Ch. 9 Chemical Bonding and Molecular Structure

Brady & Senese, 5th Ed

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Index

9.1. Molecules are three-dimensional with shapes that are built from five basic arrangements9.2. Molecular shapes are predicted using the VSEPR model9.3. Molecular symmetry affects the polarity of molecules9.4. Valence bond theory explains bondi

ng as an overlap of orbitals9.5. Hybrid orbitals are used to explain experimental molecular geometries9.6. Hybrid orbitals can be used to explain multiple bonds9.7. Molecular orbital theory explains bon

ding as constructive interference 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 basic arrangements

3

The Five Basic Electron Arrangements

Electron Domains

Shape Electron Pair Geometry

2 linear

3 trigonal planar

4 tetrahedral

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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

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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

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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 CO2?

A. linear

B. planar triangular

C. tetrahedral

D. trigonal bipyramidal

E. 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 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

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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

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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 are substituted first

• This is because the e,e bond angles are 120°, while a,e bond angles are only 90°

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9.2 Molecular shapes are predicted using the VSEPR model 11

Octahedral Geometries

• All bond angles are 90°

• Axial positions are substituted firstPersonal Use Only

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

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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 samePersonal Use Only

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 samePersonal Use Only

9.2 Molecular shapes are predicted using the VSEPR model 15

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 samePersonal Use Only

9.2 Molecular shapes are predicted using the VSEPR model 16

Your Turn!

What is the molecular geometry of C in CH4?

A. Linear

B. Square planar

C. Square pyramidal

D. Tetrahedral

E. None of these

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9.3 Molecular symmetry affects the polarity of molecules 17

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

polarPersonal Use Only

9.3 Molecular symmetry affects the polarity of molecules 18

Learning Check:

Polar or non-polar?

polar Non-polarpolar

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9.3 Molecular symmetry affects the polarity of molecules 19

Your Turn!

CH2ClCH2Cl (freon-150) is likely to be:

A. Polar

B. non-polar

C. cannot tellPersonal Use Only

9.3 Molecular symmetry affects the polarity of molecules 20

Your Turn!

Benzoyl peroxide (used in common acne medications) is likely to be:

A. polar

B. non-polar

C. cannot tellPersonal Use Only

9.4 Valence bond theory explains bonding as an overlap of atomic orbitals 21

Valence Bond Theory

• H2 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

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9.4 Valence bond theory explains bonding as an overlap of atomic orbitals 22

VB Theory And H2S

• 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

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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 type of overlap does not occur in BH3?

A. s-s

B. s-p

C. p-p

D. none of these

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9.4 Valence bond theory explains bonding as an overlap of atomic orbitals 24

Your turn!

According to VB Theory:

Which orbitals overlap in the formation of NH3?

A. s-s

B. s-p

C. p-p

D. none of these

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9.4 Valence bond theory explains bonding as an overlap of atomic orbitals 25

Difficulties With VB Theory So Far:

• Most experimental bond angles do not support those predicted by mere atomic orbital overlap

• For example: C 1s22s22p2 and H 1s1

• Experimental bond angles in methane are 109.5° and all are the same

• p orbitals are 90° apart, and not all valence e- in C 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 26

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

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9.5 Hybrid orbitals are used to explain experimental molecular geometries 27

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)Personal Use Only

9.5 Hybrid orbitals are used to explain experimental molecular geometries 28

Hybrids From s & p Atomic Orbitals take VSEPR Geometry

Hybrid Atomic Orbitals Used

Electron Geometry

sp3 s + px + py + pz

Tetrahedral, bond angles 109.5˚

sp2 s + px + py Trigonal planar, bond angles 120 ˚

sp s + px Linear,bond angles 180 ˚

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9.5 Hybrid orbitals are used to explain experimental molecular geometries 29

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 30

Determining hybridization:

1. expand all valence electrons within the valence energy level. For C, for example this means:

• 2s↑↓ 2p ↑ _ ↑ ___ [He]2s2 2p1

• Becomes:

• 2s↑ 2p ↑ _ ↑ _ ↑ __Personal Use Only

9.5 Hybrid orbitals are used to explain experimental molecular geometries 31

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 CH4 we see that there are 4 attached atoms and no lone pairs on C. Thus we will need 4 hybrid orbitals.

H

HH

H

C

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9.5 Hybrid orbitals are used to explain experimental molecular geometries 32

Hybridization (sp3)

• 3. Now analyze the atomic orbital needs. You will need to use one atomic orbital for every hybrid orbital .

• For C in CH4 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 “sp3” orbitals.

H

HH

H

C

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9.5 Hybrid orbitals are used to explain experimental molecular geometries 33

Bonding in CH4

• The 4 hybrid orbitals are evenly distributed around the C

• The H s-orbitals overlap the sp3 hybrid orbitals to form the bonds.

H

HH

H

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9.5 Hybrid orbitals are used to explain experimental molecular geometries 34

s & p hybrid shapes

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9.5 Hybrid orbitals are used to explain experimental molecular geometries 35

Your Turn!

In the compound CH3OH, what is the expected hybridization on O?

A. sp

B. sp2

C. sp3

D. O does not hybridizePersonal Use Only

9.5 Hybrid orbitals are used to explain experimental molecular geometries 36

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 octetPersonal Use Only

9.5 Hybrid orbitals are used to explain experimental molecular geometries 37

Expanded Octet hybridization

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9.6 Hybrid orbitals can be used to describe multiple bonds 38

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

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9.6 Hybrid orbitals can be used to describe multiple bonds 39

Hybridization of C in CH2O O

H HC

1. Expand all valence electrons within the same energy level. For C, for example this means:

• 2s↑↓ 2p ↑ _ ↑ ___ [He]2s2 2p1

• Becomes:

• 2s↑ 2p ↑ _ ↑ _ ↑ __Personal Use Only

9.6 Hybrid orbitals can be used to describe multiple bonds 40

Hybridization of C in CH2O O

H HC

• 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 CH2O we see that there are 3 attached atoms

and no lone pairs on C. Thus we will need 3 hybrid orbitals.

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9.6 Hybrid orbitals can be used to describe multiple bonds 41

sp2 Hybridization

• 3. Now analyze the atomic orbital needs. You will need to use one atomic orbital for every hybrid orbital. For C in CH2O 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 “sp2” orbitals. We are left with one unhybridized orbital.

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9.6 Hybrid orbitals can be used to describe multiple bonds 42

Now analyze the O:

• [He] 2s2 2p2 (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, sp2. (2s↑↓ 2p ↑ ↓_ ↑ ) _ ↑

• Again we are left with one unhybridized p orbital

O

H HC

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9.6 Hybrid orbitals can be used to describe multiple bonds 43

Pi Bonding

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9.6 Hybrid orbitals can be used to describe multiple bonds 44

H−C≡C −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

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9.6 Hybrid orbitals can be used to describe multiple bonds 45

Your Turn!

Consider a molecule of CH3CO2H:

How many pi bonds are there in the molecule?

A. 1

B. 2

C. 3

D. 4

E. There are none

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9.7 Molecular orbital theory explains bonding as constructive interference of atomic orbitals

46

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.

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9.7 Molecular orbital theory explains bonding as constructive interference of atomic orbitals

47

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.Personal Use Only

9.7 Molecular orbital theory explains bonding as constructive interference of atomic orbitals

48

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.

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9.7 Molecular orbital theory explains bonding as constructive interference of atomic orbitals

49

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 atomic orbitals

50

MO diagrams

Draw the expected MO diagram for:• O2

• BH

• He2

Which are not likely to exist, and why?

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9.8 Molecular orbital theory uses delocalized orbitals to describe molecules with resonance structures

51

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 structurePersonal Use Only

9.8 Molecular orbital theory uses delocalized orbitals to describe molecules with resonance structures

52

CO32- 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.

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