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Theories of Matter

by Ron Kurtus (revised 19 February 2016)

Although matter is defined as the substance of objects that also takes up space and has mass, there are several theories of matter that try to explain its charactertistics and properties.

One theory that explained some of the properties of matter is the Molecular Theory of Matter. This was followed by the Atomic Theory of Matter that examined matter at a microscopic level. There are new theories being developed that try to explain the true structure of matter in even more detail.

Questions you may have include:

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

Molecular or Kinetic Theory of Matter

The original Molecular Theory of Matter stated that all matter consists of tiny particles called molecules. These particles are constantly moving and bouncing off each other like billiard balls. The Molecular Theory of Matter is also called the Kinetic Theory of Matter, because of the constant movement of the molecules.

This theory provides an explanation of many characteristics of matter, including how heat works and why materials change from solid to liquid to gas.

(See Kinetic Theory of Matter for more information on that subject.)

Brownian motion

In the early 1800s Robert Brown, a botanist, discovered pollen granules suspended in water would move around in a zigzag motion, as if struck by tiny particles. This phenomenon was called Brownian motion. Years later, Albert Einstein explained the motion through the Kinetic Theory of Matter. He showed that molecules were hitting the granules and moving them, thus proving that matter consisted of molecules in motion.


The motion of molecules is responsible for the phenomenon of heat. In other words, the faster the molecules are moving, the higher the temperature.

(See Heat for more information.)

Change of state of matter

Although moving molecules have kinetic energy according to their speed, they are essentially vibrating in place in a solid. The electrostatic attraction between the molecules is much greater than the kinetic energy.

When the molecules speed up or the material is heated sufficiently, the kinetic energy overcome the molecular attraction and the substance changes its state from a solid to a liquid. Likewise, when the kinetic energy of the molecules increases further, the material can change from a liquid to a gaseous state.

(See States of Matter for more information.)

Atomic Theory of Matter

Molecules can be broken into smaller particles called atoms. The Atomic Theory of Matter states that all matter consists of extremely small particles called atoms.

It was originally thought that atoms were the smallest possible particles, but that has since been proven incorrect. Atoms consist of even smaller particles called electrons, protons, and neutrons. A combination of protons and neutrons combine to form the nucleus of an atom.

Atoms are called elements in chemistry, because they are "elementary" to molecules.

Solar system model of the atom

A popular model or picture of an atom that explains many of its properties and features is the solar system model of the atom. This model is also called the Bohr Model, named after Neils Bohr, who came up with the idea. It states that electrons rotate around the nucleus, similar to the planets revolving around the sun.

There are newer, more complex models of the atoms now, consisting of such things as probability clouds of electrons and mathematical descriptions. The solar system or Bohr model is a sufficient explanation for our studies.

Explains electricity

The Atomic Theory explains electricity. When electrons break away from their nuclei, their motion results in electricity.

Modern theories

For a long time, it was thought that the proton, neutron, and electron were the elementary particles, as well as the smallest. Since the Atomic Theory was formulated, many new particles have been discovered. The new theories concerning these particles and predicted particles attempts to explain every phenomena in physics. This is also called the Universal Theory of Matter.

Subatomic particles

Also, there have been discovered that the proton and neutron themselves are made of even smaller particles, called quarks. These particles are then held together by particles called gluons.

(See Subatomic Particles for more information.)

Finally, there is a theory that these sub-atomic particles are not particles at all, but really vibrating strings. Many scientists accepted this until recently when it was determined that there is no way to prove or disprove this theory. It is purely mathematical speculation.

Anti-matter particles

There are anti-matter particles, that are the mirror image of existing particles. For example, there is the positron, that is a (+) charged electron. There is a (−) charged proton and a neutron that spins in the opposite direction as the standard neutron.

Quantum Theory of Matter

The Quantum Theory of Matter states that at the very small sub-atomic distances, matter does not travel in continuous motion. Instead, it jumps from position to position in discrete or quantum leaps. This theory also states that particles spin in very discrete motion. The Uncertainty Principle states that with small particles, you cannot tell exactly where the particle and how fast it is going at the same time.

The study of Quantum Mechanics is uses highly advanced mathematics to formulate equations and predict behavior of matter.

String Theory

The newest theory is that matter consists of tiny strings of material, instead of round balls. Mathematically, String Theory seems to explain many phenomena for both large systems and at the quantum level. But many scientists claim that it is simply a mathematical exercise, since it cannot be proven or disproven.


The Molecular or Kinetic Theory states that matter consists of small molecules in motion. The Atomic Theory states that matter consists of smaller particles called atoms. Subatomic theories state that atoms are made of protons, neutrons and electrons, and they are made up of even smaller particles. The Quantum Theory looks at motion at small distances, and the String Theory has a new outlook on matter. They are part of a Universal Theory that is trying to explain everything.

Enjoy life by being curious about the world around you.

Resources and references

Ron Kurtus' Credentials


Matter - Wikipedia

Matter is the Stuff Around You -

Matter: Definition & the Five States of Matter -

Physics Resources


Top-rated books on Physics

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