Intermolecular Forces


Intermolecular Forces

  • Intermolecular forces are the forces of attraction between molecules
  • There are three types -
    • induced dipole-dipole forces (london forces)
    • permanent dipole-dipole forces
    • hydrogen bonds
  • The order of strength is -
    • london forces < permanent dipole-dipole < hydrogen bonding
  • The stronger the intermolecular forces, the higher the melting and boiling point

London Forces:

  • Also known as induced dipole-dipole forces
  • These are present in all molecular substances but are the only forces present in non-polar substances e.g. noble gases, halogens, hydrocarbons
  • They are caused by the movement of electrons which unbalances the charge distribution within the molecule, causing a temporary dipole
  • This in turn induces a dipole in neighbouring molecules, resulting in weak forces of attraction between molecules
  • Substances with only London forces have relatively low boiling points -
    • they are generally gases at room temperature
  • Bigger molecules (greater Mr) have more electrons so the induced dipoles are larger. This results in stronger


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