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Have you ever heard of a theory? A theory is an explanation that seeks to explain facts or events. Everything you study in science is based on the thoughts, ideas, and experiences of scientists throughout human history. In this activity, you will learn about the particle model of matter theory.

More than two thousand years ago in Greece, a philosopher named Democritus suggested that matter is made up of tiny particles too small to be seen. He thought that if you kept cutting a substance into smaller and smaller pieces, you would eventually come to the smallest possible particles — the building blocks of matter.

Many years later, scientists came back to Democritus’s idea and added to it. The theory they developed is called the particle model of matter.

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The particle model of matter theory says that the tiny particles that make up all kinds of matter move at different speeds. Think of water at a solid state in the form of an ice cube, in a liquid state inside the form of a water bottle, and in a gaseous state in the form of steam rising from a boiling kettle. If you could actually see the individual particles of matter, they would be moving at various speeds with solids being the slowest and gases being the fastest.

What does the particle model of matter teach us?

The particle model describes matter in a very specific way. It describes four important aspects of matter:

All matter is made up of particles that are incredibly small — much too small to see with the naked eye. The particles can be atoms or combinations of atoms that are bonded.
There are forces between the particles.
The particles in matter are always moving. The more energy they have, the faster they move.
The spaces between the particles in matter are empty. You might assume that the spaces between particles are filled with air, but this is not the case. They contain nothing at all.

The particle model can explain the three states of matter: solids, liquids, and gases.

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Matter can exist in one of three main states: solid, liquid, or gas.

Solid matter is composed of tightly packed particles. A solid will retain its shape; the particles are not free to move around.

Liquid matter is made of more loosely packed particles. It will take the shape of its container. Particles can move about within a liquid, but they are packed densely enough that volume is maintained.

Gaseous matter is composed of particles packed so loosely that it has neither a defined shape nor a defined volume. A gas can be compressed.