Intermolecular Force

Summary: A weak attractive force between molecules. Not to be confused with the strong covalent bonds within molecules. Tags: igcse chemistry definition bonding Created: 2026-07-14 Last Updated: 2026-07-14

Intermolecular forces are weak attractive forces that act between neighbouring molecules, in contrast to the strong covalent bonds that hold atoms together within a molecule. These forces are responsible for the relatively low melting and boiling points of simple molecular substances such as H₂O, CO₂, and CH₄, because only a modest amount of thermal energy is needed to overcome them during state changes. The strength of intermolecular forces increases with molecular size and mass, which explains why boiling points rise along a homologous series as chain length grows. A critical IGCSE distinction is that intermolecular forces are overcome when a substance melts or boils, whereas intramolecular covalent bonds remain intact — no chemical reaction has occurred. In giant covalent structures such as diamond and silicon dioxide, by contrast, very strong covalent bonds must be broken, giving them exceptionally high melting points.

Key term — IGCSE Chemistry (0620/0971) glossary.