What is Van dar Waals Force?

The force of attraction among the molecules of non polar covalent compounds is known as Van der Waals force.

Van der Waals force is very weak, much weaker than covalent bond and even weaker than dipole-dipole interaction. For example the strength of covalent bond is O2 molecule is 402 kJ mol-1, while the strength of Van der Waals force in solid oxygen is about 7 kJ mol-1.