Intermolecular forces are attractive or repulsive forces that arise between two different molecules due to the action of positively charged entities (the protons) and negatively charged entities (the electrons). Atoms consist of a positively charged nucleus surrounded by a cloud of electrons. However, the interaction with a charged species may change the manner in which the electron cloud surrounds the nucleus (the electron density may shift towards a specific region). These shifts in electron densities in the electron cloud result in several attractive and repulsive forces, which are commonly known as Van der Waal’s forces. Some important types of intermolecular interactions are listed below.
Interactions between two dipoles (dipole-dipole interactions)
Here, two polar molecules attract or repel each other based on the charges held by them. These permanent dipoles may arise in the respective molecules as a result of the differences in the electro negativities of the bound atoms.
Interactions between an ion and a dipole (ion-dipole interactions)
Here, a charged species (commonly referred to as an ion) interacts with a polar molecule featuring a dipole. Based on the charge on the ion, one end of the dipole is attracted to it and the other is repelled.
The formation of an induced dipole due to the presence of an ion (ion-induced dipole interactions): Here, the presence of a charged ion causes a shift in the electron density of an adjacent, neutral molecule. This results in the formation of a dipole in the neutral molecule, which is now called an induced dipole.
Formation of a dipole due to the presence of a dipole (dipole-induced dipole interactions)
Here, a dipole causes a shift in the electron density of an adjacent neutral molecule, resulting in the formation of an induced dipole. Now, attractive forces arise between the dipole and the induced dipole.
London Dispersion Forces
The relatively high boiling points of certain non-polar molecules such as hexane, bromine, and benzene that enables them to remain liquid at room temperatures can be accounted for by London dispersion forces. These forces also account for substances such as naphthalene and iodine maintaining a solid-state at room temperatures. London dispersion forces are intermolecular forces that arise in nonpolar molecules that were first discovered by the German physicist Fritz London. These forces arise due to temporary fluctuations in the distribution of electrons around the nucleus. A temporary increase in the density of electrons in the electron cloud on one side of the atomic nucleus can result in the formation of an instantaneous dipole. When this instantaneous dipole interacts with an adjacent neutral molecule, the interaction results in the formation of an induced dipole and, by extension, an instantaneous dipole moment. It is important to note that London dispersion forces are highly dependent on the distance between the nuclei of the neighbouring atoms and they quickly vanish when the interatomic distance increases.
Thus, the important types of intermolecular forces are briefly discussed in this article. To learn about other interesting concepts in chemistry, such as the Friedel-Crafts acylation reaction, subscribe to the BYJU’S YouTube channel and enable notifications.
