Question

How do London dispersion forces form?

Answer 1

London forces are a result of temporary dipoles that exist in molecules. They are a weak attraction that is a type of van der Waals force.

Explanation:

In a molecule that does not possess a permanent dipole, electrons are, on average, evenly distributed over time. However, from moment to moment, the electrons of the molecule can show brief uneven distributions, with the result that a temporary dipole can exist. This uneven electron distribution then creates a similar effect in nearby molecules. This is called a temporary induced dipole and we get a weak attraction between molecules due to this effect. This is the London dispersion force.

As a general rule, these forces become stronger in larger atoms, and are the reason for the increase in melting and boiling points seen in the halogens as one reads down the column.

Hope that's enough!