Answer to Question #62921 in General Chemistry for Nasia Collier

Question

What causes London Dispersion Forces? What can cause an increase London Dispersion Forces?

Answer 1

London dispersion forces are intermolecular forces that exist between ALL molecules. They are always present even if there are other forms of intermolecular attraction.
London dispersion forces exist because of the polarizability of molecules. Since there are many possible arrangements of electrons within a molecule (quantum theory and the uncertainty principle) then there is a probability that there will exist an arrangement where a nonpolar molecule will exhibit polarity, a "temporary dipole" as it is called. The temporary dipole will attract other temporary dipoles and the molecules will attract one another.
When a polar molecule is near a nonpolar molecule, it causes a change in the arrangement of the electrons so that it induces a charge in the otherwise nonpolar molecule. These opposite charges attract and you get attraction between the polar molecule and the nonpolar molecule.
Dispersion forces are always very short-range. The energy for a dispersion interaction falls off as 1/r6. Thus, the ability for molecules to pack tightly together has a huge effect on dispersion forces. Those that can get very close to one another have much stronger interaction than those that cannot. So once again, the three-dimensional geometry of the molecule has a great effect on its physical properties.
Finally, the polarizability has a larger effect on dispersion forces. The more electrons a molecule has and the farther they are from all the nuclei will affect how easily a dipole can be induced in the electron cloud (aka: polarized). This is why we perceive a molecular weight effect in boiling points. It is not that the mass of the molecules matters, it is simply that more mass implies more protons, which implies more electrons. More electrons generally lead to more dispersion forces. Higher mass can also be farther down the periodic table. This will also be more polarizable. A great example of this is the diatomic halogens. F2 is a gas, Cl2 is a gas, but Br2 is a liquid, and I2 is a solid. All four have pure covalent bond and are non-polar molecules. However, as we move down the periodic table the polarizability increases and thus the dispersion forces increase.