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Index
9.1. Molecules are three-dimensional with shapes that arebuilt from five basic arrangements
9.2. Molecular shapes are predicted using the VSEPR model9.3. Molecular symmetry affects the polarity of molecules9.4. Valence bond theory explains bonding as an overlap of
orbitals9.5. Hybrid orbitals are used to explain experimental
molecular geometries9.6. Hybrid orbitals can be used to explain multiple bonds
9.7. Molecular orbital theory explains bonding as constructiveinterference of atomic orbitals9.8. Molecular orbital theory uses delocalized orbitals to
describe molecules with resonance structure
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9.1 Molecules are three-dimensional with shapes that are built from five basicarran ements
3
The Five Basic Electron Arrangements
ElectronDomains
Shape Electron PairGeometry
2 linear
3 trigonal planar
4 tetrahedral
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9.1 Molecules are three-dimensional with shapes that are built from five basicarran ements
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 axialpositions
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9.1 Molecules are three-dimensional with shapes that are built from five basicarran ements
5
Learning Check:Identify The Electron Pair Geometry For Each Center
tetrahedral tetrahedral Trigonal
bipyramidal
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9.1 Molecules are three-dimensional with shapes that are built from five basicarran ements
6
Your Turn!
What is the electron pair geometry for C in CO 2?A. linearB. planar triangularC. tetrahedral
D. trigonal bipyramidalE. octahedral
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9.2 Molecular shapes are predicted using the VSEPR model 7
Bonding Domains And Non-bonding Domains
Bonding domains are sharedbetween nuclei
Non-bonding domains are notshared between nuclei-they exert agreater electrical field
Repulsion leads non-bondingdomains to occupy larger space
The basic shapes are distorted bynon-bonding domains to create the molecular geometry
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9.2 Molecular shapes are predicted using the VSEPR model 8
Trigonal Planar Molecular Geometries
Bonding Domains Non-bondingDomains
MolecularGeometry
3 0 trigonal planar
2 1 bent
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9.2 Molecular shapes are predicted using the VSEPR model 9
Tetrahedral Molecular geometries
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9.2 Molecular shapes are predicted using the VSEPR model 10
Trigonal Bipyramidal
Equatorial (e)positions aresubstituted first
This is becausethe e,e bondangles are 120 ,while a,e bondangles are only90
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9.2 Molecular shapes are predicted using the VSEPR model 11
Octahedral Geometries
All bond anglesare 90
Axial positionsare substitutedfirst
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9.2 Molecular shapes are predicted using the VSEPR model 12
Learning Check:
Identify the molecular geometry for each center
Trigonal
pyramidal
Non-linear,
bentLinear
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9.2 Molecular shapes are predicted using the VSEPR model 13
Your Turn!
Which require more space?A. bond pairsB. lone pairsC. both are the same
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9.2 Molecular shapes are predicted using the VSEPR model 14
Your Turn!
Which bond angles are closer in a trigonalbipyramidal structure (a= axial; e=equatorial)?
A. a-aB. a-e
C. e-eD. they are all the same
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9.2 Molecular shapes are predicted using the VSEPR model 15
Your Turn!
What is the molecular geometry of C in CH 4?A. LinearB. Square planarC. Square pyramidal
D. TetrahedralE. None of these
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9.3 Molecular symmetry affects the polarity of molecules 16
Polar Molecules Are Asymmetric
To determine the polarity, draw the structure usingthe 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
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9.3 Molecular symmetry affects the polarity of molecules 17
Learning Check:
Polar or non-polar?
polar Non-polarpolar
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9.3 Molecular symmetry affects the polarity of molecules 18
Your Turn!
CH 2ClCH 2Cl (freon-150) is likely to be:A. PolarB. non-polarC. cannot tell
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9.3 Molecular symmetry affects the polarity of molecules 19
Your Turn!
Benzoyl peroxide (used in common acnemedications) is likely to be:
A. polarB. non-polar
C. cannot tell
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9.4 Valence bond theory explains bonding as an overlap of atomic orbitals 20
Valence Bond Theory
H2 bonds form because atomic valence orbitalsoverlap
HF involves overlaps between the s orbital on Hand the 2p orbital of F
1s 1s
1s 2s 2p
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9.4 Valence bond theory explains bonding as an overlap of atomic orbitals 21
VB Theory And H 2S
Assume that theunpaired e - in S and Hare free to form apaired bond
We may assume thatthe H-S bond formsbetween an s and a porbital
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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-sB. s-p
C. p-pD. none of these
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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-sB. s-p
C. p-pD. none of these
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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 supportthose predicted by mere atomic orbital overlap
For example: C 1s 22s22p 2 and H 1s 1
Experimental bond angles in methane are 109.5
and all are the same p orbitals are 90 apart, and not all valence e - inC are in the p orbitals
How can multiple bonds form?
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9.5 Hybrid orbitals are used to explain experimental molecular geometries 25
Hybridization
The mixing of atomic orbitals to allow formationof bonds that have realistic bond angles
The new shapes that result are called hybridorbitals
The number of hybrid orbitals required = thenumber of bonding domains + the number ofnon-bonding domains on the atom
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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 previouslydefined by atomic orbitals, we name the hybrid asa combination of the orbitals used to form the newhybrid
One atomic orbital is used for every hybrid formed(orbitals are conserved)
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9.5 Hybrid orbitals are used to explain experimental molecular geometries 27
Hybrids From s & p Atomic Orbitals takeVSEPR Geometry
Hybrid Atomic
OrbitalsUsed
Electron
Geometry
sp3 s + p x + p y+ p z
Tetrahedral,bond angles
109.5 sp2 s + p x + p y Trigonal
planar, bondangles 120
sp s + p x Linear,bond angles180
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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
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9.5 Hybrid orbitals are used to explain experimental molecular geometries 29
Determining hybridization:
1. expand all valence electrons within the valenceenergy level. For C, for example this means:
2s 2p _ ___ [He]2s 2 2p1
Becomes:
2s
2p
_
_
__
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9.5 Hybrid orbitals are used to explain experimental molecular geometries 31
Hybridization (sp 3)
3. Now analyze the atomic orbital needs. You will needto 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
HH
H
C
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9.5 Hybrid orbitals are used to explain experimental molecular geometries 32
Bonding in CH 4
The 4 hybrid orbitals areevenly distributed around
the C The H s-orbitals overlap
the sp 3 hybrid orbitals toform the bonds.
H
HH
H
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9.5 Hybrid orbitals are used to explain experimental molecular geometries 33
s & p hybrid shapes
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9.5 Hybrid orbitals are used to explain experimental molecular geometries 34
Your Turn!
In the compound CH 3OH, what is the expectedhybridization on O?
A. spB. sp 2
C. sp 3
D. O does not hybridize
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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
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9.5 Hybrid orbitals are used to explain experimental molecular geometries 36
Expanded Octet hybridization
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9.6 Hybrid orbitals can be used to describe multiple bonds 37
Bonding Types
Two types of bonds result fromorbital overlap:
sigma bondsfrom head-on overlaplie along the bond axisaccount for the first bond
pi bondsfrom lateral overlap by adjacent p ord orbitals
pi bonds are perpendicular to bondaxisaccount for the second and thirdbonds in a multiple bond
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9.6 Hybrid orbitals can be used to describe multiple bonds 38
Hybridization of C in CH 2O O
H HC
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 _ _ __
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9.6 Hybrid orbitals can be used to describe multiple bonds 39
Hybridization of C in CH 2O O
H HC
2. Now analyze the bonding and lone pair needs.You will need to use one hybrid orbital forevery attached atom and one for every lone pair.
For C in CH 2O we see that there are 3 attached atomsand no lone pairs on C. Thus we will need 3 hybridorbitals.
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9.6 Hybrid orbitals can be used to describe multiple bonds 40
sp2 Hybridization
3. Now analyze the atomic orbital needs. You willneed to use one atomic orbital for every hybrid
orbital.For C in CH 2O we will need 3 hybrid orbitals.2s 2p _ _Thus, we will need to use 3 valence level atomicorbitals available to us, and one of the p orbitals willremain.(2s 2p _ ) _
s + p + p 3 new sp2
orbitals.We are left with one unhybridized orbital.
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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 threehybrid orbitals.
No expansion needed, as one unpaired e - isavailable to bond. Use 3 atomic orbitals to makethe new hybrids, sp 2. (2s 2p _ ) _
Again we are left with one unhybridized porbital
O
H HC
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9.6 Hybrid orbitals can be used to describe multiple bonds 42
Pi Bonding
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9.6 Hybrid orbitals can be used to describe multiple bonds 43
H C C H
Each C has atriple bond
and a singlebond
Requires 2
hybridorbitals, sp
unhybridized p orbitalsused to formthe pi bond
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9.6 Hybrid orbitals can be used to describe multiple bonds 44
Your Turn!
Consider a molecule of CH 3CO 2H:How many pi bonds are there in the molecule?A. 1B. 2
C. 3D. 4E. There are none
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9.7 Molecular orbital theory explains bonding as constructive interference of atomicorbitals
45
Molecular Orbital Theory
Modification of VB theory that considers that the orbitalsmay exhibit interference.
Waves may interfere constructively or destructively Bonding orbitals stabilize, antibonding destabilize.
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9.7 Molecular orbital theory explains bonding as constructive interference of atomicorbitals
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.
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9.7 Molecular orbital theory explains bonding as constructive interference of atomicorbitals
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Filling MO diagrams
1. Electrons fill the lowest-energy orbitals that areavailable.
2. No more than two electrons, with spins paired,can occupy any orbital.
3. Electrons spread out as much as possible, withspins unpaired, over orbitals that have the sameenergy.
4. Bond order = e - in bonding orbital-e - innonbonding orbitals.
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9.7 Molecular orbital theory explains bonding as constructive interference of atomicorbitals
48
Diatomic MO diagrams differ by group
A) I - V B) VI-VIIIA
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9.7 Molecular orbital theory explains bonding as constructive interference of atomicorbitals
49
MO diagrams
Draw the expected MO diagram for: O2
BH
He 2
Which are not likely to exist, and why?
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9.8 Molecular orbital theory uses delocalized orbitals to describe molecules withresonance structures
50
Delocalized Electrons
Lewis structures use resonance to explain that theactual molecule appears to have several equivalent
bonds, rather than different possible structures MO theory shows the electrons being delocalized
in the structure
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