Intermolecular Forces

A number of intermolecular (Van der Waals') forces exist between molecules in a substance.

Dispersion

These forces exist between all molecules, and occur due to the formation of 'temporary dipoles'. Temporary dipoles are formed due to an imbalance in the positions of electrons in an atom's shells; this causes the atom to temporarily become weakly charged, which in turn polarises the neighbouring atoms and so on. These dipoles only last for very short periods of time.

Dispersion forces are proportional to the number of electrons in a molecule.

Dispersion forces are the weakest intermolecular forces.

Dipole-dipole

Dipole-dipole forces occur between two polar molecules, between a negative region of one and a positive region of the other.

Dipole-dipole forces are much stronger than dispersion forces.

Hydrogen bonding

Hydrogen bonding occurs when a Hydrogen atom bonded to either $\text{N}$, $\text{O}$ or $\text{F}$ is attracted to another highly electronegative atom in another molecule.

Hydrogen bonds may be considered as a stronger form of dipole-dipole forces.

Ion-dipole

Ion-dipole forces occur between an ionic molecule and a dipole.

These are much stronger than dipole-dipole forces and hydrogen bonds.

Ionic

These forces occur between two ionic molecules, for example ${\text{Na}}^{+}$ and ${\text{Cl}}^{-}$.

These are the strongest intermolecular forces.