Sigma and Pi Bonds

Concept of Overlapping

• Overlapping is the core concept in understanding sigma and pi bonds.
• Two main types of overlapping:
  • Head-to-Head (Axial) Overlapping
    • Example: The "head" of a pencil joined with the "head" of another pencil.
    • Forms Sigma (σ) Bonds.
  • Side-to-Side (Parallel) Overlapping
    • Example: The "side" of a pencil attached to the "side" of another pencil.
    • Forms Pi (π) Bonds.

Sigma (σ) Bonds

• Formed through axial (head-to-head) overlapping.
• Example: Overlapping of two p orbitals headwise.
• Possible Combinations:
  • s orbital + s orbital → Sigma bond
  • s orbital + p orbital → Sigma bond
  • p orbital + p orbital → Sigma bond (if headwise)

Pi (π) Bonds

• Formed through parallel (side-to-side) overlapping.
• Example: Overlapping of two p orbitals sidewise.
• Possible Combinations:
  • p orbital + p orbital → Pi bond (if sidewise)

Importance of Studying Sigma and Pi Bonds

• Explain bond energy, bond strength, and bond length.
• Sigma bonds have maximum overlapping, hence stronger than pi bonds.
• Pi bonds have minimum overlapping, hence weaker.

Bond Strength and Energy

• Depends on the extent of overlapping.
• Order of Bond Strength:
  • Sigma bond (pp orbitals) > Sigma bond (sp orbitals) > Sigma bond (ss orbitals) > Pi bond (pp orbitals).
• Bond Length:
  • Higher bond energy/strength = shorter bond length.
  • Example: Sigma bond of pp orbitals has high overlapping, short bond length.

MCQ Example

• Which bond has more bond energy? 1s2s, 1s2p, 1s3s.
  • Answer: 1s2s (due to maximum overlapping).

Finding Number of Sigma and Pi Bonds in Molecules

• Rule: First bond is always sigma; second and third can be pi.
• Examples:
  • Water (H2O):
    • 2 Sigma bonds, 0 Pi bonds.
  • Ethene (C2H4):
    • 5 Sigma bonds, 1 Pi bond.
  • Ethyne (C2H2):
    • 3 Sigma bonds, 2 Pi bonds.
  • Propanoic Acid (C3H5OH):
    • 10 Sigma bonds, 1 Pi bond.
  • Sulfur Trioxide (SO3):
    • Count based on given rules.

Summary

• Sigma bonds are stronger due to more overlapping.
• Knowing how to identify sigma and pi bonds helps understand molecular bonding properties.