## Van der Waals bonding (London dispersion forces)

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.

### Formation of Van der Waals forces or London dispersion forces

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:
• In the first image the electrons have moved to the left hand side of the helium atom, this will make the left side of the atom δ- and the opposite side of the atom δ+
• In the second and third images of the helium atom the electrons are on the other side of the atom, but this will still create a dipole with δ+ and δ- ends to the atom. These temporary induced dipoles are very short lived as the electrons are in contant motion but they can influence the electron distribution in other atoms that are close by. The ability of an atom or molecule to have its electron density distorted or altered in this way is called polarisation. Generally the more electrons that are in an atom or molecule and the larger the atom or molecule the more easily it is polarised.

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.

### Shape and Van der Waals bonding

It is not only the size and number of electrons present that will influence the amount of Van der Waals bonding present, the shape 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.

Finally it is worth noting that Van der Waals forces act between all atoms and molecules and they are in addition to any other intermolecular forces or bonds that are also acting. However you should bear in mind that Van der Waals bonding is very weak in comparison to intramolecular bonding such as ionic or covalent bonding.

## Key Points

• Van der Waals forces or London dispersion forces are a weak form of intermolecular bonding present in all substances. They are caused by the uneven or asymmetrical distribution of the electrons to create dipoles within the atoms or molecules.
• These Van der Waals forces only act over short distances. The atoms/molecules must be close before they will have an affect
• The size of the Van der Waals forces increases with increasing molecular size and atomic size (as the number of electrons increase). The Van der Waals forces for example are larger in an iodine molecule than a much smaller fluorine molecule
• The shape of molecules is also an importnat consideration in deciding on the amount of Van der Waals bonding present. Molecules with large surface areas will have more Van der Waals bonding present than similar molecules with different shapes.
• Polarisation is the ability of an atom or molecules electron density to be squashed or altered in some way.