Van der Waals forces
(aka London forces and dispersion forces) are the
forces holding together
molecules with no permanent
dipole moment. Molecules such as
dinitrogen,
methane, and
fullerene are held in liquid and solid states by Van der Waals forces. Van der Waals forces can be thought of as very weak
intermolecular covalent bonds, resulting from
momentary dipoles forming due to
asymmetry in
electron cloud density. This electron cloud
polarization causes a
perturbation of nearby molecules' electron clouds. The
interaction of these
temporary dipoles results in a net
attractive interaction. Were it not for these intermolecular forces, molecules with no permanent dipole moment would exist solely in the
gaseous phase.
All molecules, even those with permanent dipole moments, are held together to some degree by Van der Waals forces. However, the intermolecular forces from electrostatic attraction from dipole-dipole interactions are at least an order of magnitude stronger than Van der Waals interactions, even for small dipole moments. Thus, Van der Waals' contribution to properties such as heat of vaporization for molecules with a dipole moment is almost negligible.
Van der Waals forces only have about a tenthousandth of a percent of the strength of a typical covalent bond.