There are three major types of intermolecular forces: Van der Waals forces; Dipole-dipole attractions;
and hydrogen bonding.
Van der Waals Forces
These are the weakest of the intermolecular forces. Their existence is deduced from the fact that non-
polar substances can be liquified and solidified. This suggests that there are forces of attraction
between the atoms and molecules of non-polar elements, as energy is required to transform the non-
polar elements between their various states, suggesting some force of attraction must need to be
overcome. The explanations for the existence of these Van der Waal’s forces are as follows. On
average the arrangement of electrons in a given atom will be symmetrical. But at any given instant the
electron arrangement may be unsymmetrical, causing a temporary dipole, which can attract the
electron cloud of another molecule. This means that both molecules will have dipoles and the
direction of the dipoles will be such that they attract each other. The speed with which electrons move
means that each dipolar attraction will only have a temporary existence, but there will be a constant
stream of new, temporary dipolar attractions.
As the size of a molecule increases, the number of electrons also increases, making the
molecule more polarisable and producing greater induced dipole attractions. Further, where the
surface area of a molecule is greater (such as in long hydrocarbon chains), the Van der Waals forces
are greater as the potential points of contact are greater.
This intermolecular force occurs between polar molecules which have a permanent dipole. The polar
molecules will align in such a way that the partial positive charges are adjacent to partial negative
charges giving a net attractive force. Dipole-dipole interactions are non-directional. They are simply
weak electrostatic attractions between charges on different molecules.