The strength of intermolecular forces

Intermolecular forces originate from either temporary or permanent molecular dipoles. The forces coming from permanent dipoles tend to be stronger than those from temporary dipoles, although this is not always the case. The reason is that their strength depends on different factors, which can vary a lot.

What does the strength depend on?

The van der Waals dispersion force is mainly dependent on the size of electron clouds, which usually correlates with the size of the species (atoms or molecules). Thus, large molecules, either consisting of a few large atoms, or many small ones, experience strong van der Waals forces. The strength of permanent dipole-dipole forces, on the other hand, depends mainly on the difference in electronegativity between the elements in the molecules. But remember, that is only the case as long as the geometry of the molecules does not cause the individual dipoles to cancel each other out. Thus, determining which intermolecular force is the strongest is not always straightforward.

Hydrogen bonding is special

Despite the complicated picture painted above, one intermolecular force stands out in terms of strength: hydrogen bonding. Even though hydrogen bonding is just a subcategory of permanent dipole-dipole forces, we often separate it from the crowd because of its unique character. The large electronegativity difference and small atomic radii make hydrogen the strongest intermolecular force!