Materials and their structure

Look around you. The chair you sit on, the water in your glass, even the air you cannot see — all of these are matter.

Matter is the scientist's word for "stuff". Something counts as matter if it has two things:

• Mass — there is an amount of it; it would tip a balance.
• Volume — it takes up space.

A rock is matter. So is a puddle of water. So is the air inside a balloon, even though you cannot see it — blow up a balloon and you can feel it pushing back.

A few things are not matter: light, sound and heat are forms of energy, not stuff. They have no mass and take up no space.

Matter does not always look or behave the same way. The same material can appear in different forms — solid, liquid or gas. We call these the three states of matter, and learning about them is the doorway into all of chemistry.

Matter comes in three everyday states. You meet all three before breakfast — ice, water and steam are the same material in three different states.

• A solid keeps its own shape and size. A pencil stays pencil-shaped wherever you put it.
• A liquid keeps the same volume but takes the shape of its container. Pour juice into a tall glass or a wide bowl — the amount is the same, but the shape changes.
• A gas has no fixed shape or volume. It spreads out to fill whatever container it is in, and it can be squashed into a smaller space.

Why do solids, liquids and gases behave so differently? Scientists explain it with the particle model.

The particle model has one big idea: all matter is made of tiny particles. These particles are far too small to see, even with most microscopes. A single grain of sugar contains more particles than you could count in a lifetime.

The particles are always moving. The hotter the matter, the more energy the particles have and the faster they move.

Imagine a crowd of people:

• In a packed lift, people are squeezed together and can barely move — that is like a solid.
• At a busy market, people are close but can shuffle past each other — that is like a liquid.
• In an empty playground, people run around freely with lots of space — that is like a gas.

The model is not something we can photograph — it is a picture in our minds that scientists use because it explains what we observe. A good model lets you predict what matter will do.

The secret is in how the particles are arranged and how they move.

• In a solid, particles are packed close together in a neat, regular pattern. They cannot move around — they only vibrate on the spot. That is why a solid keeps its shape.
• In a liquid, particles are still close together but in a messy, random arrangement. They can slide past each other, so a liquid flows and takes the shape of its container.
• In a gas, particles are spread far apart and move quickly in all directions. There is lots of empty space between them, so a gas can be squashed and will fill any container.

When we say matter is made of particles, what are those particles really? The smallest ones are called atoms.

An atom is incredibly tiny. If you could line up atoms side by side, it would take millions of them just to stretch across the full stop at the end of this sentence.

Everything you can touch is built from atoms — and there are about 100 different kinds. A substance made of only one kind of atom is called an element, like gold, oxygen or iron.

Atoms can join together to make molecules. A water molecule, for example, is made of three atoms joined together. Think of atoms like the letters of an alphabet — a small set of letters can spell millions of different words. In the same way, a small set of atoms builds every material in the universe.

So the full picture is: matter → particles → atoms. Atoms are the bottom layer, the basic building blocks of everything around you.

Once you can picture particles, the rest of chemistry becomes much clearer:

• Changes of state — melting, freezing and boiling are all explained by particles gaining or losing energy.
• Properties of materials — why some materials are hard, light or good conductors comes back to their particles.
• Mixtures and separating — dissolving, filtering and evaporating all make sense once you think in particles.
• Chemical reactions — when atoms rearrange to make new substances, you will rely on this picture again.

Whenever a chemistry idea feels tricky later, try drawing the particles. Take your time with this topic — really understanding the particle model now is worth far more than rushing ahead.