Molecular Orbitals Lecture Notes

Introduction to Molecular Orbitals

• Atomic Orbitals: Fundamental units of atomic structure.
• Hybridization: Process of combining atomic orbitals to form molecular orbitals.
• Covalent Bonds: Formed by overlapping hybridized orbitals; example, H2 molecule.

Hydrogen Molecule (H2)

• 1s Orbital: Each hydrogen atom has one electron in the 1s orbital.
• Sigma Bond: Formed by overlapping atomic orbitals to create a molecular orbital.
• Energy Consideration: H2 molecule is lower in energy than separate hydrogen atoms due to electrostatic interactions.
• Bonding and Antibonding Orbitals: Formation conserves the number of orbitals.
  • Bonding Orbital: Lower energy, occupied in covalent bonds.
  • Antibonding Orbital (sigma star): Higher energy, occupied if electrons are excited.

Role of Hybridization

• Multiple Bonds: Atoms utilize hybrid orbitals for multiple bonds.
• Carbon Example: Uses 2s and 2p orbitals to form sp3 hybridized orbitals.
  • Promotion: One 2s electron is promoted to 2p before hybridization.
  • sp3 Hybridization: Forms four degenerate orbitals.

Types of Hybridization

• sp3 Hybridization: For four sigma bonds (e.g., methane).
• sp2 Hybridization: Involves three electron domains (e.g., ethylene, C2H4).
  • Pi Bond Formation: Unhybridized p orbitals overlap to form pi bonds.
• sp Hybridization: Involves two electron domains (e.g., acetylene, C2H2).
  • Triple Bond Formation: Two pi bonds from unhybridized p orbitals.

Orbital Diagrams

• Purpose: Visualize the electron configuration in molecules.
• H2 Molecule: 1s electrons fill bonding orbital.
• Bond Order Calculation: (Electrons in bonding orbitals - Electrons in antibonding orbitals)/2.
  • H2 Example: Bond order of 1.

Oxygen Molecule (O2)

• Orbital Configuration: 1s and 2s orbitals form sigma and sigma star orbitals.
• 2p Orbitals: Form sigma, pi, pi star, and sigma star.
• Bond Order Calculation: 10 bonding electrons minus 6 antibonding electrons, divided by 2, gives bond order of 2 (double bond).

Nitrogen Molecule (N2)

• Bond Order: Calculation predicts a bond order of 3, explaining the triple bond in N2.

• Conclusion: Orbital diagrams help predict bonding behavior for molecules.
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