Shapes of Molecules

• Linear

  Angles between electron pairs: 180

  Example: BeCl2
• Trigonal Planar

  Angles between electron pairs: 120

  Example: BF3
• Tetrahedral

  Angles between electron pairs: 109.5

  Example: NH4
• Trigonal Bipyramidal

  Angles between electron pairs: 90 + 120

  Example: PCl5
• Octahedral

  Angles between electron pairs: 90

  Example: SF6
• Non-bonding pairs


  Non-bonding pairs have a greater repulsive effect than those which are bonding pairs so their presence in a molecule causes slightly distorted shapes
• Describe how to determine if there are non-bonding pairs 


  Subtract the number of atoms surrounding the central atom from the number of electron pairs. Any pairs leftover must be non-bonding
• Name other common shapes of molecules

  Seesaw

  Linear

  Square pyramidal
• VSEPR (valence shell electron pair repulsion) theory can be used to predict the shapes of molecules and polyatomic ions.
• The number of electron pairs surrounding a central atom can be found by:

  • Taking the total number of valence (outer) electrons on the central atom and adding one for each atom attached
  • Adding an electron for every negative charge
  • Removing an electron for every positive charge
  • Dividing the total number of electrons by two to give the number of electron pairs
• Electron pairs are negatively charged and repel each other. They are arranged to minimise repulsion and maximise separation.
• The arrangement of electron pairs around a central atom is:

  • Linear for 2 electron pairs
  • Trigonal planar for 3 electron pairs
  • Tetrahedral for 4 electron pairs
  • Trigonal bipyramidal for 5 electron pairs
  • Octahedral for 6 electron pairs
• Shapes of molecules or polyatomic ions are determined by the shapes adopted by the atoms present based on the arrangement of electron pairs.
• Electron pair repulsions decrease in strength in the order:

  non-bonding pair : non-bonding pair > non-bonding pair : bonding pair > bonding pair : bonding pair