The particle model: Year 7 KS3 introduction

Particle model or particle theory?

A model is a simplified representation used to explain observations and make predictions. The particle model, sometimes called particle theory, uses diagrams to represent arrangement, spacing, movement and attraction. The familiar circles-in-a-box diagram is not a literal view through a microscope: it leaves out many details of real atoms and molecules.

Use the model in a disciplined order. First describe what the particles are doing; then connect that feature to the observable property. For example, gas particles are far apart and move rapidly in random directions, so a gas spreads through the available space. Saying only that particles are random is incomplete because it does not identify the spacing, motion or collision that causes the observation.

Explaining solids, liquids and gases with particles

In a solid, particles are closely packed in a regular or fixed arrangement and vibrate around fixed positions. Strong attractions keep neighbouring particles together, so the solid keeps a fixed shape and volume. The particles are still moving: vibration is motion, even though each particle remains close to the same position. A solid is difficult to compress because there is little empty space between particles.

In a liquid, particles remain close together but are arranged less regularly and can move past one another. This explains why a liquid flows and takes the shape of its container while keeping nearly the same volume. In a gas, particles are much farther apart and travel rapidly in random directions. A gas therefore fills its container, has no fixed shape or volume and can be compressed because its particles have substantial space between them.

Density, pressure, changes of state and conservation of mass

Density describes how much mass is contained in a given volume. The particle model helps explain broad density differences because closely packed particles usually place more mass into the same volume than widely spaced particles. This is a useful explanation, but pupils should not assume every solid is denser than every liquid: ice floats on water because its solid structure is less dense than liquid water. Measurements and the density equation are needed for specific comparisons.

Gas pressure comes from particles colliding with the walls of a container. Heating a sealed gas increases the particles’ average kinetic energy, so collisions become more frequent and harder; pressure rises if the container volume cannot change. Compressing the gas puts the same particles into a smaller space, also increasing collision frequency. The particles do not themselves become larger when pressure rises.

During a change of state, particle arrangement and movement change, but the substance remains the same. During melting or boiling at constant pressure, a pure substance absorbs energy to overcome attractions without its temperature rising. In a closed system, mass is conserved: particles may spread out or change arrangement, but they are not created or destroyed. A measured mass loss from an open container can occur when gas escapes.

BBC Bitesize solids, liquids and gases: checked support

The checked BBC Bitesize guides below cover particle arrangement, density, fluid pressure, gas pressure, changes of state and conservation of mass. Start with the solids, liquids and gases overview, then choose the density or conservation guide if that matches your question.

Oak’s downloadable slides and worksheets provide structured KS3 practice, while the clearly labelled Twinkl resources require account access. For each practice question, pupils should use a complete chain of reasoning: identify the particle feature, state the resulting behaviour and connect it to the observation. Finish by checking that the explanation does not claim particles vanish, expand or change identity during a physical change.