Lesson 5: Molecular Shapes

• Lone pairs


  Push against each other and other bonds the most, which is why they are more repulsive
• Triple bonds


  Have the same repulsive force as a single bond
• Stereochemistry


  The study of the shapes of chemical species
• VSEPR Theory

  The Valence Shell Electron Pair Repulsion theory can be used to predict the geometry of molecules
• Valence electron pairs are arranged around the central atom of a molecule in such a way as to minimize repulsion
• Repulsive force magnitude from most to least repulsive

  • Lone Pair-Lone Pair (most repulsive)
  • Lone Pair-Bonding pair
  • Bonding Pair-Bonding pair (least repulsive)
• Double and triple bonds count as one electron group
• If all electron groups are bonding pairs then molecule will be one of the five basic shapes, however varying numbers of lone pairs will alter the molecular shapes
• Double and triple bonds have the same magnitude of repulsive force as a single bond
• Predicting Molecular Shape

  1. Draw Lewis structure of molecule based on given formula

  2. Determine the total number of electron groups attached to the central atom

  3. Determine one of the 5 geometrical arrangements of possible from the number of electron groups

  4. Determine the molecular shape from the positions of lone pairs (LP) and bonded pairs (BP)
• Ronald J Gillespie and Richard Bader of McMaster University in Hamilton, Ontario have both contributed to furthering the VSEPR model of molecular geometry through electron density calculations
• A triangle coloured in means the atom is projecting out/forward, a dashed line triangle means the atom is projecting backwards, and a straight line means same plane
• Types of isomers:

  • structural (constitutional) isomers
  • stereoisomers/spatial isomers
  • enantiomers: molecules that are mirror reflections of each other
  • diastereomers
  • cis/trans isomers
  • conformers
  • rotamers