Intermolecular forces are the forces of attraction and repulsion between interacting particles (atoms and molecules). This term does not include the electrostatic forces that exist between the two oppositetic charged ions and the forces that hold atoms of a molecule together i.e., covalent bonds.
Attractive intermolecular forces are known as Van der Waals forces, in honour of Dutch scientist Johannes van der Waals (1837- 1923), who explained the deviation of real gases from the ideal behaviour through these, gases from the ideal behaviour through these forces. We will learn about this later in this unit. Van der Waals forces vary considerably in magnitude and include dispersion forces or London forces, dipole-dipole forces, and dipole-dipole interaction is hydrogen bonding.
Only a few elements can participate in hydrogen bond formation, therefore it is treated as a separate category.
At this point, it is important to note that attractive forces between an ion and these are not van der Waals forces. We will now learn about different types of Van der Waals forces.
Dispersion Forces of London forces
Atoms and nonpollar molecules are electrically symmetrical and have no dipole moment because their electronic charge cloud is symmetrically distribution. But a dipole may develop momentarily even in such atoms and molecule. This can be understood as follows.
Suppose we have two atoms 'A' and 'B' in the close vicinity of each other.
It so happen that momentarily electronic charge distribution in one of the atoms, say 'A' becames unsymmetrical i.e., the charge cloud is more on one side than the other.
This results in the development of instantaneous dipole on the atom 'A' for a very short time. This instantaneous or transient dipole distorts the electron density of the other atoms 'B' which is close to it and as a consequence a dipole is induced in the atom 'B'.
