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Kinetic Theory of Matter

by Ron Kurtus (revised 27 March 2016)

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. A simplified version of the theory is the Kinetic Theory of Gases.

There had been various theories in history stating that matter consisted of moving particles, but it wasn't until 1905 when Albert Einstein explained that moving molecules were causing the motion.


The particles of matter are widely spaced and in constant motion. The assumption is that the collisions between the particles are elastic, meaning no energy is lost when they collide. At the atomic level, this can be a valid approximation.

The greater their velocity, the greater their kinetic energy (KE) and temperature. In a gas, the KE and spacing are the greatest. They are closer together and slower in a liquid, and close enough in a solid that motion is primarily molecular vibrations.



By making some basic assumptions, such as the idea that matter is made of widely spaced particles in constant motion and that collisions between particles are elastic, the theory helps to explain the behavior of matter.

Increased energy and temperature determines the state

Questions you may have include:

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



Initial assumptions

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 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.


Undergo elastic collisions. Do not exert forces on each other.

Obey Newton's Laws of motion

States of matter

The Kinetic Theory of Matter provides a good explanation of the states or phases of matter.


In a gas, molecules are in constant, random linear motion. They are so far apart on the average that attractive forces between them is negliable. Collisions between molecules are considered elastic such that no kinetic energy is lost and turned into potential energy,

The lack of attraction or molecular forces allows the gas to expand unless confined in a container.

See Kinetic Theory of Gases


Molecules will flow or glide over one another, but stay toward the bottom of the container. Motion is a bit more random than that of a solid. They have enough kinetic energy to slip out of the ordered arrangement of a solid.

Gravity affects the molecules.


Molecules are held close to each other by their attractions of charge. They will bend and/or vibrate, but will stay in close proximity. The molecules have an ordered arrangement.


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

Kinetic theory of gases - Wikipedia

Brownian motion - Wikipedia

Physics Resources


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

Top-rated books on Thermodynamics

Questions and comments

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