Here is an image I found which illustrates dispersion forces:

enter image description here

One helium has its electron cloud repelled by the other helium's electron cloud, which exposes the nucleus and gives an induced dipole which holds the atoms together weakly. But when two helium atoms approach each other, what determines which atom will have its electron cloud repelled? I would think that both electron clouds would equally repel each other. Also I am guessing that this effect will get stronger as temperature is lowered (since helium is a liquid at very low temperatures), but I am not sure why.


You are correct that the electron clouds would repel each other equally.

The induction as shown in your second picture would require another outside entity, polarizing one of the He atoms, which would then affect the other.

At an arbitrarily show distance, we might describe a molecular orbital formation between the two helium nuclei, as shown here:

Helium orbitals
(source: mhhe.com)

In this case, without the external perubation, the orbital shape (and hence the electron clouds) is (are) quite symmetrically distorted.

However, it becomes nonesense to describe a system of two atoms. In liquid helium there are manifold other charge fields distorting these nice shapes, and you may get some asymmetry as shown in your picture above.

  • $\begingroup$ when you say outside entity, I'm guessing that can't be other helium atoms because they would repel each other equally, so if you had a vial of liquid helium how could the atoms be staying together? are the atoms at the walls of the vial polarised by the molecules that make up the vial, or by molecules in the air, which then has a domino effect on the rest of the atoms? or is there another factor. same thing for something like iodine? for heterogeneous molecules e.g. hexane, are the dispersion forces due to the electronegativity differences between carbons and hydrogens? $\endgroup$ – k-- Apr 9 '16 at 6:46
  • $\begingroup$ It could be other helium atoms, because they are moving and the 'chaos' of the situation could result in one being closer to anther than a third. And then a fourth makes it more 'perturbed'. And a fifth, etc. You are also correct the container walls will also matter. The field for describing this is statistical mechanics. $\endgroup$ – Lighthart Apr 10 '16 at 1:26

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