Cohesion (or cohesive attraction, or cohesive force) is the tendency of different parts of a substance to hold together (being mutually attractive). Cohesion is due to forces acting between its molecules: a molecule will repel one close to it but attract one that is farther away. This situation results in both cohesion and adhesion. The attractive action is due to intermolecular forces and takes on very different values, depending on the state of aggregation of matter. Therefore, in the solid state the cohesion is very intense, in the liquid state it is less, and it is almost zero in the aeriform state; its strength decreases with rising in temperature.

Cohesion and adhesion forces are of great importance in explaining some phenomena such as surface tension and capillarity (which are also called “surface phenomena”). Cohesion is an intrinsic property of a substance that is caused by the shape and structure of its molecules, which makes the distribution of surrounding electrons irregular when molecules get close to one another, creating electrical attraction that can maintain a microscopic structure such as a water drop. In other words, cohesion allows for surface tension, creating a “solid-like” state upon which light-weight or low-density materials can be placed. Molecules in the liquid state experience strong intermolecular attractive forces. When those forces are between like molecules, they are referred to as cohesive forces.

The Lennard-Jones model can be used to estimate the attraction forces that are present in gases at an interatomic and intermolecular level.

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