Lewis Dot
Chapter 8
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.
The LE Model
 Description of the electrons in a
molecule using Lewis model.
 Prediction of the geometry of the
molecule using valence shell electronpair repulsion (VSEPR).
 Description of atomic orbitals used by
atoms to share electrons or hold lone
pairs (Molecular Orbital model in
chapter 9).
Lewis Dot Structure
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
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.
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.
How about CO2?
 How many Valence
electrons?
 16
 Bond the three
atoms.
 O-C-O
 How to arrange the
remaining e-’s?
More examples
 What is the Lewis
structure for:
 HF
 N2
 NH4+
 CH4
 CF4
 NO+
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.
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.
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
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.
Other exceptions
 Beryllium and
nitrogen are also
exceptions.
 Exception are not
reserved for less
than octet.
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.
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.
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.