Ch.

9 Chemical Bonding and

Molecular Structure
Brady & Senese, 5th Ed
Index
9.1. Molecules are three-dimensional with shapes that are
built from five basic arrangements
9.2. Molecular shapes are predicted using the VSEPR model
9.3. Molecular symmetry affects the polarity of molecules
9.4. Valence bond theory explains bonding as an overlap of
orbitals
9.5. Hybrid orbitals are used to explain experimental
molecular geometries
9.6. Hybrid orbitals can be used to explain multiple bonds
9.7. Molecular orbital theory explains bonding as constructive
interference of atomic orbitals
9.8. Molecular orbital theory uses delocalized orbitals to
describe molecules with resonance structure
9.1 Molecules are three-dimensional with shapes that are built from five basic
arrangements
3
The Five Basic Electron Arrangements
Electron
Domains
Shape Electron Pair
Geometry
2 linear
3 trigonal planar
4 tetrahedral
9.1 Molecules are three-dimensional with shapes that are built from five basic
arrangements
4
The Five Basic Electron Arrangements (Cont.)
Electron Domains Shape Electron Pair Geometry
5 trigonal bipyramidal
has equatorial and axial
positions.
6 octahedral
has equatorial and axial
positions
9.1 Molecules are three-dimensional with shapes that are built from five basic
arrangements
5
Learning Check:
Identify The Electron Pair Geometry For Each Center
tetrahedral tetrahedral
Trigonal
bipyramidal
9.1 Molecules are three-dimensional with shapes that are built from five basic
arrangements
6
Your Turn!
What is the electron pair geometry for C in CO
2
?
A. linear
B. planar triangular
C. tetrahedral
D. trigonal bipyramidal
E. octahedral
9.2 Molecular shapes are predicted using the VSEPR model 7
Bonding Domains And Non-bonding Domains
Bonding domains are shared
between nuclei
Non-bonding domains are not
shared between nuclei-they exert a
greater electrical field
Repulsion leads non-bonding
domains to occupy larger space
The basic shapes are distorted by
non-bonding domains to create the
molecular geometry
9.2 Molecular shapes are predicted using the VSEPR model 8
Trigonal Planar Molecular Geometries
Bonding Domains Non-bonding
Domains
Molecular
Geometry
3 0 trigonal planar
2 1 bent
9.2 Molecular shapes are predicted using the VSEPR model 9
Tetrahedral Molecular geometries
9.2 Molecular shapes are predicted using the VSEPR model 10
Trigonal Bipyramidal
Equatorial (e)
positions are
substituted first
This is because
the e,e bond
angles are 120,
while a,e bond
angles are only
90
9.2 Molecular shapes are predicted using the VSEPR model 11
Octahedral Geometries
All bond angles
are 90
Axial positions
are substituted
first
9.2 Molecular shapes are predicted using the VSEPR model 12
Learning Check:
Identify the molecular geometry for each center
Trigonal
pyramidal
Non-linear,
bent
Linear
9.2 Molecular shapes are predicted using the VSEPR model 13
Your Turn!
Which require more space?
A. bond pairs
B. lone pairs
C. both are the same
9.2 Molecular shapes are predicted using the VSEPR model 14
Your Turn!
Which bond angles are closer in a trigonal
bipyramidal structure (a= axial; e=equatorial)?
A. a-a
B. a-e
C. e-e
D. they are all the same
9.2 Molecular shapes are predicted using the VSEPR model 15
Your Turn!
What is the molecular geometry of C in CH
4
?
A. Linear
B. Square planar
C. Square pyramidal
D. Tetrahedral
E. None of these
9.3 Molecular symmetry affects the polarity of molecules 16
Polar Molecules Are Asymmetric
To determine the polarity, draw the structure using
the proper molecular geometry
Draw the bond dipoles
If they cancel, the molecule is non-polar
If the molecule has uneven dipole distribution, it is
polar
9.3 Molecular symmetry affects the polarity of molecules 17
Learning Check:
Polar or non-polar?
polar Non-polar polar
9.3 Molecular symmetry affects the polarity of molecules 18
Your Turn!
CH
2
ClCH
2
Cl (freon-150) is likely to be:
A. Polar
B. non-polar
C. cannot tell
9.3 Molecular symmetry affects the polarity of molecules 19
Your Turn!
Benzoyl peroxide (used in common acne
medications) is likely to be:
A. polar
B. non-polar
C. cannot tell
9.4 Valence bond theory explains bonding as an overlap of atomic orbitals 20
Valence Bond Theory
H
2
bonds form because atomic valence orbitals
overlap
HF involves overlaps between the s orbital on H
and the 2p orbital of F
1s 1s
1s 2s 2p
9.4 Valence bond theory explains bonding as an overlap of atomic orbitals 21
VB Theory And H
2
S
Assume that the
unpaired e
-
in S and H
are free to form a
paired bond
We may assume that
the H-S bond forms
between an s and a p
orbital
9.4 Valence bond theory explains bonding as an overlap of atomic orbitals 22
Your turn!
According to VB Theory:
Which type of overlap does not occur in BH
3
?
A. s-s
B. s-p
C. p-p
D. none of these
9.4 Valence bond theory explains bonding as an overlap of atomic orbitals 23
Your turn!
According to VB Theory:
Which orbitals overlap in the formation of NH
3
?
A. s-s
B. s-p
C. p-p
D. none of these
9.4 Valence bond theory explains bonding as an overlap of atomic orbitals 24
Difficulties With VB Theory So Far:
Most experimental bond angles do not support
those predicted by mere atomic orbital overlap
For example: C 1s
2
2s
2
2p
2
and H 1s
1
Experimental bond angles in methane are 109.5
and all are the same
p orbitals are 90apart, and not all valence e
-
in
C are in the p orbitals
How can multiple bonds form?
9.5 Hybrid orbitals are used to explain experimental molecular geometries 25
Hybridization
The mixing of atomic orbitals to allow formation
of bonds that have realistic bond angles
The new shapes that result are called hybrid
orbitals
The number of hybrid orbitals required = the
number of bonding domains + the number of
non-bonding domains on the atom
9.5 Hybrid orbitals are used to explain experimental molecular geometries 26
What Shall We Call These New Orbitals?
Since we have annexed the spaces previously
defined by atomic orbitals, we name the hybrid as
a combination of the orbitals used to form the new
hybrid
One atomic orbital is used for every hybrid formed
(orbitals are conserved)
9.5 Hybrid orbitals are used to explain experimental molecular geometries 27
Hybrids From s & p Atomic Orbitals take
VSEPR Geometry
Hybrid Atomic
Orbitals
Used
Electron
Geometry
sp
3
s + p
x
+ p
y
+ p
z
Tetrahedral,
bond angles
109.5
sp
2
s + p
x
+ p
y
Trigonal
planar, bond
angles 120
sp s + p
x
Linear,
bond angles
180
9.5 Hybrid orbitals are used to explain experimental molecular geometries 28
Learning Check:
Identify The Hybrid Orbitals In The Following, Based
On Their VSEPR Geometry
9.5 Hybrid orbitals are used to explain experimental molecular geometries 29
Determining hybridization:
1. expand all valence electrons within the valence
energy level. For C, for example this means:
2s 2p _ ___ [He]2s
2
2p
1
Becomes:
2s 2p _ _ __
9.5 Hybrid orbitals are used to explain experimental molecular geometries 30
Hybridization
2. Now analyze the bonding and lone pair needs.
You will need to use one hybrid orbital for every
bonding domain and one for every non-bonding
domain.
For C in CH
4
we see that there are 4 attached
atoms and no lone pairs on C. Thus we will need 4
hybrid orbitals.
H
H
H
H
C
9.5 Hybrid orbitals are used to explain experimental molecular geometries 31
Hybridization (sp
3
)
3. Now analyze the atomic orbital needs. You will need
to use one atomic orbital for every hybrid orbital .
For C in CH
4
we will need 4 hybrid orbitals.
2s 2p _ _
Thus, we will need to use all valence level atomic
orbitals available to us.
(2s 2p _ _ )
S + p + p + p 4 new equivalent sp
3
orbitals.
H
H
H
H
C
9.5 Hybrid orbitals are used to explain experimental molecular geometries 32
Bonding in CH
4
The 4 hybrid orbitals are
evenly distributed around
the C
The H s-orbitals overlap
the sp
3
hybrid orbitals to
form the bonds.
H
H H
H
9.5 Hybrid orbitals are used to explain experimental molecular geometries 33
s & p hybrid shapes
9.5 Hybrid orbitals are used to explain experimental molecular geometries 34
Your Turn!
In the compound CH
3
OH, what is the expected
hybridization on O?
A. sp
B. sp
2
C. sp
3
D. O does not hybridize
9.5 Hybrid orbitals are used to explain experimental molecular geometries 35
Expanded Octet Hybridization
Can be predicted from the geometry as well
In these situations, d orbitals are be needed to
provide room for the extra electrons
One d orbital is added for each pair of electrons in
excess of the standard octet
9.5 Hybrid orbitals are used to explain experimental molecular geometries 36
Expanded Octet hybridization
9.6 Hybrid orbitals can be used to describe multiple bonds 37
Bonding Types
Two types of bonds result from
orbital overlap:
sigma bonds
from head-on overlap
lie along the bond axis
account for the first bond
pi bonds
from lateral overlap by adjacent p or
d orbitals
pi bonds are perpendicular to bond
axis
account for the second and third
bonds in a multiple bond
9.6 Hybrid orbitals can be used to describe multiple bonds 38
Hybridization of C in CH
2
O
O
H H
C
1. Expand all valence electrons within the same energy level.
For C, for example this means:
2s 2p _ ___ [He]2s
2
2p
1
Becomes:
2s 2p _ _ __
9.6 Hybrid orbitals can be used to describe multiple bonds 39
Hybridization of C in CH
2
O
O
H H
C
2. Now analyze the bonding and lone pair needs.
You will need to use one hybrid orbital for
every attached atom and one for every lone pair.
For C in CH
2
O we see that there are 3 attached atoms
and no lone pairs on C. Thus we will need 3 hybrid
orbitals.
9.6 Hybrid orbitals can be used to describe multiple bonds 40
sp
2
Hybridization
3. Now analyze the atomic orbital needs. You will
need to use one atomic orbital for every hybrid
orbital.
For C in CH
2
O we will need 3 hybrid orbitals.
2s 2p _ _
Thus, we will need to use 3 valence level atomic
orbitals available to us, and one of the p orbitals will
remain.
(2s 2p _ ) _
s + p + p 3 new sp
2
orbitals.
We are left with one unhybridized orbital.
9.6 Hybrid orbitals can be used to describe multiple bonds 41
Now analyze the O:
[He] 2s
2
2p
2
(2s 2p _ ) _

The O is has one bonding domain and 2 non-

bonding domains, hence it will require three
hybrid orbitals.
No expansion needed, as one unpaired e
-
is
available to bond. Use 3 atomic orbitals to make
the new hybrids, sp
2
. (2s 2p _
) _
Again we are left with one unhybridized p
orbital
O
H H
C
9.6 Hybrid orbitals can be used to describe multiple bonds 42
Pi Bonding
9.6 Hybrid orbitals can be used to describe multiple bonds 43
HCC H
Each C has a
triple bond
and a single
bond
Requires 2
hybrid
orbitals, sp
unhybridized
p orbitals
used to form
the pi bond
9.6 Hybrid orbitals can be used to describe multiple bonds 44
Your Turn!
Consider a molecule of CH
3
CO
2
H:
How many pi bonds are there in the molecule?
A. 1
B. 2
C. 3
D. 4
E. There are none
9.7 Molecular orbital theory explains bonding as constructive interference of atomic
orbitals
45
Molecular Orbital Theory
Modification of VB theory that considers that the orbitals
may exhibit interference.
Waves may interfere constructively or destructively
Bonding orbitals stabilize, antibonding destabilize.
9.7 Molecular orbital theory explains bonding as constructive interference of atomic
orbitals
46
MO diagrams
Show atomic energy level diagram for each atom
Show molecular orbitals (bonding and
antibonding*)
1 MO for each Atomic orbital.
Show electron occupancy of the orbitals.
9.7 Molecular orbital theory explains bonding as constructive interference of atomic
orbitals
47
Filling MO diagrams
1. Electrons fill the lowest-energy orbitals that are
available.
2. No more than two electrons, with spins paired,
can occupy any orbital.
3. Electrons spread out as much as possible, with
spins unpaired, over orbitals that have the same
energy.
4. Bond order = e
-
in bonding orbital-e
-
in
nonbonding orbitals.
9.7 Molecular orbital theory explains bonding as constructive interference of atomic
orbitals
48
Diatomic MO diagrams differ by group
A) I - V B) VI-VIIIA
9.7 Molecular orbital theory explains bonding as constructive interference of atomic
orbitals
49
MO diagrams
Draw the expected MO diagram for:
O
2
BH
He
2
Which are not likely to exist, and why?
9.8 Molecular orbital theory uses delocalized orbitals to describe molecules with
resonance structures
50
Delocalized Electrons
Lewis structures use resonance to explain that the
actual molecule appears to have several equivalent
bonds, rather than different possible structures
MO theory shows the electrons being delocalized
in the structure
9.8 Molecular orbital theory uses delocalized orbitals to describe molecules with
resonance structures
51
CO
3
2-
Hybridization
Carbonate has three equivalent resonance
structures. What are they, and which electrons are
delocalized?
Draw the hybrid molecule to indicate the
delocalization of these electrons.