Molecules such as H2, N2, O2 and F2 also form molecular solids because all the valence electron orbitals are used either for intramolecular bonding or are occupied with nonbonding electrons. Thus any intermolecular bonding that holds molecules together in the solid must be weak compared with the strength of intramolecular bonding in molecules. The weak forces that contribute to intermolecular bonding are called van der Waals forces after the Dutch physicist, Johannes van der Waals.