Dispersion Forces


  • The temporary dipoles
  • London forces
  • Dipole
  • Polarisability
  • Instantaneous dipole
  • Electrically symmetrical
  • Dipole moment

Definition of Dispersion Forces

London dispersion force is a weak intermolecular force between two atoms or molecules in close proximity of each other

More About Dispersion Forces

  • Atoms and nonpolar molecules are electrically symmetrical and have no dipole moment because their electronic charge cloud is symmetrically distributed. But a dipole may develop momentarily even in such atoms and molecules
  • Suppose we have two atoms ‘A’ and ‘B’ in the close vicinity of each other
  • It may so happen that momentarily electronic charge distribution in one of the atoms, say ‘A’, becomes unsymmetrical. That is, the charge cloud is more on one side than the other
  • This results in the development of instantaneous dipole on the atom ‘A’ for a very short time
  • This instantaneous or transient dipole distorts the electron density of the other atom ‘B’, which is close to it and as a consequence a dipole is induced in the atom ‘B’
  • The temporary dipoles of atom ‘A’ and ‘B’ attract each other. Similarly temporary dipoles are induced in molecules also
  • It is known as London force or dispersion force, as it was first proposed by German physicist Fritz London
  • These forces are always attractive and the interaction energy is inversely proportional to the sixth power of the distance between two interacting particles
  • These forces are important in short distances (≈500pm)
  • Their magnitude depends on the polarisability of the particle
  • The London dispersion force is the weakest of the van der Waals forces and is the force that causes nonpolar atoms or molecules to condense into liquids or solids as temperature is lowered
  • The London dispersion force is the weakest intermolecular force
  • The London dispersion force is a temporary attractive force that results when the electrons in two adjacent atoms occupy positions that make the atoms form temporary dipoles
  • Larger and heavier atoms and molecules exhibit stronger dispersion forces than smaller and lighter ones
  • In a larger atom or molecule, the valence electrons are, on average, farther from the nuclei than in a smaller atom or molecule. They are less tightly held and can more easily form temporary dipoles
  • The ease with which the electron distribution around an atom or molecule can be distorted is called the polarizability
  • London dispersion forces tend to be: stronger between molecules that are easily polarized and weaker between molecules that are not easily polarized. The shapes of molecules also affect the magnitudes of dispersion forces between them
  • At room temperature, neopentane (C5H12) is a gas whereas n-pentane (C5H12) is a liquid
  • London dispersion forces between n-pentane molecules are stronger than those between neopentane molecules even though both molecules are nonpolar and have the same molecular weight.
  • The somewhat cylindrical shape of n-pentane molecules allows them to come in contact with each other more effectively than the somewhat spherical neopentane molecules