Structures and Bonding: Intermolecular Forces

Understanding Intermolecular Forces

Intermolecular forces are forces of attraction that exist between molecules.
These forces are responsible for many physical properties of substances like melting and boiling points, solubilities, and vapour pressures.
The different types of intermolecular forces include London dispersion forces, dipole-dipole interactions, and hydrogen bonds.

London Dispersion Forces

London dispersion forces, also known as van der Waals forces, are the weakest type of intermolecular force.
They arise due to temporary fluctuations in the electron distribution within atoms or molecules, creating a temporary dipole.
These forces are present in all molecules, regardless of polarity, but are particularly significant in nonpolar substances.
London dispersion forces increase with the size of the molecule or the number of electrons it has.

Dipole-Dipole Forces

Dipole-dipole forces exist in molecules that are polar, where there is an uneven distribution of electron density.
These forces arise from the electrostatic attraction between the positive end of one polar molecule and the negative end of another.
Dipole-dipole forces are stronger than London dispersion forces but weaker than hydrogen bonds.

**Hydrogen Bonding **

Hydrogen bonding is the strongest form of intermolecular force.
It occurs when a hydrogen atom is bonded to a highly electronegative atom (nitrogen, oxygen, or fluorine), and this hydrogen is attracted to another electronegative atom in another molecule.
Hydrogen bonding gives substances specific properties like high boiling points. It’s responsible for the unique properties of water, including its ability to expand upon freezing.

Impact and Importance of Intermolecular Forces

The strength and type of intermolecular forces present in a substance greatly determine its physical properties.
Substances with strong intermolecular forces typically have higher boiling and melting points, as more energy is required to overcome the forces.
Understanding intermolecular forces allows prediction and explanation of trends in the physical properties of substances.