The Shapes of Molecules: VSEPR Theory

• The shape (geometry) of a molecule influences physical and chemical properties, including melting point, boiling point, and reactivity

• Valence Shell Electron Pair Repulsion (VSEPR) Theory: based on Lewis structures and the principles that:

  • Valence electron pairs repel each other

  • The geometry around the central atom will minimize the repulsion

• VSEPR Nomenclature

  • A = central atom

  • X = bonding atom

  • E = lone pair

• If a molecule has 2 or more resonance structures, the VSEPR model can be applied to any one of them

• If there’s more than one central atom in a molecule, consider the bonding about each atom independently

• Steric number = number of atoms bonded to central atom + number of lone pairs

  • Used to predict geometries

Molecules Without Lone Pairs

Molecules With Lone Pairs

• Electrons in bonds have less “spatial distribution that lone pairs, meaning:

  • Electrons in bonds take up less space

  • Lone-pair electrons can be anywhere

  • Lone pair electrons take up more space and therefore experience more repulsion

  • Repulsive forces for molecules with lone pairs

    • Lone pair/lone pair repulsion (most repulsion) -> lone pair/bonding pair repulsion -> bonding pair/bonding pair repulsion (least repulsion)

  • In molecules with lone-pair electrons, angles between bonded atoms tend to be smaller

  • When atomic size increases, lone pairs occupy larger spatial volumes, so angles between bonded atoms tend to be even smaller