Intermolecular Bonds

Intermolecular Forces

Intermolecular forces exist between separate molecules. They are weaker than the covalent bonds within molecules but are responsible for important physical properties.
Three types of such forces exist: Van der Waals (dispersion) forces, dipole-dipole interactions, and hydrogen bonding.

Van der Waals’ forces are weak attractive forces seen in all molecules, whether they are polar or non-polar.
They occur as a result of random, instantaneous shifts in electron distribution within a molecule, creating a momentary imbalance, a dipole.
This dipole then induces a dipole in a neighbouring molecule, and attraction occurs between them. This is known as an instantaneous dipole-induced dipole interaction or London dispersion force.
The strength of Van der Waals’ forces depends on the number of electrons within the molecule; heavier molecules have stronger dispersion forces.

Dipole-dipole interactions occur between polar molecules where a partially positive hydrogen or another atom is attracted to a partially negative atom.
They are stronger than Van der Waals’ forces but still considerably weaker than covalent or ionic bonds.

Hydrogen bonding is a special type of intermolecular bond that occurs when a hydrogen atom is covalently bonded to a strongly electronegative atom such as nitrogen, oxygen, or fluorine.
This bond is highly polar, resulting in the hydrogen atom carrying a significant positive charge. It is then attracted to the lone pair of electrons on a neighbouring electronegative atom.
Hydrogen bonds are the strongest intermolecular forces and are responsible for many unusual properties, such as the high boiling point of water compared to other molecules of a similar size.
The role of hydrogen bonding in our atmosphere is extremely significant, particularly in the behaviour of water and nitrogen oxides. This is often a focus of exam questions within the topic of ‘The Ozone Story’.