London Dispersion Forces (Van Der Waals Forces): Weak Intermolecular Forces

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  • 0:05 London Dispersion Force
  • 1:16 Weakness of Force
  • 2:01 Effects on Properties
  • 3:53 Lesson Summary
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Lesson Transcript
Instructor: Amy Meyers

Amy holds a Master of Science. She has taught science at the high school and college levels.

Learn how London dispersion forces are created and what effect they have on properties such as boiling and melting points. Discover this weak intermolecular force and how it is one of the Van der Waals forces.

London Dispersion Force

You know that every atom and molecule has electrons and that these electrons are in constant motion. At any one instant in time, these electrons can be more towards one side of a molecule than another. When the electrons are concentrated more at one end of a molecule, that end becomes slightly negative. The other end, where the electrons are not as concentrated, becomes slightly positive. At this instant, this molecule is a temporary dipole. This dipole can encourage a nearby molecule to also become dipole because the negative side of the first molecule will cause the electrons to run away on the other molecule (since negative (-) detests negative (-)).

A molecule can become a temporary dipole due to the movement of its electrons
A dipole molecuel

Now these two adjacent dipoles created from the movement of electrons are attracted to each other. This very weak intermolecular force is called London dispersion force. London dispersion force is the weak intermolecular force that results from the motion of electrons that creates temporary dipoles in molecules. The London dispersion force is sometimes called a 'Van der Waals force.' Van der Waals force is a general term that describes any attractive intermolecular force between molecules and includes both the London dispersion force and the dipole-dipole force discussed elsewhere.

The London dispersion force causes the attraction of adjacent dipoles.
Diploes Attracted to Each Other

Weakness of Force

As you can imagine, this is an extremely weak force - much weaker than hydrogen bonding or any other intramolecular force. As I mentioned, this force works between all atoms and molecules. This is the only intermolecular force that works on noble gases and nonpolar molecules.

A London dispersion force works because of the movement of electrons. As you can imagine, the more electrons in the atoms, the stronger the force. Also, in a larger atom, the electrons are going to be filling the higher energy shells, which means they will be farther away from the nucleus and thus be less tightly held by the nucleus' attraction. The electrons farthest away are able to move more freely, creating a dipole easier. This is why the London dispersion force increases with increasing atomic mass.

Effects on Properties

Intermolecular forces affect the properties of a substance. The stronger the bonds between - and among - molecules in a substance, the greater that substance sticks together. For instance, helium, He, is a noble gas. The only intermolecular force it has between molecules is a London dispersion force. This force is very weak, so it doesn't hold those molecules together very strongly. That is why helium has such a low boiling point of -452° F.

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