Feline1 gave a rather good explanation here: http://forums.webelements.info/viewtopic.php?p=5108#5108

Thank you very much. I think i know it now.

:evil: honestly that topic really sucked i made a better one- maybe you should you delete this :evil:

I think that the V der W force increases as you go down the Periodic Table because you need more energy to break the intermolecular forces or is it that the V der W force decreases as you go down. Sorry I've just confused myself maybe someone can answer that question?

As you go down the periodic table, the valence electrons move further away from the nucleus, and are sheilded more.
So they are held less strongly.....how does that relate to VdW forces..?

because as you go down the periodic table it takes more or les (can't recall which) energy to break the bonds between the electrons.

The point I was trying to guide people to there was that if the valence electrons are not held as strongly, they are more polarisable, and an instantaneous dipole created in one will induce instantaneous dipoles in the others faster when the electrons are easier to manipulate, because they are held less tightly.