Key concept · Models

Unit 1: The Particle Model of Matter

Everything around you — your desk, the air you breathe, the water in your bottle — is made of tiny particles. A model is a simple picture scientists use to explain something they cannot see directly. In this unit, you will use the particle model to explain why matter behaves the way it does.

01 What is a scientific model?

We cannot see atoms with our eyes. So scientists build a model — a simplified picture — that matches what we observe in experiments. A good model lets us predict what will happen next.

The particle model says: all matter is made of tiny particles that are always moving. The particles are so small we cannot see one alone.

02 States of matter

The particle model explains the three states of matter:

State	Arrangement	Movement
Solid	Particles are close together, in fixed positions	Vibrate in place only
Liquid	Particles are close but can slide past each other	Move around slowly
Gas	Particles are far apart	Move fast, in every direction

Easy way to remember: Solid = a full, silent bus (everyone in a seat). Liquid = people walking slowly in a crowded room. Gas = people running freely in an empty field.

03 Changing state

When you heat a substance, you give its particles more energy. They vibrate or move faster. If they gain enough energy, they break free from their neighbours and the substance changes state (melting, boiling). Cooling does the opposite.

Important: when ice melts into water, the particles themselves do not change — only their arrangement and movement change.

04 Inside the atom

Particles of matter are usually atoms or groups of atoms called molecules. An atom has:

A tiny, dense nucleus in the centre, made of protons (positive charge) and neutrons (no charge)
Electrons (negative charge) moving in the space around the nucleus

An atom has no overall charge because the number of protons equals the number of electrons.

V Key vocabulary

Term	Meaning
Particle	A very small piece of matter, such as an atom or molecule.
Model	A simplified idea that explains and predicts what we observe.
Nucleus	The small, dense centre of an atom.
Molecule	Two or more atoms joined together.

? Quick check

Try to answer before you tap. Answers are hidden underneath.

Why do scientists use models instead of just describing what they see?

Because we cannot observe atoms directly. A model gives us a simple picture that explains and predicts behaviour we can test in experiments.

A gas is heated inside a sealed container. What happens to its particles?

The particles gain kinetic energy and move faster, colliding with the container walls more often and more strongly (this is why pressure increases).

Why does an atom have no overall electric charge?

Because the number of positive protons in the nucleus equals the number of negative electrons around it, so the charges cancel out.

next →Unit 02: Forces and How They Interact