eli5 London Dispersion


I cannot for the life of me figure out how it works.

Edit: got it, thanks guys

In: 1

Most molecules have some sort of electric “dipole” – the electrons prefer to spend their time on one side of the molecule and so it has one negatively charged end and one positively charged end. This helps the molecules stick together in the liquid and solid phase.

But what about perfectly symmetric molecules like carbon dioxide?

We know it can still solidify when it gets cold enough, but it doesn’t have any sort of permanent dipole that would help this along. What’s holding the molecules together?

This is where the dispersion forces come in. The molecule has no *permanent* dipole but electrons are jumpy little things. At any given moment they may randomly shift to one end of the molecule or the other, creating a temporary negatively charged bias on that end.

This then encourages a reaction in the next molecule over, repelling those electrons and producing a positive. Repeat down the chain. The molecules slightly magnetize themselves by sheer randomness, and stick together in a chain reaction of self-reinforcing temporary dipoles.

It’s the weakest of the intermolecular forces, but in large volumes at low temperatures it still adds up. It’s what holds liquid oxygen and dry ice together.

You have to imagine that electrons around an atomic nucleus are evenly distributed only on average.

It happens that while moving around the nucleus the electrons find themselves all ‘on the same side’ of the nucleus, so that this symmetrical distribution is broken.

When this happens the atom gains a partial positive electrical charge on the side that lacks electrons and a partial negative one on the side that has an over-abundance of electrons.

This instantaneous charges can affect atoms which are near the first atom, polarizing it.
(If the second atom is near to the negatively-charged side of the first one, it will gain a positive charge on the side near the first atom).

So you’ll have two electrical charges of opposite sign near each other, this will create a electrostatic force that will attracts the atoms to each other.

This situation of course lasts just a moment, the second after the electrons on the first atom will be differently distributed, so the force will fade away, this is why these forces are very weak.