Valence Bond Theory Lecture Notes

Introduction

• Previous Knowledge: Chapter 10 covered covalent bonding and predicting structures using Lewis structures and VSEPR theory.
• In Chapter 11: Introducing valence bond theory, orbital hybridization, sigma and pi bonding.
  • Goal: To deepen understanding of bonding in covalent species.
  • Preparation for: Organic chemistry.

Covalent Bonds and Molecular Orbitals

• Covalent Bond Formation: Occurs when atomic orbitals overlap to create a bonding (molecular) orbital.
  • Bonding Orbital: Can accommodate two electrons with opposite spins.
  • Bond Strength: Stronger when interacting orbitals have closely matched energies.
    • Dependence: On principal quantum number (n) and type of orbitals (s, p, d, f).
  • Best Bonds: S orbitals bond best, p orbitals well, d and f orbitals weakly.
• Example - H₂ Molecule:
  • Formation creates a bonding orbital more stable by 436 kJ/mol.
  • Comparison: Ionization energy of H is 2600 kJ/mol.

Orientation and Bond Types

• Sigma Bonds: Form from end-to-end overlap of orbitals.
  • Example: H and F, both form strong sigma bonds.
• Pi Bonds: Result from side-on interactions; not possible with H-F.

Concept of Orbital Hybridization

• Hybrid Orbitals: Remixed atomic orbitals to make predictions without complex molecular orbital theory.
  • Not Necessary: But simplifies predictions.
  • Formation: Number of hybrid orbitals equals number of atomic orbitals mixed.
• Hybridization Types:
  • SP: Linear, 2 regions of electron density.
  • SP2: Trigonal planar, 3 regions.
  • SP3: Tetrahedral, 4 regions.

Examples of Hybridization

• Beryllium Chloride (BeCl₂):
  • Structure: Linear, forms SP hybrid orbitals.
• Boron Trifluoride (BF₃):
  • Structure: Trigonal planar, forms SP2 hybrid orbitals.
• Methane (CH₄):
  • Structure: Tetrahedral, forms SP3 hybrid orbitals.

Sigma and Pi Bonding

• Single Bond: Always sigma.
• Double Bond: One sigma, one pi.
• Triple Bond: One sigma, two pi.
• Pi Bonds: Weaker than sigma due to side-on overlap, important in organic reactions.

Organic Chemistry Context

• Reactivity Sites: Pi bonds often sites of reactivity.
• Sigma Bonds: Form framework, can also break in reactions.

Practice and Examples

• Molecule Analysis: Determine hybridization, count sigma and pi bonds.
• Example: DEET molecule analysis.

Conclusion

• Review Concepts: Ensure understanding of hybridization and molecular structures.

These notes summarize key points and concepts from the lecture on valence bond theory and should serve as a study guide for mastering the material presented.