The particle model of matter

Where the idea came from

Around 2400 years ago the Greek thinker Democritus argued that you can't keep cutting forever. Eventually you would reach tiny pieces that can't be divided. He called them atomos, Greek for "uncuttable", which is where our word atom comes from. Democritus was a philosopher. He reasoned his way to the idea but had no way of testing it, and most people at the time didn't believe him.

About 2000 years later, in the early 1800s, the English chemist John Dalton looked at the results of many chemistry experiments and saw a pattern. Substances always combined in simple, fixed ratios. Water, for example, always forms from hydrogen and oxygen in the same 2 to 1 ratio. That only makes sense if matter is built from separate particles. Dalton's version of atomic theory said:

• All matter is made of tiny particles called atoms.
• Atoms can't be created, destroyed or split.
• All atoms of one element are identical, and different from the atoms of other elements.
• Atoms join in simple whole-number ratios to make compounds.

The difference between the two men matters. Democritus had an idea. Dalton had an idea backed by evidence, which is what turns a guess into science.

We now know Dalton was wrong about one thing: atoms can be split. Each atom has a tiny central nucleus made of protons (positive charge) and neutrons (no charge), with even smaller electrons (negative charge) moving around it. You'll study this properly in Year 9. For now, the important part is that atoms can join together into bigger particles called molecules, such as oxygen gas (O₂) or carbon dioxide (CO₂). In these notes "particle" means whatever the smallest unit of a substance is, whether that's a single atom or a molecule.

The particle model in five ideas

Scientists combined Dalton's work with later discoveries into the particle model of matter. Everything else in this chapter is explained with these five ideas, so they are worth learning word for word.

1. All matter is made of tiny particles, far too small to see.
2. Particles of the same substance are identical. Different substances have different particles.
3. Particles are always moving. The hotter they are, the faster they move.
4. Particles attract each other. These forces of attraction are strongest in solids and weakest in gases.
5. There is empty space between particles. Not air: nothing at all.

Particles in solids, liquids and gases

Analogies help. A solid is like a packed cinema: everyone fidgets in their seat, but nobody changes seats. A liquid is like the crowd leaving the cinema: still shoulder to shoulder, but everyone is shuffling past everyone else. A gas is like a hundred bouncy balls thrown into an empty classroom, flying in every direction and bouncing off the walls, except that gas particles never slow down and stop.

Why use a model at all?

Particles are too small to see with an ordinary microscope, so scientists use a model: a simplified picture that explains what we observe and lets us make predictions. The particle model is excellent at explaining things like squashing, flowing and spreading. But every model leaves things out, and good scientists know the limits of theirs.