Chapter 11
Theories of Covalent Bonding
Molecular Geometry and Hybridization of Atomic Orbitals
Goals and Objectives of Chapter 10
• 1. Relate Lewis structures to the molecular geometry.
• 2. Predict molecular geometry of a molecule or ion from its
Lewis structure.
• 3. Assign bond angles by understanding geometry and
electron interactions.
• 4. Explain why nonbonding electrons in a molecule exert a
greater repulsive interaction on other bonds within a
molecule.
• 5. Predict, from the molecular shape and the
electronegativities of the atoms involved, whether a
molecule can have a dipole moment.
• 5. Understand Valence Bond theory and how it complements
VSEPRT
The 3-D geometry of a molecule is one of five basic
arrangements of electron groups (domains).
Valence Shell
Electron Pair
Repulsion Theory: the
optimum arrangements
of a given number of
electron domains is the
one that minimizes
repulsion among them.
Note that each shape
has a specific “bond
angle”
An electron group (or domain) is either a pair of
bonding electrons or a pair of non-bonding
electrons surrounding a central atom. Multiple
bonds only count as 1-group or domain.
••
••
P
••
Cl •
••
F
N
F
4 electron groups
3 bonding
1 non-bonding
F
••
••
Cl •
••
••
P
••
••
••
••
Cl
••
•• Cl
••Cl
Cl
••
••
••
•
5 electron groups
5 bonding
0 non-bonding
[
O
C
O
O
3 electron groups
3 bonding
0 non-bonding
••
4 electron groups ••Cl
3 bonding
1 non-bonding
••
••
Cl
2 electron groups
bonding
••
Be
Cl
Cl
]
2-
•
We can “code” the bonding/non-bonding
information into shorthand notation called AXE
classification.
A = Central Atom
AX2E0 = AX2
X = # of Bonded
Domains
E = # Non-Bonded
Domains
It’s implied that E = 0
F
N
F
Cl
Be
F
4 electron groups
3 bonding
1 non-bonding
Cl
2 electron groups
bonding
AX3E1
AX2
Chemists use Valence Shell Electron Pair Repulsion
Theory to predict the shapes of molecules using
these five electron group geometries.
1. Draw Lewis Structure
from chemical formula.
2. Count all electron
domains to get AXE code.
3. Group domains into
bonding and non-bonding
pairs of electrons.
4. Match the number of
bonding and non-bonding
domains to the proper
VSEPRT geometry.
The electron geometry is the geometry of all
electron domains whereas the molecular geometry
describes the geometry of only the atoms bonded to
the central atom.
AX3E1 =
Tetrahedral
electron
geometery with
109.5˚ bond
angles.
Molecular
Geometry is
trigonal
pyramidal bond
angles <109.5˚
The total number of electron groups (domains)
defines one of the five basic geometries.
3 EG
4 EG
2 EG
5 EG
6 EG
How Predict Geometry Using VSEPRT
1. Draw a plausible Lewis structure for the molecule.
2. Determine the total number of electron domains and
identify them as bonding or lone pairs.
3. Use the total number of electron domains to establish the
electron geometry from one of the five possible
geometric shapes.
4. Establish the AXnEm designation to establish the
molecular geometry (or do both electron and molecular
geometry together simultaneously)
5. Remember bond angles in molecules are altered by lone
pairs of electrons (repulsion forces reduce angles).
6. Molecules with more than one central atom can be
handled individually.
2 Electron Groups = Linear Electron Geometry
and 1-Possible Molecular Geometry
Bond Angle
Cl
Be
Cl
S
C
N
O
C
O
Other Examples:
AX2E0 = AX2
A = Central Atom
X = # of Bonded
Domains
CS2, HCN, BeF2
E = # Non-Bonded
Domains
3 Electron Groups = Trigonal Planar Electron
Geometry and 2-Possible Molecular Geometries
AX3
Examples:
SO3, BF3,
NO3-, CO32-
A
Bond
Angle
3-Electron Domain
Examples:
SO2, O3,
PbCl2, SnBr2
A
AX2E1
4 Electron Groups = Tetrahedral Electron
Geometry and 3-Possible Molecular Geometries
AX4
Bond
Angle
AX3E1
NH3
PF3
ClO3
H3O+
AX2E2
Examples:
CH4, SiCl4,
SO42-, ClO4-
H 2O
OF2
SCl2
5 Electron Groups = Trigonal Bipyramial Electron
Geometry and 4-Possible Molecular Geometries
PF5
AsF5
AX5
AX4E1
SF4
XeO2F2
SOF4
IF4+
IO2F2-
Equatorial
Position
Axial
Position
ClF3
AX2E3
BrF3
AX3E2
XeF2
I3 IF2-
6 Electron Groups = Octahedral Electron
Geometry and 3-Possible Molecular Geometries
AX6
BrF5
TeF5XeOF4
AX5E1
SF6
IOF5
XeF4
AX4E2
ICl4-
Non-bonding electrons repulse bonding
electrons and alter the bond angles in molecules.
Electron lone pairs render the normal 109˚ tetrahedral
angle less than 109!
bonding-pair vs. bonding
lone-pair vs. bonding
lone-pair vs. lone pair
<
<
pair repulsion
pair repulsion
repulsion
Double-bonds and/or triple bonds in molecules
also decrease bond angles in molecules (think
repulsion by electron rich regions).
H
120°
H
C
H
H
116°
C
H
Predicted Bond Angles
Double-bond vs. Single-bond
>
repulsion
122°
C
H
H
C
H
Actual Bond Angles
Single-bond vs, Single-bond
repulsion
Predicting Molecular Shapes with Two, Three, or Four Electron
Groups
PROBLEM:
SOLUTION:
Draw the molecular shape and predict the bond angles (relative
to the ideal bond angles) of (a) PF3 and (b) COCl2.
(a) For PF3 - S = N - A;, N = 4 X 8 = 32 e-;
A = 4 + 3(7) = 26 e-
S = N - A = 32 - 26 = 6 e- ......or 6/2 = 3 bonds around central atom
# Non-bonded e- = 26 - 6 = 20 e2. Draw the Lewis structure
3 Count the electron domains and establish electron geometry from 5 shapes
4. There are 4 electron domains so the electron geometry is tetrahedral
5. The molecular geometry designation is AX3E1 so the molecular geometry is
trigonal pyramidal.
6. The F-P-F bond angles should be
<109.50 due to the repulsion of the
nonbonding electron pair.
<109.50
Predicting Molecular Shapes with Two, Three, or Four Electron Groups
(b) For COCl2, C has the lowest EN and will be the center atom.
There are 24 valence e-, 3 atoms attached to the center atom.
SOLUTION: (a) For COCl2 - S = N - A;, N = 4 X 8 = 32 e-;
A = 4 + 6 + 2(7) = 24 e-
S = N - A = 32 - 24 = 8 e- ......or 8/2 = 4 bonds around central atom
# Non-bonded e- = 24 - 8 = 16 e-
Type AX3
3 Count the electron domains and establish electron geometry from 5 shapes
4. There are 3 electron domains so the electron geometry is trigonal planar
5. The molecular geometry designation is AX3E0 so the molecular geometry is
also trigonal planar (no lone pairs).
6. The Cl-C-Cl bond angle will be less than
1200 due to the electron density of the C=O.
124.50
1110
Determine the molecular shape and predict the bond
angles (relative to the ideal bond angles) of (a) SbF5
and (b) BrF5.
Determine the molecular shape and predict the bond
angles (relative to the ideal bond angles) of (a) SbF5
and (b) BrF5.
SOLUTION:
(a) SbF5 - 40 valence e-; all electrons around central
atom will be in bonding pairs; shape is AX5 - trigonal
bipyramidal.
(b) BrF5 - 42 valence e-; 5 bonding pairs and 1 nonbonding pair on central
atom. Shape is AX5E, square pyramidal.
More Than One Central Atom
• In acetic acid, CH3COOH, there are three central atoms.
• We assign the geometry about each central atom
separately.
What is the geometry
around these atoms?
Take one atom at a time and apply the
rules of electron domains.
More Than One Central Atom
ethane
CH3CH3
tetrahedral
electron
domain and
molecular
geometry
ethanol
CH3CH2OH
Predicting the Molecular Shape With Multiple
Central
Predicting Molecular Shapes
withAtoms
More Than One Central Atom
PROBLEM:
PLAN:
Determine the shape around each of the central atoms in
acetone, (CH3)2C=O.
Find the shape of one atom at a time after writing the Lewis
structure.
SOLUTION:
tetrahedral
tetrahedral
trigonal planar
>1200
<1200
Electronegativity is an element’s inherent property to
draw electrons to itself when chemically bonded to
another atom in a molecule. The units are
dimensionless (all relative measurements to Li).
Rank
F
O
N
Cl
Br
Differences in elements electronegativity
between bonding atoms result in the formation
of polar-covalent bonds and net dipole
moments in molecules.
Polar Bond
d
n
o
B
r
Polar Bond
la
o
P
No Net Dipole Moment
Po
la
rB
on
d
Net Dipole Moment
Think of the dipole moment as a molecule with separated
charges + and -.
For a poly-atomic molecule we must consider the
vector sum of polar bonds in the molecule to see if
there is a net dipole moment.
Dipole
Moment
Dipole
Moment
No Net
Dipole
Moment
Dipole
Moment
No Net
Dipole
Moment