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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 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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Orbitals on bonding atoms overlap directly between bonding atoms Sigma ( ) Bonding Between s orbitals Between p orbitals
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Consider VSEPR Shapes and bonding: Sigma (s) Bonding
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What’s wrong with this picture? Atoms bond by having their valence orbitals overlap
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Orbitals don’t go in same directions as atomic orbitals. 2s 2p z 2p x 2p y Orbitals in CH 4 Orbitals on C
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Orbitals don’t go in same directions as atomic orbitals. 2s 2p z 2p x 2p y Orbitals in CH 4 Conclusion: Atomic orbitals change shape when they make molecules.
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Hybrid Orbitals Atomic valence orbitals “combine and mix” to form new “Hybrid Orbitals” Hybrid orbitals go in the VSEPR electron geometry directions.
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Types of Hybrid Orbitals
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Types of Hybrid Orbitals: 2 Pairs
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Types of Hybrid Orbitals: 3 Pairs
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Types of Hybrid Orbitals: 4 Pairs
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Types of Hybrid Orbitals
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Sigma Bonding Orbitals overlap directly between two nuclei:
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Sigma Bonding involves hybrid orbitals and/or H 1s orbitals. Pi bonding involves unhybridized p orbitals. Sigma vs. Pi Bonding
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Pi Bond Formation
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Bonding in Ethene
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Bonding in Acetylene
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Bonding in Formaldehyde
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Bonding in Benzene
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Advanced: Allene Each end carbon is a flat trigonal. Are they co-planar or perpendicular?
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Bonding in Allene
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Conformations vs. Isomers Isomers: Molecules with same formula but different structure Conformers: Different temporary shapes of the same molecule
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Bond Rotations CAN happen around single bonds Cannot happen around double bonds
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Cis-Trans Isomerization When two groups are on a “side” of a molecule. cis trans
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Can these molecules have cis-trans isomers?
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