Measuring Van der Waals Forces

Van der Waals forces are among the weakest known chemical interactions, yet they are everywhere around us, making things stick together. In the world of nanotechnology, where particles can be very close together, Van der Waals forces can become the dominant interaction.

Van der Waals forces occur between atoms because, although an atom has no net overall charge, it is made up of different charged particles which are all moving and those movements cause tiny differences in charge that fluctuate very quickly. An atom that is experiencing a momentary tiny positive charge will then cause an adjacent atom to experience a momentary tiny negative charge, the oppositely charged atoms are attracted to each other.

Scientists have been studying Van der Waals forces by passing atoms through a grating. The atoms are attracted to the bars which separate the grating's slits by Van der Waals forces, and, depending on the strength of the interaction, the path the atoms are following is changed. The scientists can measure the degree to which this path has changed.

Reference
Vincent Lonij, Catherine Klauss, William Holmgren, Alexander Cronin. Atom Diffraction Reveals the Impact of Atomic Core Electrons on Atom-Surface Potentials. Physical Review Letters, 2010; 105 (23) DOI: 10.1103/PhysRevLett.105.233202

Further Reading
Intermolecular Forces
Intramolecular Forces

Study Questions

1. What is meant by the term intermolecular force?
2. How is an intermolecular force different to a intramolecular force?
3. Name three intermolecular forces.
4. Place the intermolecular forces above in order of increasing strength.
5. Name the type of intermolecular force that is predominant in interactions between molecules of:
   
• water
• paraffin wax
• methane
• ethanol (ethyl alcohol)
• trichloromethane
• nitrogen
• ammonia
• tetrachloromethane
6. For each pair of substances below, which will have the higher boiling point?
   
• octane and water
• ammonia and nitrogen
• ethanol (ethyl alcohol) and octanol
• chloromethane and methane
• hydrogen bromide and hydrogen fluoride
7. Explain why Van der Waals forces are considered to be very weak while ionic bonds, which depend on the attraction of oppositely charged ions, are considered to be very strong.