van der Waals' force

In chemistry, the term Van der Waals force refers to a particular class of intermolecular forces. The term originally referred to all such forces, and this usage is still sometimes observed, but it is now more commonly used to refer to those forces which arise from the polarization of molecules into dipoles. This includes forces that arise from fixed or angle-averaged dipoles (Keesom forces) and free or rotation dipoles (Debye forces) as well as shifts in electron cloud distribution (London forces). The name refers to the Dutch physicist and chemist Johannes Diderik van der Waals, who first documented these types of forces. The Lennard-Jones potential is often used as an approximate model for the Van der Waals force as a function of distance.

Van der Waals interactions are observed in noble gases, which are very stable and tend not to interact. This is why it is difficult to condense them into liquids. However, the larger the atom of the noble gas (the more electrons it has) the easier it is to condense the gas into a liquid. This happens because, when the electron cloud surrounding the gas atom gets large, it does not form a perfect sphere around the nucleus. Rather, it is only spherical if averaged over longer times and generally forms an ellipsoid, which has a slight negative charge on one side of the major axis and a slight positive charge on the other. The atom becomes a temporary dipole. This induces the same shift in neighboring atoms and spreads from one atom to the next. Unlike charges attract, and the induced dipoles are held together by dispersion force (or Van der Waals force). Van der Waals forces are responsible for certain cases of pressure broadening (Van der Waals broadening) of spectral lines.




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Interaction energy of argon dimer. The long-range part is due to London dispersion forces
London dispersion forces, named after the German-American physicist Fritz London, are weak intermolecular forces that arise from the attractive force between transient dipoles (or better multipoles) in molecules without permanent multipole moments. London dispersion forces are also just called dispersion forces or London forces and sometimes van der Waals forces.

London forces can be exhibited by nonpolar molecules because electron density moves about a molecule probabilistically, see quantum mechanical theory of dispersion forces. There is a high chance that the electron density will not be evenly distributed throughout a nonpolar molecule. When an uneven distribution occurs, a temporary multipole is created. This multipole may interact with other nearby multipoles. London forces are also present in polar molecules, but they are usually only a small part of the total interaction force.