Chapter 9Chemical Bonding Theories

Valence Bond Theory: Uses Lewis StructuresBonds form using shared electrons between overlapping orbitals on adjacent atoms.Orbitals arrange around central atom to avoid each other.Two types of bonds: sigma and pi.

Molecular Orbital Theory: Uses MO DiagramsOrbitals on atoms “mix” to make molecular orbtials, which go over 2 or more atoms. Two electrons can be in an orbital.Orbitals are either: bonding, antibonding, or nonbonding.Bonds are either: sigma or pi.

Orbitals on bonding atoms overlap directly between bonding atoms

Sigma () Bonding

Between s orbitals

Between p orbitals

Consider VSEPR Shapes and bonding:

Sigma (s) Bonding

What’s wrong with this picture?

Atoms bond by having their

valence orbitals overlap

Enter question text...

1. 1

2. 2

3. 3

Orbitals don’t go in same directions as atomic orbitals.

2s

2pz 2px 2py

Orbitals in CH4Orbitals on C

Orbitals don’t go in same directions as atomic orbitals.

2s

2pz 2px 2py

Orbitals in CH4

Conclusion: Atomic orbitals change shape when they make molecules.

Hybrid Orbitals

Atomic valence orbitals “combine and mix” to form new “Hybrid Orbitals”

Hybrid orbitals go in the VSEPR electron geometry directions.

Types of Hybrid Orbitals

Types of Hybrid Orbitals: 2 Pairs

Types of Hybrid Orbitals: 3 Pairs

Types of Hybrid Orbitals: 4 Pairs

Hybrid orbitals in H2O?

1 2 3 4 5

19%

35%

0%4%

42%1. sp

2. sp2

3. sp3

4. sp3d

5. sp3d2

Hybrid orbitals in O3?

1 2 3 4 5

9%

70%

0%4%

17%

1. sp

2. sp2

3. sp3

4. sp3d

5. sp3d2

Hybrid orbitals in CO2?

1 2 3 4 5

76%

9%

0%1%

13%

1. sp

2. sp2

3. sp3

4. sp3d

5. sp3d2

Hybrid orbitals in SF4?

1 2 3 4 5

3% 0%7%

80%

10%

1. sp

2. sp2

3. sp3

4. sp3d

5. sp3d2

Hybrid orbitals used for sigma bonds and lone pairs.

Unhybridized p orbitals used for pi bonds, or they are empty.

Sigma Bonding

Orbitals overlap directly between two nuclei:

NH3

CH2O

Sigma Bonding involves hybrid orbitals and/or H 1s orbitals.

Pi bonding involves unhybridized p orbitals.

Sigma vs. Pi Bonding

Pi Bond Formation

Bonding in Ethene

Bonding in Acetylene

Bonding in Formaldehyde

How many bonds in acetonitrile?

1. 7 sigma, 0 pi

2. 6 sigma, 1 pi

3. 5 sigma, 2 pi

4. 4 sigma, 3 pi

Bonding in Benzene

Advanced: Allene

Each end carbon is a flat trigonal.

Are they co-planar or perpendicular?

Bonding in Allene

Conformations vs. Isomers

• Isomers: Molecules with same formula but different structure

• Conformers: Different temporary shapes of the same molecule

Bond Rotations• CAN happen around single bonds

• Cannot happen around double bonds

Examples: dichloroethane and diclhloroethene

Types of Isomers

• Constitutional: Different bond connections

• Geometric/Stereoisomers: Same connections, different directions

Cis-Trans Stereoisomerization

When two groups are on a “side” of a molecule.

cis trans

Can these molecules have cis-trans isomers?

What are these?

1. Constitutional isomers

2. Stereoisomers

3. Conformations

4. None of these

What are these?

1. Constitutional isomers

2. Stereoisomers

3. Conformations

4. None of these

What are these?

1. Constitutional isomers

2. Stereoisomers

3. Conformations

4. None of these

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