Molecular Orbital Theory (MO Theory)

Overview

• MO theory describes chemical bonding using quantum mechanics.
• Electrons in molecules occupy molecular orbitals formed from atomic orbitals.
• Molecular orbitals can be:
  • Bonding: Stabilizes the molecule.
  • Antibonding: Destabilizes the molecule.
  • Non-bonding: Neither stabilizes nor destabilizes.
• MO theory is more comprehensive than valence bond theory.

Key Concepts

1. Combining Atomic Orbitals

• Atomic orbitals combine to form new molecular orbitals when atoms bond.
• This is done using the Linear Combination of Atomic Orbitals (LCAO) method.

2. Bonding vs. Antibonding Orbitals

• Bonding Orbitals:
  • Formed when atomic orbitals combine constructively.
  • Lower in energy than parent atomic orbitals.
  • Increase electron density between nuclei, stabilizing molecules.
• Antibonding Orbitals:
  • Result from destructive combination of atomic orbitals.
  • Higher in energy than parent atomic orbitals.
  • Decrease electron density, destabilizing molecules.

3. Molecular Orbital Diagrams

• Visual diagrams that show energy levels of molecular orbitals.
• Illustrate electron occupancy.

4. Bond Order

• Calculated by the difference between number of bonding vs. antibonding electrons divided by two.
• Indicates the strength of the bond and molecule stability.

5. Delocalization

• Electrons are not confined to specific bonds but are spread out over the entire molecule.

Examples

• Hydrogen (H2):

  • Two 1s atomic orbitals form one bonding and one antibonding orbital.
  • Electrons fill the bonding orbital, stabilizing the molecule.

• Oxygen (O2):

  • Explained by MO theory's paramagnetism, due to two unpaired electrons.

• Ethene (C2H4):

  • Pi bonds described by combining p orbitals to form bonding and antibonding pi orbitals.

Advantages of MO Theory

• Accurate Prediction of Magnetic Properties:

  • MO theory predicts magnetic properties, such as paramagnetism in molecules like oxygen.

• More Accurate than Valence Bond Theory:

  • Provides a better description of complex bonding situations.

• Delocalized Electrons:

  • Views electrons as spread over the entire molecule, offering a more accurate depiction of electron distribution.

Conclusion

• MO Theory is a powerful model for understanding chemical bonding and molecular structure.
• It provides insights into electron behavior that are not easily explained by valence bond theory.
• Useful for predicting the physical and chemical properties of molecules.

These notes summarize key aspects of MO Theory as explained primarily through Chemistry LibreTexts and other educational resources.