Question

What causes London dispersion forces?

Answer 1

London dispersion / van der Waals forces are caused by distribution of electrons throughout the molecule / atom of the compound.

Explanation:

One must first distinguish how this comes about from the explanation of dipole - dipole interactions. Whereas dipole - dipole interactions arise from overall distribution of electrons, and refer to permanent dipoles, London dispersion forces occur because electrons are in constant movement. We know that electrons must be in continuous movement because they would be otherwise unable to overcome the pull of the nucleus, and be absorbed into it.

However, in a given 'freeze frame' image of a molecule or atom, the electrons will not always be in a position where they are drawn towards a positive region. The fact that electrons move at random within their orbital regions means that the electron cloud - which refers to electron distribution throughout the atom or molecule as a whole - will be distributed at random and be continually changing.

When this happens in one molecule or atom, it results in an instantaneous dipole, because it is a dipole that exists only in that instant whilst the electron clouds is distributed in that way, before the always-moving electrons disrupt this dipole and the dipole ceases to be.

Whilst this instantaneous dipole is in effect, it has an effect on the electron clouds of neighbouring molecules / atoms and causes the location of electrons to be altered. This results, in these other molecules / atoms, in an induced dipole, because it is a dipole that has been temporarily induced by the original instantaneous dipole.

The key notion here is that these dipoles are only temporary. Although induced dipoles will induce dipoles in molecules / atoms local to them, they exist only for an instant, after which the electron cloud will have changed in distribution and lost that particular dipole.

Now we must consider that the electron cloud is constantly being redistributed in such a way that instantaneous dipoles are created, which in turn causes induced dipoles to occur through the structure. This is why London dispersion forces do have an effect on the structure's properties.