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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 Liquid s. 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 Oval , which has a slight negative charge on one side 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. The Van der Waals force is the force to which the Gecko 's climbing ability is attributed. A gecko can hang on a glass surface using only one toe. Efforts continue to create a synthetic "gecko tape" that exploits this knowledge. So far, research has produced some promising results - early research yielded an Adhesive Tape product, which only obtains a fraction of the forces measured from the natural material, and new research [http://www.uakron.edu/news/articles/uamain_1293.php has yielded a discovery that purports 200 times the adhesive forces of the natural material. LONDON DISPERSION FORCE London dispersion forces, named after the German-American physicist Fritz London , are weak Intermolecular Force s that arise from the attractive force between transient Dipole s (or better multipoles) in Molecule s without permanent Multipole moments. London dispersion forces are also just called '''dispersion forces''' or '''London forces''' and sometimes ''' Van Der Waals Force s'''. London forces can be exhibited by nonpolar molecules because electron density moves about a molecule probabilistically. 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. Electron Density in a molecule may be redistributed by proximity to another multipole. Electrons will gather on the side of a molecule that faces a positive charge and retreat from a negative charge. Hence, a transient multipole can be produced by a nearby polar molecule, or even a transient multipole in another nonpolar molecule. London forces are weaker than other Intermolecular Force s such as ionic interactions, Hydrogen Bond ing, or permanent dipole-dipole interactions. This phenomenon is the only attractive intermolecular force at large distances present between neutral atoms (e.g., Helium ), and is the major attractive force between non-polar molecules, such as Nitrogen or Methane (to name a couple). Without London forces, there would be no attractive force between Noble Gas atoms, and they could not then be obtained in a liquid form. London forces become stronger as the atom (or molecule) in question becomes larger. This is due to the increased polarizability of molecules with larger, more dispersed electron clouds. This trend is exemplified by the and Chlorine are Gas es at room temperature, Bromine is a liquid, and Iodine is a solid. RELATION TO THE CASIMIR EFFECT The London-Van der Waals force is related to the Casimir Effect for dielectric media, the former the microscopic description of the latter bulk property. First detailed calculations of this were done 1955 by E. M. Lifshitz . This equivalence gained attention and some new papers, in the recent discussion of Sonoluminescence . SEE ALSO REFERENCES
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