intermolecular

Intermolecular refers to interactions that occur between separate molecules, as opposed to intramolecular forces that bind atoms within a single molecule. The study of intermolecular interactions explains how substances behave in different phases, mix with others, and determine properties such as boiling point, melting point, solubility, and viscosity. While intramolecular bonds such as covalent, ionic, and metallic bonds are responsible for a molecule's identity and structure, intermolecular forces govern how molecules attract or repel one another in condensed phases and solutions.

The main types of intermolecular forces are van der Waals forces, which include London dispersion forces, dipole-dipole interactions, and hydrogen bonding. London dispersion forces arise from temporary fluctuations in electron density that create instantaneous dipoles; they are present in all molecules, but are especially important in nonpolar substances. Dipole-dipole interactions occur between polar molecules with permanent dipole moments, aligning the molecules to maximize electrostatic attraction. Hydrogen bonding is a stronger, directional interaction that occurs when hydrogen is bonded to highly electronegative elements such as fluorine, oxygen, or nitrogen and interacts with lone pairs on neighboring molecules.

Other relevant intermolecular interactions include ion-dipole forces, between ions and polar molecules, and dipole-induced dipole effects, where a polar molecule induces a dipole in a nonpolar partner. The cumulative effect of these forces determines phase behavior, solubility, surface tension, and crystallization patterns in materials and biological systems, from water and ethanol to proteins and polymers.