Valence Bond Theory and Hybridization

• In valence bond theory, bonds result from the pairing of unpaired electrons from the valence shell atomic orbitals
• σ bond: cylindrically symmetric about the bond axis; no nodal plane along the bond axis
• 𝜋 bond: electron density in 2 lobes with a single nodal plane along the bond axis
• Single bond: 𝜋 bond
• Double bond: σ bond + 𝜋 bond
  • Molecules can’t rotate around a double bond
• Triple bond: σ bond + 𝜋 bond + 𝜋 bond

Electron Promotion and Hybridization of Atomic Structures

• Electron promotion serves to increase the number of unpaired electrons
  • If you have an empty orbital you can promote one of your electrons to that orbital
• The hybrid orbitals only differ in their orientation in space
• Energy for the initial electron promotion comes from bonding
• (# of bonded atoms) + (# of lone pairs) = # of hybrid orbitals
• 2 hybrid orbitals = sp
• 3 hybrid orbitals = sp²
• 4 hybrid orbitals = sp³
• Exception: single-bonded terminal atoms
• Don’t hybridize single-bonded terminal atoms