London dispersion force

References:

• Wikipedia: https://en.wikipedia.org/wiki/London_dispersion_force
• Khan academy: https://www.khanacademy.org/science/ap-chemistry-beta/x2eef969c74e0d802:intermolecular-forces-and-properties/x2eef969c74e0d802:intermolecular-forces/v/london-dispersion-forces

Also known as 'induced dipole' forces. Electronically neutral atoms/molecules/whatever still attract each other because:

1. Random movements of electrons create a momentary 'dipole' - an imbalance where more electrons are on one side of the atom than the other, so there's a difference in charge across the atom.
2. This induces a dipole in a neighboring atom, because the positive pole of the first atom attracts the electrons in the second atom.
3. (presumably?) the induced dipole then induces / reinforces the dipole in the first atom, following the same mechanism.
   • when does this stop?
4. The dipoles attract each other.

These are called "dispersion" forces because they are caused by the dispersion of the electron cloud within an atom (even though the effect of the force is that the atoms attract, rather than disperse).

The strength of the forces is related to the polarizability of the substance, i.e., how strong of a dipole can it form. Bigger atoms tend to be more polarizable, because the electron clouds are more spread out, less tightly held to the nucleus, so there is more room to arrange all electrons on a given side of the nucleus without them all repelling each other.