Types of Intermolecular Forces

Three forces, in order of strength

• London dispersion — from instantaneous, fluctuating dipoles. Present in every substance, and it grows quickly with molecular size and electron count.
• Dipole–dipole — between molecules with permanent dipoles.
• Hydrogen bonding — a special, much stronger dipole–dipole case, but only when H is bonded directly to N, O or F.

The distinction that matters most

These are forces between molecules. Boiling water breaks hydrogen bonds between $\text{H}_2\text{O}$ molecules; it does not break the O–H bonds inside them. That is why steam is still water.

Size can beat type: $\text{I}_2$ is a solid at room temperature with only London forces, while polar $\text{HCl}$ is a gas — because iodine's electron cloud is far more polarisable.