The Particle Model of Matter

The particle model of matter says that all matter is made of tiny particles that are always moving, with spaces between them and forces of attraction that hold them together to different degrees. In brief: the model explains why solids keep their shape, why liquids flow and why gases can be squashed, using the arrangement, movement and spacing of particles.

The particles may be atoms, molecules or ions, depending on the substance. The particle model, also called the kinetic particle theory, is a simple tool that links what we see in the everyday world to what happens at an invisible scale. This article lists the main ideas of the model, shows the evidence for particles, compares the states in a table, works through a scale example, explains the limits of the model and gives a short history.

What are the main ideas of the particle model?

• Matter is made of particles. Each substance has its own kind of particle, and the particles of one substance are all alike.
• Particles are in constant motion. They vibrate, slide or move freely, depending on the state.
• There are spaces between particles. In gases the spaces are large. In solids and liquids they are small.
• Particles attract each other. The attraction is strong in solids, weaker in liquids and negligible in gases at ordinary pressure.
• Temperature is related to motion. Hotter matter has particles that move faster, on average.

What is the evidence for particles?

We cannot see atoms with our eyes, but many observations support the particle model.

• Diffusion: the smell of perfume spreads across a room because gas particles move and mix.
• Dissolving: sugar disappears in tea but makes it sweet, because sugar particles spread between water particles.
• Compression of gases: a syringe with trapped air can be pushed in, because there is empty space between particles.
• Brownian motion: pollen grains or smoke particles seen under a microscope jiggle because they are hit by invisible, moving particles.
• Direct images: since the 1980s, scanning tunneling microscopes have produced images of individual atoms on surfaces.

How does the model describe solids, liquids and gases?

In a solid, the particles are packed closely in a fixed pattern and vibrate in place. They cannot move past each other, so the solid keeps its shape and volume. In a liquid, the particles are close together but not in a fixed pattern. They can slide past each other, so a liquid flows and takes the shape of its container, but its volume is fixed. In a gas, the particles are far apart and move quickly and randomly in all directions. They fill any container and can be compressed.

Particle model summary table

Worked example: how small and how many?

Estimating the number of particles shows why they are invisible. A glass of water of 250 g has about 250 ÷ 18 = 13.9 mol of water, since the molar mass of water is 18 g/mol.

1. Number of molecules. One mole contains 6.02 × 10²³ particles, so 13.9 mol contains 13.9 × 6.02 × 10²³ = 8.4 × 10²⁴ molecules.
2. Comparison. With a world population of roughly 8 × 10⁹ people, that is about 8.4 × 10²⁴ ÷ (8 × 10⁹) ≈ 1 × 10¹⁵ molecules for every person alive on Earth. In other words, each person would receive roughly a million billion molecules.
3. Size. A water molecule is about 0.3 nanometers (3 × 10⁻¹⁰ m) across. A human hair is about 70,000 nm wide, so about 70,000 ÷ 0.3 = 233,000 molecules fit across the width of a hair.

Such numbers explain why even a tiny drop contains an enormous number of particles and why matter looks continuous to us.

What are the limits of the model?

The simple model draws particles as solid spheres, and this is a simplification. Real particles are atoms, molecules or ions with internal structure. The model does not show that particles in solids and liquids can differ in size and shape, nor that the forces between them vary between substances. It also does not describe the electrons and the nucleus inside atoms. It still explains many properties of matter, but later topics refine the model.

How did the particle idea develop?

In ancient Greece, Leucippus and Democritus suggested that matter consists of tiny, indivisible atoms. This idea was not widely accepted for many centuries. In 1738 Daniel Bernoulli explained gas pressure using moving particles. John Dalton proposed an atomic theory in 1808, and Robert Brown observed the jiggling of pollen grains in 1827. In 1905 Albert Einstein explained Brownian motion with a quantitative theory based on molecular collisions, and Jean Perrin’s experiments around 1908 confirmed it, which convinced almost all scientists that atoms and molecules are real.

Which mistakes are common?

Mistake 1: Thinking that particles themselves expand when heated

The particles do not grow. They move faster or vibrate more, and the spaces between them increase.

Mistake 2: Putting air or “nothing” between particles in a gas

The space between gas particles is empty (a vacuum), not filled with more air.

Mistake 3: Saying that particles in a solid do not move

They vibrate around fixed positions.

Mistake 4: Giving particles properties of the bulk material

A single atom of copper is not “hot”, “shiny” or “soft”. Properties such as these belong to large collections of particles.