Shapes of molecules

• Valence Shell Electron Pair Repulsion Theory (VSEPR)


  Theory that helps to explain molecule shapes by considering electron-pair repulsion
• Charge clouds


  • Electrons occupy orbitals, which are regions of space where there is a 95% chance the electron is located
  • Electrons can be viewed as charge clouds - regions of space where the electrons move around
• Electron repulsion


  • Electrons are negatively charged and repel each other
  • Electrons will try to stay as far apart from each other as possible
  • This determines the geometry of a molecule as the electrons in the bonds repel and try to stay as far from each other as possible
• Lone pairs vs bonding pairs


  • Lone pairs are held closer to the nucleus and repel each other more than bonding pairs
  • Repulsion strength: lone pair-lone pair > lone pair-bonding pair > bonding pair-bonding pair
• Electron Pairs and Geometry

  • There is an optimal geometry for every number of electron pairs
• Linear geometry


  1. Central atom has two electron pairs

  2. Bond angle is 180°
• Trigonal planar geometry


  1. Central atom has three electron pairs

  2. Bond angle is 120°
• Tetrahedral geometry


  1. Central atom has four electron pairs

  2. Bond angle is 109.5°
• Trigonal bipyramidal geometry


  1. Central atom has five electron pairs

  2. Bond angle around equator is 120°

  3. Bond angle from equator to apex is 90°
• Octahedral geometry


  1. Central atom has six electron pairs

  2. Bond angle is 90°