As two atoms come into close proximity they show a weak bonding interaction due to their fluctuating electrical charges. This force is known as van der Waals attraction (Fig. A.). van der Waals attraction (per atom) is 0.1 kcal/mole both in a vacuum and in water.
If two atoms come too close to each other they will be strongly repelled (Fig. A.).
The most effective distance between atoms can be calculated by adding the van der Waals radius of the two interacting atoms (Fig. B.).
Even though van der Waal's attractions are weak, large complimentary surfaces can be held together readily if many atoms are involved at the same time.