Introduction
So who was he?
What are Van der
Waals forces?
What's his equation
all about?
Get to the lizards already
Other stuff
Links and references
Graphite, and other examples
Zoom in or out by holding down the shift key and the left mouse button.
Image taken from
http://www.pbs.org/wgbh/nova/diamond/insidegraphite.html
The image above shows a single sheet of graphite. Graphite itself is composed of many sheets of hexagonal carbon arrays. When graphite pencils are used the motion of the tip against the paper overcomes the London force, which is the only force holding the sheets together, leaving a thin layer of a few sheets of graphite on the paper - a pencil mark.
Ice
Water also makes use of Van der Waals forces in both its solid and liquid phase. The image above shows the arrangement of water molecules in ice. Note that all the δ+ hydrogen atoms are pointing towards δ- oxygen atoms as hydrogen bonds have formed. These bonds are stronger than the attractions caused by London or Keesom forces, but weaker than ionic or covalent bonds. They are the result of the large ΔEN between hydrogen and oxygen. These bonds force the water molecules to space themselves out, forming the orderly pattern shown above. It can be seen that the array has large gaps in its structure.
Water molecules in liquid form are able to move faster, and so are not affected by hydrogen bonding to such a large degree. The molecules are not forced to adopt an orderly pattern, and so can actually position themselves closer together. This is why ice is less dense than water in its liquid form, and therefore why ice floats in water. Hydrogen bonds do affect liquid water in a lesser way, by increasing the boiling point of water from that which would be expected if hydrogen bonding did not occur.