Question

How do Van der Waals forces arise between molecules?

Answer 1

Electron dipoles

Explanation:

At any point, electrons are moving 'randomly' in orbitals around atoms, so at any one point, more electrons may be on one side of the atom than the other, forming a small, instantaneous electron dipole where one side of the atom is negative and the other side is more positive.
This forms a temporary bond with other atoms which also have small, instantaneous electron dipoles.

Thus, the more electrons a molecule has, the stronger the Van der Waals forces can be said to be, as there is a greater probability of these small dipoles forming, and they will likely be more polar with more electrons being present.