Valence-Shell Electron Pair Repulsion Theory (VSEPR)

• A model that predicts the arrangement of electron pairs around a central atom to minimize repulsions and achieve the lowest energy configuration.
• Electron Pair Geometry: describes the three-dimensional arrangement of bonding pairs and lone pairs around a central atom.
• Molecular Geometry: describes the three-dimensional arrangement of atoms in a molecule.
• If there are no lone pairs, Electron-Pair Geometry = Molecular Geometry.

Key Points About VSEPR

• Steric Number (SN): The sum of atoms bonded to a central atom and lone pairs on that atom.
• SN determines Electron-Pair Geometry:
  • 2 = Linear
  • 3 = Trigonal Planar
  • 4 = Tetrahedral
  • 5 = Trigonal Bipyramidal
  • 6 = Octahedral

Difference Between Electron-Pair and Molecular Geometry

• When lone pairs are present, Molecular Geometry differs from Electron-Pair Geometry.
• Molecular Geometry is based on the positions of bonded atoms.

Bond Angles

• Defined by the angle between two atoms bonded to a third.
• Electron-pair geometry provides theoretical bond angles.
• Actual bond angles vary due to VSEPR effects:
  • Lone pair-bonding pair repulsion is greater than bonding pair-bonding pair repulsion.
  • Lone pair repulsion is greater than double bond repulsion.
  • Double bond repulsion is greater than single bond repulsion.
  • Two lone pairs on a central atom exert greater repulsion than one lone pair.

SN = 2 (Linear)

• Electron Pair Geometry: Linear

• Bonded Atoms: 2

• Lone Pairs: 0

• Molecular Geometry: Linear
• Bond Angles: 180°
• Example: CO₂
• Hybridization: sp

SN = 3 (Trigonal Planar)

• Electron Pair Geometry: Trigonal Planar

• Bonded Atoms: 3

• Lone Pairs: 0

• Molecular Geometry: Trigonal Planar

• Bond Angles: 120°

• Example: CH₂O

• Hybridization: sp²

• Electron Pair Geometry: Trigonal Planar

• Bonded Atoms: 2

• Lone Pairs: 1

• Molecular Geometry: Bent

• Bond Angles: Less than 120°

• Example: SO₂

• Hybridization: sp²