London Dispersion Forces

There is no good reason to think that non-polar molecules would attract each other, and yet they do.

We know that they do because when non-polar gases are cooled to VERY low temperatures, they condense into liquids. That would only happen if they are attracted in some way.

In the same vein, wax is composed of long non-polar chains of hydrogen atoms and carbon atoms, but it is a solid at room temperature.

The question that remains then is why? Why would two non-polar molecules attract each other?

We'll look at two He atoms to explore how an attraction might develop, although the same argument applies to any non-polar particles (atoms or molecules). A helium atom is simply a nucleus (of two protons and two neutrons) with 2 electrons around it (in the 1s orbital).

In this image, the electrons are represented by the blue circle.

The electrons are centered around the nucleus, as would be expected. However, we need to remember that that is actually a probability plot--a picture of where the electrons are likely to be, not where they are. That means that the electrons might (for no apparent reason) go to the left side.

Another way to think about this is to picture a track where the neighborhood folks go to run. Assuming that they are each running independently, they will be (on average) spread evenly around the track. But, assuming that they aren't all moving the same speed, the faster folks will pass the slower folks. So there will be moments when the runners are "clumped" one one side of the track. If the runners are electrons...we end up with the picture below.

If/when that happens, one side of the atom will be slightly negative (more electrons than normal) and the other side will be slightly positive (less electron density than normal).


If this happens to two atoms that are near to each other, the slightly negative end of one can attract the slightly positive part of the other. This attraction is called London Dispersion Forces.

Note: The name comes from the man who thought of it (Fritz London), his name for the random uneven distribution of electrons (dispersion) and the attraction created (force).

If this seems desperately random, keep in mind that the second atom will be influenced by the first through induction. So it's not quite random.

This force is generally very weak because it requires electrons to "slosh" randomly from one side to the other with no real reason. There is also no reason for those electrons to stay where they are, so the force is short-lived.

It is important to know that not all London Dispersion Forces are created equal. The strength of the attraction depends on the size of the non-polar particles (atoms or molecules) and on the surface area of the particles.