Intermolecular Forces

Basic Concept

Intermolecular forces are the forces of attraction between individual molecules in a substance.
These forces are weaker than the strong covalent bonds within the molecules.
Every substance experiences some type of intermolecular force but the type and strength of these forces can vary greatly.

Types of Intermolecular Forces

Three main types of intermolecular forces exist: London dispersion forces, dipole-dipole attractions, and hydrogen bonds.
London dispersion forces are the weakest intermolecular force and occur between all atoms and molecules. These are due to temporary fluctuations in electron distribution within atoms or molecules.
Dipole-dipole attractions are stronger and occur between polar molecules. These arise due to permanent differences in electron distribution within polar molecules resulting in positively and negatively charged ends.
The strongest intermolecular force is the hydrogen bond. This occurs between molecules where hydrogen is bonded to highly electronegative elements like nitrogen, oxygen or fluorine.

Correspondence to Properties

Boiling and melting points: Substances with stronger intermolecular forces have higher boiling and melting points as more energy is required to overcome these forces.
Volatility: Substances with weaker intermolecular forces are more likely to be volatile as less energy is needed for the molecules to escape into the gas phase.
Viscosity: Substances with stronger intermolecular forces have higher viscosity as the molecules find it more difficult to move past each other.
Surface tension: Stronger intermolecular forces result in a higher surface tension as the molecules on the surface of a liquid are drawn into the liquid by the intermolecular forces.

Remember that understanding intermolecular forces is crucial to the understanding of properties of different substances and their interactions, and lays the groundwork for further topics such as states of matter and solutions. It forms the basis of many aspects of chemistry, from why oil and water don’t mix to why certain combinations of chemicals can be dangerous.