When two wet glass plates are pressed together, they cannot be separated easily. This is because of the force of attraction existing between the atoms and molecules, and are electrical in origin.
All mechanical systems have to acquire a state of minimum potential energy, to attain stability. This is achieved by the force of attraction and repulsion acting between the atoms. This can be understood by the following graph showing the variation of potential energy with interatomic distances.
As the two isolated Hydrogen atoms approach each other, the nucleus of one atom attracts the valence electrons of the other and vice versa. As the hydrogen atoms come closer, the attractive force tends to decrease the potential energy of the system. When the potential energy reaches a minimum value, sharing of electrons take place between the two, resulting in a covalent bond. This stage is represented by the distance 'ro' in the graph. It is interesting to note that when the distance between the two atoms becomes even lesser, their nuclei repel each other, thus preventing them from collapsing. In other words, the force of repulsion increases the potential energy of the system.
At distances equal to ro, the two forces balance each other, resulting in a molecule. Similarly, forces of attraction exist between molecules, binding them together. These are intermolecular forces. The intermolecular forces are weaker than interatomic forces, as the former forces are the Van der waals forces.