary
Second row elements C, N, O & F
always follow the octet rule.
Second row B, & Be may have less
than 8. These electron deficient
molecules are highly reactive.
2nd row elements may never exceed
the octet since they do not have
access to d orbitals in the second
energy level.
19. Summary continued
Third row and higher elements often
satisfy the octet rule but may exceed
the rule as they have access to the d
orbital.
When writing the Lewis structure,
follow the octet rule. If electrons
remain, only then may you add extra
electron pairs to the element having
available d orbitals.