Intermolecular Forces (1) Van der Waals forces

Intermolecular forces: van der Waals forces

Here’s a question to get you thinking: how is it that monomolecular substances like the Noble gases can both liquefy and freeze?

If these substances are composed of single atoms how can these particles attract each other?

All Noble gases exist at room temperature in the gaseous state. 

There must be then at best fairly weak forces between the atoms of these gases.

What must be happening to allow the molecules to get closer and then to liquefy at lower temperatures?

What can explain the fact that even helium the lightest and smallest of the atoms of the Noble gases will enter the liquid state at 4K or –269^oC?

Or, though you may never have considered this, why can Gordon the Gecko climb a wall or even a glass window without dropping off?

These and several other phenomena are the result of van der Waals forces.

They are named after Johannes Van Der Waals the Dutch Physicist.

These forces are the result of slight imbalances of electrical charge in individual but otherwise electrically neutral atoms.

In the diagram below you can see how at any instant in time the charge distribution within the atom allows for the formation of a temporary dipole.

If two of these dipoles line up correctly as in the diagram then there is a slight weak attraction between the two particles. 

The imbalance of charge in one atom could also induce an imbalance of charge in another atom again the result being a slight weak force of attraction between the two particles. 

These weak induced dipoles result in London forces or dispersion forces.

 

So when Noble gas atoms are cooled these forces begin to act between the atoms and result in these gases liquefying and solidifying at very low temperatures.

I guess the values of their b.p. and m.p. tell you that these forces are pretty weak. 

Still they are the forces that act when Gordon Gecko climbs a glass ceiling.

It’s all in his feet.

Thousands of little hairs called setae combine with the glass surface using van der Waals forces.

There is more here.

These are the forces between polythene molecules and that should tell you that if there are enough of these minute forces acting together between two long polythene chains then the overall effect can be considerable.

Hence polythene is a solid at room temperature with a reasonable melting point.  

Same for the layers of graphite which are held together by thousands of van der Waals forces 

See the diagram below:

Van der Waals forces are the weakest of the intermolecular forces in existence in nature.

Van der Waals forces exist between alkane molecules and between halogen molecules in fact between all molecules.

The strength of these forces depends on the number of electrons in the molecule or atom the greater the number of electrons the stronger the potential dipole induced or otherwise. 

You can argue then that these van der Waals forces sometimes called London or dispersion forces are responsible for the variation in the boiling and melting points of the alkanes, the halogens etc….