Particle theory

This lesson covers: 

  1. How the particles in matter behave
  2. How the particles differ in solids, liquids and gases

Particle behaviour determines the state

The properties of a substance depend on the behaviour of its particles.


The key factors are:

  • Energy of the particles.
  • Strength of forces between particles.
  • Arrangement of the particles.


The same material can exist as a solid, liquid or gas - its state depends on its particles.

Particles in solids

Diagram showing tightly packed particles in a solid with strong forces between them.
  • Particles in a solid have low energy and vibrate in fixed positions.
  • Strong forces between the particles, keep particles in an orderly, close-packed arrangement.


  • The particles can't move freely so solids keep a fixed shape and volume.
  • Solids are hard to compress as the particles are already tightly packed.
  • Solids are usually dense as many particles occupy a small volume.

Particles in liquids

Illustration showing particles in a liquid state, demonstrating their ability to move around each other while remaining close due to forces of attraction.
  • Particles in a liquid have more energy and can move around each other.
  • Forces of attraction between the particles, keep particles close but they can slide past one another.


  • Liquids flow to fill a container but keep a constant volume.
  • Liquids are hard to compress as the particles still close together.
  • Liquids are quite dense but less dense than solids.

Particles in gases

Illustration showing particles in a gas moving in all directions with high energy and widely spaced.
  • Particles in a gas have high energy and move very fast in all directions.
  • There are almost no forces of attraction between the widely spaced particles.


  • Gas have no fixed shape or volume so can expand to fill any container.
  • Gases are easy to compress as particles have lots of space between them.
  • Gases are very low density as particles spread through a large volume.