Chapter 9 Chemical Bonding Theories Valence Bond Theory: Uses Lewis Structures Bonds 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 Diagrams Orbitals 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.
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Chapter 9 Chemical Bonding Theories Valence Bond Theory: Uses Lewis Structures Bonds form using shared electrons between overlapping orbitals on adjacent.
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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