Lecture 11: Bond Order

Definition of Bond Order

• Bond Order is defined as the number of bonds divided by the number of bonding regions.
• Bonding Regions are areas of electron density between two atoms.

Examples of Diatomic Gases

• Hydrogen (H₂):
  • 1 bond within 1 bonding region → Bond Order = 1 (Single bond)
• Oxygen (O₂):
  • 2 bonds within 1 bonding region → Bond Order = 2 (Double bond)
• Nitrogen (N₂):
  • 3 bonds within 1 bonding region → Bond Order = 3 (Triple bond)

Carbon-Carbon Bonds in Compounds

• Ethane (C₂H₆):
  • Single bond between the two carbons → Bond Order = 1
• Removing Hydrogens:
  • C₂H₄ (remove 2 H) → Bond Order = 2 (Double bond)
  • C₂H₂ (remove 4 H) → Bond Order = 3 (Triple bond)

Bond Characteristics

• Bond Lengths:
  • Single bond (C-C): 1.54 Å
  • Double bond (C=C): 1.33 Å
  • Triple bond (C≡C): 1.20 Å
• Bond Energies:
  • Single bond: 347 kJ/mol
  • Double bond: 612 kJ/mol
    • Note: 612 kJ/mol is not double 347 kJ/mol due to differences in sigma and pi bonds.
  • Triple bond: 820 kJ/mol

Patterns in Bond Order

• Higher Bond Order:
  • Shorter bond length
  • Stronger bond
  • Higher bond energy

Class Demonstration

• Demonstration with Rope:
  • Single bond: weak and long
  • Double bond: medium strength and length
  • Triple bond: strong and short

Summary

• Single bonds are the weakest and longest.
• Triple bonds are the strongest and shortest.

Exercise

• Question:
  • Identify structures with the longest and strongest nitrogen to nitrogen bonds based on bond order.