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The Basics[]

  • Officially all intermolecular forces are Van  der Waals forces but we use this term only to describe the weakest intermolecular forces caused by spontaneous dipoles.
  • You should know that all molecules and atoms have Van der Waals forces even if they also have stronger Intermolecular forces.
  • You should know that the heavier the atom or molecule the stronger the Van der Waals forces will be. You should also be able to explain why.
  • You should also know that, for two Isomers of the same mass, the least branched molecule will have the stronger Intermolecular forces because the molecules will pack together more closely.
  • You should be able to draw and explain the origin of Van der Waals forces. (See below).

Exam Hint[]

A very common question is to explain how Van der Waals forces are formed.

These questions generally centre on a non-polar molecule like Chlorine (Cl2)

Points to make in your explanation would be:

  1. Electrons are free to move.
  2. Sometimes this leaves to an uneven distribution of electron density.
  3. The atom with a bigger share of the electron density is δ-, the other is δ+
  4. This will induce a an opposite set of partial charges in neighbouring molecules if they are close enough.
  5. The two molecules will then have an electrostatic force of attraction to each other.
Van der waals1

One spontaneous dipole induces equal and opposite dipoles in neighbours.

Group trends in Van der Waals forces[]

Van der Waals questions often focus on the halogens.

This is probably because the elements move from gaseous, through liquid to solid at room temperature as we go down the group.

  • You should be able to explain this in terms of increasing molecular mass.
  • You should also mention that, for example, outer electrons in Iodine are more shielded and further from the nucleus than in Chlorine. this means they are more able to move about and more likely to form spontaneous large dipoles.
  • You should probably also be able to sketch the lattice formed by Iodine.
I2
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