Particle diagrams come up in every science course from Year 7 to IGCSE, and in Class 9 CBSE (Matter in Our Surroundings and Is Matter Around Us Pure), and they are marked on rules, not on artistic skill. This page gives the rules for each diagram, draws all eight of them, lists the mistakes that cost the mark, and ends with a printable worksheet of ten diagrams to draw with the answers.
Download the particle diagram worksheet as a PDF (ten diagrams to draw or identify, with answers and the marking rules; A4, free).
What a particle diagram shows, and what it does not
A particle diagram represents each particle (atom, molecule or ion) as a circle inside a box that stands for the container. It shows two things: the state (how close together and how ordered the particles are) and the kind of substance (whether the particles are all the same, joined into units, or a mix). It does not show the real size of atoms, what they look like, or their electrons; the question that asks "what is the purpose of a particle diagram" wants "a simplified model of the arrangement of particles in a substance", not "to show the exact size of the particles".
The three states
| Solid | Liquid | Gas | |
|---|---|---|---|
| Arrangement | Regular, in rows (a lattice) | Random, no pattern | Random |
| Spacing | Touching | Touching, with small gaps | Far apart (about ten diameters) |
| Movement (write it in words next to the drawing) | Vibrate about fixed positions | Slide past each other, move freely but stay in contact | Move fast in all directions, in straight lines until they collide |
| Fills the box? | No, keeps its own shape | Takes the shape of the bottom of the container | Fills the whole container |
| Compressible? | No | Almost not | Yes |
The three rules the examiner checks for each: regular or random, touching or apart, and how much of the box is filled. Get those three right and the mark is yours; a beautiful drawing with the particles of a liquid in neat rows gets nothing.
Element, compound, mixture
Now the circles get colours or shading. One colour = one type of atom.
| Diagram | What you must draw | Real example |
|---|---|---|
| Element, monatomic | All circles the same, separate | Helium, neon, argon; solid iron or copper as a lattice |
| Element, diatomic | Pairs of identical circles joined, all pairs the same | H2, O2, N2, Cl2 |
| Compound | Every unit identical, each unit made of two or more different circles joined | HCl (one of each), H2O (two small + one large), CO2 |
| Mixture of elements | Two or more different circles, not joined, in no fixed ratio | Air (simplified), a mixture of gases |
| Mixture of a compound and an element | Some joined units of different atoms, plus some single or paired circles of another kind | Oxygen dissolved in water |
| Mixture of compounds | Two or more different kinds of joined unit, not joined to each other | Sugar dissolved in water, carbon dioxide in air |
The ratio matters for a compound and not for a mixture: if a box has three H2O units and one HCl unit, that is a mixture of two compounds, not a new compound.
What changes on melting, boiling and dissolving
- Melting (solid → liquid): the particles stay touching and stay the same particles; only the arrangement changes from regular to random, and they start to slide. Draw the same number of circles.
- Boiling or evaporating (liquid → gas): the spacing changes: the particles spread far apart and fill the container. Draw far fewer circles in the same box, none touching.
- Dissolving (solute in solvent): the solute particles separate and spread evenly among the solvent particles. Draw many solvent circles touching randomly (a liquid) with a few solute circles of another colour scattered evenly through them, none clumped. The particles do not change; they mix. That is why a solution is drawn as a mixture, and why it is not a chemical reaction.
- Chemical reaction: the atoms rearrange into new units. Before: two different diatomic pairs. After: joined units of two different circles. Same total number of circles of each colour, because atoms are conserved; the pairing is what changes.
The five mistakes that lose the mark
- Gas particles close together. A gas has about a tenth as many particles in the box, with wide gaps. If your gas looks like a messy liquid, it is a liquid.
- Liquid particles in rows. Rows mean a solid. Liquids are random.
- A compound drawn as a mixture. The different atoms must be joined (circles touching or drawn overlapping as one unit), and every unit must be the same.
- Changing the particles during a state change. Water molecules do not become smaller, split or change colour when ice melts. Same circles, new arrangement.
- Leaving out the words. Beside the diagram, write the movement: "vibrate about fixed positions", "slide past each other", "move rapidly and randomly". In many mark schemes the description carries a mark of its own.
Practice: draw these ten
The worksheet in the PDF gives a blank box for each and the answers on the second page.
- Solid iron 2. Liquid water 3. Steam 4. Neon gas 5. Oxygen gas 6. Hydrogen chloride gas 7. Air, simplified as nitrogen and oxygen 8. Salt dissolved in water 9. Ice melting: before and after 10. Hydrogen burning in oxygen to make water: before and after
Check each of your drawings against three questions: regular or random? touching or apart? all the same, joined-different, or unjoined-different? If all three are right, the drawing is right.
How we teach the particle model
A 1-on-1 online class where the child draws each diagram on paper while the tutor watches, and we correct the arrangement and spacing on the spot, then the words to write beside it. One class covers the states; a second covers element, compound, mixture and the changes; the third does past-paper questions on it. The first 30-minute class is free: send the topic and the last test on WhatsApp or the contact page. The definitions of solution, solute and solvent that go with diagram 8 are in solution, solute and solvent; the subject page is online chemistry tutoring.
Questions parents ask
1What is a particle diagram?
2How do you draw the particle diagram of a liquid?
3What is the difference between a compound and a mixture in a particle diagram?
4Why does the particle diagram of a gas have so few particles?
See how we teach this, 1-on-1 online →
Shobha
Founder and lead tutor, Science with Shobha
Teaching maths, science and English to children online since 2018. 500+ students, 20+ tutors, families in nine countries.