Consider the atoms in a noble gas for a second. In year 7 science you learn that the forces of attraction between solid particles are greater than the forces of attraction between liquid particles which are greater than those between gas particles, but what are these forces which exist between particles? If we cool down a noble gas such as radon it liquefies. This must mean that there are forces of attraction between the radon atoms, but what are they? The radon atoms are electrical neutral, non-polar atoms. We know that noble gases don't react easily since they have full electron shells and they consist of individual atoms, but in order to liquefy radon or even to solidify it there must be forces of attraction present between the particles.
The electrons within an atom or a molecule are in constant motion; this means that the distribution of charge within the atom or molecule is going to be random and asymmetrical at times. This uneven distribution of electrons within an atom or a molecule creates of partial negative charge (δ-) where the density of the electrons is high and other areas of partial positive charge (δ+) where the electron density is lower. This random movement of electrons will then create dipoles within the atoms/molecules. However these dipoles will only be temporary since the electrons are in continual motion. However they will last long enough to influence the electron distribution in any other atoms/molecules they come close to. This will induced dipoles in these neighbouring atoms/molecules. This is shown below using helium atoms as an example.You can see that:
However it is not only the electrons that are in constant motion, the atoms themselves in the gas phase move at high speed in a random manner. Even though there are large spaces between the atoms as they cool they get closer to each other and this can also lead to the formation of temporary induced dipoles within the atoms or molecules. As shown below:
These temporary induced dipoles have a sort of chain reaction affect in that they generate dipoles in neighbouring
atoms; the attraction of one atom/molecule to the oppositely charged end of another molecule is called
a Van der
Waals force or London dispersion force.
The size of these Van der Waals forces increases as the number of electrons in the atom/molecule increases, it also increases with molecular size. In the diagram below the red and green ovals surroundings the molecules represent the skin of electron density which we can imagine as covering the molecules. Due to uneven electron distribution within the molecules at any one time dipoles are temporarily generated, the magnitude and number of these Van der Waals forces increases with increasing number of electrons and increasing surface are of the molecules. There is much more Van der Waals bonding present in pentane than in the much smaller ethane molecules as shown below.
It is not only the size and number of electrons
present that will influence the amount of Van der Waals bonding present, the
of the molecules is also an important consideration. Now Van der Waals is
a form of intermolecular bonding that relies on
the molecules/atoms being able to get close enough to each other to influence the electron distribution. As an
example of how important shape is
consider pentane, a hydrocarbon molecule with the formula C5H12. The image below shows
2 isomers of
pentane. The straight chain isomer will be able to get much closer to another neighbouring molecule than will the branched isomer. This means that
there will be much more Van der Waals bonding present in the straight chain isomer of pentane. This means that the straight chain isomer will
have slightly differ physical properties, for example it will have a higher boiling point and viscosity than the branched isomer due to the presence of
this additional intermolecular Van der Waals bonding.