Chemical Bonding II:
Molecular Geometry and
Hybridization of Atomic Orbitals
Chapter 10
1
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Valence shell electron pair repulsion (VSEPR) model:
Predict the geometry of the molecule from the electrostatic
repulsions between the electron (bonding and nonbonding)
pairs.
Class
# of atoms
bonded to
central atom
# lone
pairs on
central atom
Arrangement of
electron pairs
Molecular
Geometry
AB2
2
0
linear
linear
B
B
2
0 lone pairs on central atom
Cl
Be
Cl
2 atoms bonded to central atom
3
VSEPR
Class
# of atoms
bonded to
central atom
# lone
pairs on
central atom
Arrangement of
electron pairs
Molecular
Geometry
AB2
2
0
linear
linear
0
trigonal
planar
trigonal
planar
AB3
3
4
Boron Trifluoride
5
VSEPR
Class
# of atoms
bonded to
central atom
# lone
pairs on
central atom
Arrangement of
electron pairs
Molecular
Geometry
AB2
2
0
linear
linear
trigonal
planar
tetrahedral
AB3
3
0
trigonal
planar
AB4
4
0
tetrahedral
6
Methane
7
VSEPR
Class
# of atoms
bonded to
central atom
# lone
pairs on
central atom
Arrangement of
electron pairs
Molecular
Geometry
AB2
2
0
linear
linear
trigonal
planar
AB3
3
0
trigonal
planar
AB4
4
0
tetrahedral
tetrahedral
AB5
5
0
trigonal
bipyramidal
trigonal
bipyramidal
8
Phosphorus Pentachloride
9
VSEPR
Class
# of atoms
bonded to
central atom
# lone
pairs on
central atom
Arrangement of
electron pairs
Molecular
Geometry
AB2
2
0
linear
linear
trigonal
planar
AB3
3
0
trigonal
planar
AB4
4
0
tetrahedral
tetrahedral
AB5
5
0
trigonal
bipyramidal
trigonal
bipyramidal
AB6
6
0
octahedral
octahedral
10
Sulfur Hexafluoride
11
12
bonding-pair vs. bonding- lone-pair vs. bondinglone-pair vs. lone-pair
<
<
pair repulsion
pair repulsion
repulsion 13
VSEPR
Class
# of atoms
bonded to
central atom
# lone
pairs on
central atom
AB3
3
0
AB2E
2
1
Arrangement of
electron pairs
Molecular
Geometry
trigonal
planar
trigonal
planar
trigonal
planar
bent
14
VSEPR
Class
# of atoms
bonded to
central atom
# lone
pairs on
central atom
AB4
4
0
AB3E
3
1
Arrangement of
electron pairs
Molecular
Geometry
tetrahedral
tetrahedral
tetrahedral
trigonal
pyramidal
15
VSEPR
Class
# of atoms
bonded to
central atom
# lone
pairs on
central atom
AB4
4
0
Arrangement of
electron pairs
Molecular
Geometry
tetrahedral
tetrahedral
AB3E
3
1
tetrahedral
trigonal
pyramidal
AB2E2
2
2
tetrahedral
bent
16
VSEPR
Class
AB5
AB4E
# of atoms
bonded to
central atom
5
4
# lone
pairs on
central atom
Arrangement of
electron pairs
Molecular
Geometry
0
trigonal
bipyramidal
trigonal
bipyramidal
1
trigonal
bipyramidal
distorted
tetrahedron
17
VSEPR
Class
AB5
# of atoms
bonded to
central atom
5
# lone
pairs on
central atom
0
AB4E
4
1
AB3E2
3
2
Arrangement of
electron pairs
Molecular
Geometry
trigonal
bipyramidal
trigonal
bipyramidal
trigonal
bipyramidal
trigonal
bipyramidal
distorted
tetrahedron
T-shaped
18
VSEPR
Class
# of atoms
bonded to
central atom
# lone
pairs on
central atom
AB5
5
0
AB4E
4
1
AB3E2
3
2
AB2E3
2
3
Arrangement of
electron pairs
Molecular
Geometry
trigonal
bipyramidal
trigonal
bipyramidal
trigonal
bipyramidal
trigonal
bipyramidal
distorted
tetrahedron
trigonal
bipyramidal
linear
T-shaped
19
VSEPR
Class
# of atoms
bonded to
central atom
# lone
pairs on
central atom
AB6
6
0
octahedral
octahedral
AB5E
5
1
octahedral
square
pyramidal
Arrangement of
electron pairs
Molecular
Geometry
20
VSEPR
Class
# of atoms
bonded to
central atom
# lone
pairs on
central atom
AB6
6
0
octahedral
octahedral
AB5E
5
1
octahedral
AB4E2
4
2
octahedral
square
pyramidal
square
planar
Arrangement of
electron pairs
Molecular
Geometry
21
22
Predicting Molecular Geometry
1. Draw Lewis structure for molecule.
2. Count number of lone pairs on the central atom and
number of atoms bonded to the central atom.
3. Use VSEPR to predict the geometry of the molecule.
23
Example 10.1
Use the VSEPR model to predict the geometry of the following
molecules and ions:
(a) AsH3
(b) OF2
(c)
(d)
(e) C2H4
Dipole Moments and Polar Molecules
electron poor
region
electron rich
region
H
F
d+
d-
m=Qxr
Q is the charge
r is the distance between charges
1 D = 3.36 x 10-30 C m
25
Behavior of Polar Molecules
field off
field on
26
Bond moments and resultant dipole moments in NH3 and NF3.
27
28
Example 10.2
Predict whether each of the following molecules has a net
dipole moment:
(a) BrCl
(b) BF3 (trigonal planar)
(c) CH2Cl2 (tetrahedral)
Change in Potential Energy of Two Hydrogen Atoms
as a Function of Their Distance of Separation
30
Change in electron density as two hydrogen atoms
approach each other.
31
Hybridization – mixing of two or more atomic
orbitals to form a new set of hybrid orbitals
1. Mix at least 2 nonequivalent atomic orbitals (e.g. s
and p). Hybrid orbitals have very different shape
from original atomic orbitals.
2. Number of hybrid orbitals is equal to number of
pure atomic orbitals used in the hybridization
process.
3. Covalent bonds are formed by:
a. Overlap of hybrid orbitals with atomic orbitals
b. Overlap of hybrid orbitals with other hybrid
orbitals
32
Formation of sp Hybrid Orbitals
33
Formation of sp2 Hybrid Orbitals
34
Formation of sp3 Hybrid Orbitals
35
Formation of Covalent Bonds in CH4
36
sp3-Hybridized N Atom in NH3
Predict correct
bond angle
37
How do I predict the hybridization of the central atom?
1. Draw the Lewis structure of the molecule.
2. Count the number of lone pairs AND the number of
atoms bonded to the central atom
# of Lone Pairs
+
# of Bonded Atoms
Hybridization
Examples
2
sp
BeCl2
3
sp2
BF3
4
sp3
CH4, NH3, H2O
5
sp3d
PCl5
6
sp3d2
SF6
38
39
Example 10.3
Determine the hybridization state of the central (underlined)
atom in each of the following molecules:
(a) BeH2
(b) AlI3
(c) PF3
Describe the hybridization process and determine the molecular
geometry in each case.
sp2 Hybridization of Carbon
41
Unhybridized 2pz orbital (gray), which is perpendicular
to the plane of the hybrid (green) orbitals.
42
Bonding in Ethylene, C2H4
Sigma bond (s) – electron density between the 2 atoms
Pi bond (p) – electron density above and below plane of nuclei
of the bonding atoms
43
Sigma (s) and Pi Bonds (p)
Single bond
1 sigma bond
Double bond
1 sigma bond and 1 pi bond
Triple bond
1 sigma bond and 2 pi bonds
44
Another View of p Bonding in Ethylene, C2H4
45
sp Hybridization of Carbon
46
Bonding in Acetylene, C2H2
47
Another View of the Bonding in Acetylene, C2H2
48
Example 10.5
Describe the bonding in the formaldehyde molecule whose
Lewis structure is
Assume that the O atom is sp2-hybridized.
Experiments show O2 is paramagnetic
O
O
No unpaired eShould be diamagnetic
Molecular orbital theory – bonds are formed from
interaction of atomic orbitals to form molecular
50
orbitals.
Energy levels of bonding and antibonding molecular
orbitals in hydrogen (H2).
A bonding molecular orbital has lower energy and greater
stability than the atomic orbitals from which it was formed.
An antibonding molecular orbital has higher energy and
lower stability than the atomic orbitals from which it was
51
formed.
General molecular orbital energy level diagram for the
second-period homonuclear diatomic molecules Li2, Be2, B2,
C2, and N2.
52
Delocalized molecular orbitals are not confined between
two adjacent bonding atoms, but actually extend over three
or more atoms.
Example: Benzene, C6H6
Delocalized p orbitals
53
Bonding in the Carbonate Ion, CO32-
54