How do Van der Waals forces affect the boiling point of helium?
1 Answer
The attractive van der Waals forces between helium atoms are so weak that the normal boiling point of liquid helium is only about 4.2 K, but without them, helium could not liquify at all!
Van der Waals forces are attractive interactions between atoms and molecules that are much weaker than ordinary chemical bonds. Most molecules have an electric dipole moment that results from unsymmetrical distribution of charges of the protons and electrons. The attractive interactions of dipoles with each other, combined with weaker dipole/induced-dipole and dispersion forces, accounts for the ability of most substances to condense into liquids and solids.
Hydrogen bonding is a particularly strong type of van der Waals interaction, which gives water an unusually high boiling point, but those forces are still weak compared with chemical bonds.
Helium has no permanent dipole moment, so the attractive interactions are derived almost entirely of instantaneous induced-dipole interactions where the motions of electrons on one atom are correlated with those on another to produce a very slight net attractive force between two transient dipole moments.
