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…