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Kinetic Theory of Matter

by Ron Kurtus (updated 8 February 2022)

The Kinetic Theory of Matter states that matter is composed of a large number of small particles—individual atoms or molecules—that are in constant motion. This theory is also called the Kinetic-Molecular Theory of Matter and the Kinetic Theory of Gases.

By making some simple assumptions, such as the idea that matter is made of widely spaced particles in constant motion, the theory helps to explain the behavior of matter.

Questions you may have include:

This lesson will answer those questions. Useful tool: Units Conversion

Matter consists of small particles

The first assumption in this theory is that matter consists of a large number a very small particles—either individual atoms or molecules.

All matter (solid, liquid, and gas) is made up of tiny particles called atoms, or atoms that are joined to form molecules.

Large separation between particles

The next assumption concerns the separation of the particles.

In a gas, the separation between particles is very large compared to their size, such that there are no attractive or repulsive forces between the molecules.

In a liquid, the particles are still far apart, but now they are close enough that attractive forces confine the material to the shape of its container.

In a solid, the particles are so close that the forces of attraction confine the material to a specific shape.

Particles are in constant motion

Another assumption is that each particle is in constant motion.

In gases, the movement of the particles is assumed to be random and free.

In liquids, the movement is somewhat constrained by the volume of the liquid.

In solids, the motion of the particles is severely constrained to a small area, in order for the solid to maintain its shape.

The velocity of each particle determines its kinetic energy. There is an exchange or transfer of energy between particles—both atoms and molecules—during a collision between them.


The Kinetic Theory of Matter states that matter is composed of a large number a small particles that are in constant motion. It also assumes that particles are small and widely separated. They collide and exchange energy. The theory helps explain the flow or transfer of heat and the relationship between pressure, temperature and volume properties of gases.

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Resources and references

Ron Kurtus' Credentials


Kinetic Theory - HyperPhysics

A New Perspective for Kinetic Theory and Heat Capacity - Progress in Physics

The Kinetic Molecular Theory -

Kinetic theory of gases - Wikipedia

Brownian motion - Wikipedia

Matter Resources

Physics Resources


(Notice: The School for Champions may earn commissions from book purchases)

Introduction to Thermodynamics and Kinetic Theory of Matter by Anatoly I. Burshtein; Wiley-Interscience (1995)

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