Within the three types of intermolecular forces, there will be subtle variations in their strengths. While dipole-dipole interactions are the mid-strength type of interactions, some dipole-dipole interactions are stronger than others. Although London Dispersion Forces are the weakest interactions, some are stronger than other London Dispersion Forces.

What factor is responsible for these variations in the strengths of the different types of intermolecular forces?

  • The larger the radii of the atoms that are carrying the partial charges, the stronger the forces of attraction will be.
  • The larger the dimensions of the molecules that have the dipoles, or partial charges, the stronger the forces of attraction will be.
  • Basically, bigger is stronger!!!

Studies have shown that larger atoms, or molecules, are able to sustain partial charges better. This translates into the idea that the attractions between these types of charges will be stronger. So, bigger atoms, or molecules, will have stronger attractions, and, thus, higher melting points, boiling points, and be harder. The reverse is true for smaller systems.

 If the dimensions of the nonpolar halogens were plotted against their melting points, the resulting graph would be similar to the one shown here. It demonstrates that as the molecules become larger, the strength of the interactions increases. As a result, the melting points increase.  
In conclusion, the strength of the intermolecular forces in systems that use dipole-dipole interactions or London Dispersion Forces will increase with the increasing size of the atoms, or molecules, within the systems.