Intermolecular Bonds

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

  • 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

  • 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

  • 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’.