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

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

The Lennard-Jones model can be used to estimate the attractive forces present in gases at the interatomic and intermolecular levels.

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