Bonding – General Concepts
Lewis Structures
Shows how valence electrons are arranged
among atoms in a molecule.
Reflects central idea that stability of a
compound relates to noble gas electron
configuration.
Completing a Lewis Structure -CH3Cl
Make carbon the central atom
Add up available valence electrons:
Join peripheral atoms
to the central atom
with electron pairs.
H
..
..
Complete octets on
H
atoms other than
hydrogen with remaining
electrons
C
..
H
Total = 14
..
Cl
..
..
C = 4, H = (3)(1), Cl = 7
Multiple Covalent Bonds:
Double bonds
H
H
C
H
H
C
H
C
H
H
Ethene
Two pairs of shared electrons
C
H
Multiple Covalent Bonds:
Triple bonds
H
C
C
H
H
C
C
H
Ethyne
Three pairs of shared electrons
Resonance
Resonance is invoked when more than one valid Lewis
structure can be written for a particular molecule.
H
Benzene, C6H6
H
H
H
H
H
H
H
H
H
H
H
The actual structure is an average of the resonance
structures.
The bond lengths in the ring are identical, and
between those of single and double bonds.
Resonance Bond Length and Bond Energy
Resonance bonds are shorter and stronger
than single bonds.
H
H
H
H
H
H
H
H
H
H
H
H
Resonance bonds are longer and weaker than double
bonds.
Resonance in Ozone, O3
O
O
O
O
O
O
Neither structure is correct.
Oxygen bond lengths are identical, and
intermediate to single and double bonds
Resonance in Polyatomic Ions
Resonance in a carbonate ion:
Resonance in an acetate ion:
Localized Electron Model
Lewis structures are an application of the
“Localized Electron Model”
L.E.M. says: Electron pairs can be
thought of as “belonging” to pairs of
atoms when bonding
Resonance points out a weakness in the
Localized Electron Model.
Models
Models are attempts to explain how
nature operates on the microscopic level
based on experiences in the macroscopic
world.
Models can be physical
as with this DNA model
Models can be mathematical
Models can be theoretical
or philosophical
Fundamental Properties of Models
A model does not equal reality.
Models are oversimplifications, and are
therefore often wrong.
Models become more complicated as they
age.
We must understand the underlying
assumptions in a model so that we don’t
misuse it.
MOLECULAR GEOMETRY
VSEPR
• Valence Shell Electron
Pair Repulsion theory.
• Most important factor in
determining geometry is
relative repulsion
between electron pairs.
Molecule
adopts the
shape that
minimizes the
electron pair
repulsions.
VSEPR – Valence Shell Electron Pair
Repulsion
X+E
Overall Structure
2
Linear
3
4
5
6
Trigonal Planar
AX3, AX2E
Tetrahedral
AX4, AX3E, AX2E2
Trigonal bipyramidal
AX5, AX4E, AX3E2, AX2E3
Octahedral
AX6, AX5E, AX4E2
Forms
AX2
AX
A = central atom
X = atoms bonded to A
E = nonbonding electron pairs on A
VSEPR: Linear
AX2
CO2
Figure 9.2 Molecular Shapes 2,3,4
electron pairs.
John A. Schreifels
Chemistry 211
Chapter 10-16
Figure 9.3 Molecular shapes 5, 6
electron pairs
John A. Schreifels
Chemistry 211
Chapter 10-17
• AX2
- Linear
• AX
- Linear
• Draw CO2
and
HF
AX3
Triganol
planar
AX2E
Bent
BF3
SnCl2
VSEPR: Tetrahedral
AX4
CCl4
tetrahedral
AX3E
PCl3
Triangular Pyramidal
AX2E2
Bent
Cl2O
VSEPR: Trigonal Bi-pyramidal
AX5
PCl5
Triangular bipyramidal
AX4E
See-saw
AX3E2
SF4
ClF3
T-shaped
AX2E3
Linear
I3-
VSEPR: Octahedral
AX6
SF6
Octahedral
AX5E
BrF5
Square pyramidal
AX4E2
Square planar
ICl4-
Are the following molecules polar or
nonpolar ? Which will dissolve in
water?
•
OF2
SO3
PCl3
SF6
NH3
Formal charge (FC) in Lewis structures
Comparison of the formal number of valence electrons about an
atom in the Lewis structure of a molecule and comparison of FN
with the number of valence electrons (VE) in the neutral atom.
Computation of FC:
FC = VE (neutral atom) - LE (atom in molecule) - 1/2BE (atom in molecule)
where
VE
LE
BE
24
= the number of valence electrons in the neutral atom
= the number of lone pair electrons on the atom in the molecule
= the number of bonding electrons on the atom in the molecule
Example of formal charge computation: ozone,
O3
VE (atom)
=
6
6
6
1/2 BE (molecule)=
-2
-3
-1
LE (molecule)
=
-4
-2
-6
FC
=
0
+1
-1
O
O
O
Note: New charge for molecule = 0, sum of
formal charges must = 0.
25
Isomers: Same composition, two different
constitutional Lewis structures
HCN = atomic compositional structure
HCN possesses 10 VE = Lewis compositional
structures
Two possible Lewis constitutional structures:
H-C-N or H-N-C
Both need to have 10 VE in their Lewis structure
Problem: Try to achieve an acceptable Lewis
structure (duet and octet rule followed) for
26
both.
HCN = atomic compositional structure
HCN: 10 VE = Lewis compositional structures
Two possible Lewis constitutional structures
H-C-N or H-N-C
Any acceptable Lewis structure for HCN needs to show 10 VE
Try to achieve an acceptable Lewis structure (duet and octet rules
obeyed) for all isomeric structures.
Two acceptable Lewis structures. Which is
better?
H
27
C
N
H
N
C
Use formal charges to decide on the stability
of isomeric Lewis structures
H
C
N
VE (atom)
1
4
5
1/2 BE (molecule)
-1
-4
-3
H
N
C
1
5
4
-1
-4
-3
UE (molecule)
0
0
-2
0
0
-2
________________________________________________________
FC on atom
0
0
0
0
+1
-1
H
C
N
H
N
C
Important: the net charge of composition HCN = 0,
so the sum of the formal charges in any acceptable
28
Lewis structure must be = 0 also.