Molecular Orbitals

1
Molecular Orbitals

• An overview

2
MO Theory

• Robert Mullikan won the Nobel Prize in 1966 for
  developing this theory.
• This theory describes the electrons in orbitals
  belonging to the entire molecule.

3
Molecular Orbitals

• A molecular orbital (MO) can hold a maximum of
  two electrons.
• A MO has a definite energy.
• We can represent an MO with an electron-density
  cloud.

4
LCAO

• Linear Combination of Atomic Orbitals
• Whenever two atomic orbitals overlap, two
  molecular orbitals form.
• in out
• Energy is conserved, so
• One orbital will be lower in energy
• One orbital will be higher in energy

5
The Hydrogen Molecule
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The Two Orbitals

• s Antibonding orbital
• Very little electron density between the nuclei
• Higher in energy
• Destabilizes bond formation
• s Bonding orbital
• Promotes bond formation
• Electron density is between the nuclei
• Lower in energy

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Bonding and Antibonding Orbitals
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MO Diagrams
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H2 and He2
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Complex MO Diagrams
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Bond Order

• In MO theory, bond stability of a covalent bond
  is related to its bond order.
• Bond order ½( bonding electrons -
  antibonding electrons)
• Bond order can be an integer or a fraction

Bond Order of bonds
0 No bond exists
1 Single bond
2 Double bond
3 Triple bond
12
Calculate Bond Order for He2
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Compare bond orders

• Which would be more stable He2 or He2?
  Justify your answer.