Molecular Orbital Diagrams and Predicting Properties

Introduction

• Molecular Orbital (MO) Diagrams: Used to depict electron configuration in molecules and predict molecular properties.
• Filling MO Diagrams: Electrons are added following rules akin to atomic orbitals.

Example: Beryllium (Be₂)

• Beryllium Atom: Each has 2 electrons.
• Beryllium Molecule (Be₂): Total of 4 electrons.
• Filling Process:
  • Fill from the bottom up.
  • Follow rules of opposite spins in the same orbital.
• Electron Configuration:
  • MO diagram shows electron configuration for Be₂.

MO Diagrams for Periodic Table Row 2

• Atoms like Lithium and Beryllium: Similar orbital structures.
• Oxygen: Notable swapping of orbitals due to orbital mixing.
  • Sigma 2pₓ decreases in energy.

Example: Nitrogen (N₂)

• Nitrogen Atom: Each has 5 electrons.
• Nitrogen Molecule (N₂): Total of 10 electrons.
• Filling Process:
  • Fill degenerate orbitals with parallel spins first.
• Bond Order Calculation:
  • Formula: ( \frac{\text{Bonding electrons} - \text{Antibonding electrons}}{2} )
  • For N₂: Bond order = 3.
  • Matches Lewis structures and valence bond theory.

Bond Order and Stability

• Higher Bond Order: Stronger interactions between atoms.
• Be₂ Bond Order: Zero, indicating no net bonding.

Molecular Hydrogen vs. Helium

• Hydrogen (H₂):
  • Bonding and antibonding orbitals from valence 1s electrons.
  • Bond order = 1.
• Helium (He₂):
  • 4 electrons result in bond order = 0.
  • Explains non-existence of He₂.

Helium-2 Ion (He₂⁺)

• Helium-2 Ion (He₂⁺):
  • 3 electrons after positive charge adjustment.
  • Bond order = 0.5, slight bonding interaction.
• Existence in Sun:
  • Helium-2 positive exists as a stable species in the sun.

Conclusion

• MO diagrams provide valuable insight into molecular structure and behavior.
• Understanding electron configurations aid in predicting molecular properties and stabilities.