Molecular Orbital Theory - Chemistry 2e

Learning Objectives

• Outline the quantum-mechanical approach to deriving molecular orbitals from atomic orbitals.
• Describe traits of bonding and antibonding molecular orbitals.
• Calculate bond orders based on molecular electron configurations.
• Write molecular electron configurations for first- and second-row diatomic molecules.
• Relate electron configurations to molecular stabilities and magnetic properties.

Introduction

• Lewis Structures: Useful for predicting electron-pair geometry, molecular geometry, and bond angles in covalent molecules.
• Problem with O2: Lewis structure does not account for its paramagnetism (attraction to magnetic fields).

Magnetic Properties

• Paramagnetism: Attraction to magnetic fields due to unpaired electrons.
• Diamagnetism: Weak repulsion from magnetic fields due to paired electrons.
• O2 has unpaired electrons, contrary to its Lewis structure prediction.

Molecular Orbital Theory (MO Theory)

• Explains bonding and paramagnetism in molecules like O2.
• Uses combination of atomic orbitals to form delocalized molecular orbitals.
• Differentiates between bonding and antibonding interactions based on orbital filling.

Key Concepts

• Valence Bond Theory: Localized bonds, uses hybrid orbitals.
• Molecular Orbital Theory: Delocalized electrons, combination of atomic orbitals.

Molecular Orbitals

• Homonuclear Diatomic Molecules: e.g., H2, Cl2.
• Linear Combination of Atomic Orbitals (LCAO): Combining atomic orbitals to create molecular orbitals.
• Types:
  • Sigma (σ) and Sigma-star (σ*) from s orbitals.
  • Pi (π) and Pi-star (π*) from p orbitals.

Electron Configuration and Bond Order

• Molecular Orbital Diagrams: Shows energy levels of atomic and molecular orbitals.
• Bond Order: Calculated as (bonding electrons - antibonding electrons)/2.
• Examples:
  • H2: Bond order = 1 (stable).
  • He2: Bond order = 0 (unstable).

Diatomic Molecules of the Second Period

• Electron Configurations: Predict using molecular orbital diagrams and s-p mixing.
• Bond Order Examples:
  • Li2: Stable.
  • Be2, Ne2: Unstable.
  • O2: Explains paramagnetism with unpaired electrons.

Band Theory

• Energy Bands: Valence band and conduction band in solids.
• Conductors vs. Insulators vs. Semiconductors: Determined by size of the band gap.

Applications

• Computational Chemistry: Applications in drug design using MO theory.
• Nobel Laureates: Walter Kohn's contributions to electronic structure and density functional theory.

Examples

• Molecular Orbital Diagrams: Help predict stability, bond order, and magnetic properties of molecules.
• Ion Predictions: Use diagrams to predict stability and magnetic properties of ions like Be2 and C2.

Summary

• MO theory provides a comprehensive framework for understanding molecular structure and properties.
• Explains phenomena like paramagnetism and helps in the design of pharmaceuticals and understanding of electronic materials.