O-Level and SEC G3 Chemistry K324
C2: The Particulate Nature of Matter
Use particle and atomic models to explain states, diffusion, elements, compounds, mixtures, and electronic structure.
Reviewed for the 2027 cohort on 19 July 2026. The official syllabus remains authoritative for assessable wording and paper details.
Core notes
The particulate model connects observations to invisible particles. K324 combines state changes, diffusion, atomic structure, nuclide notation, isotopes, and ions, so explanations must move carefully between particle arrangement, energy, charge, and the numbers of subatomic particles.
States, energy and particle motion
In a solid, particles are closely packed in fixed positions and vibrate. In a liquid, they remain close but move past one another. In a gas, they are far apart and move rapidly in random directions. Melting, boiling, condensation, freezing, and sublimation change arrangement and motion without changing the chemical identity of the substance.
Heating supplies energy. During a change of state, the energy changes the interactions and separation between particles rather than raising temperature continuously. Cooling reverses this process. Describe the particle change and energy transfer, not merely that particles become larger or disappear.
Diffusion and its rate
Diffusion is the net movement of particles from a region of higher concentration to lower concentration because particles move randomly. It explains the spread of perfume through air and soluble substances through water. The evidence is the gradual spread even without stirring, not Brownian motion, which K324 does not require.
Higher temperature gives particles more kinetic energy and increases diffusion rate. At the same temperature, lower molecular mass generally gives faster gas diffusion. Keep concentration gradient, distance, temperature, and medium in mind when comparing experiments.
Atoms, isotopes and ions
An atom has protons and neutrons in a nucleus, with electrons in shells. A proton has relative charge +1 and relative mass 1; a neutron has charge 0 and relative mass 1; an electron has charge -1 and a much smaller relative mass. Proton number identifies the element, while nucleon number counts protons plus neutrons.
Isotopes are atoms of the same element with the same proton number but different neutron numbers. For a neutral atom, electrons equal protons. A positive ion has lost electrons and a negative ion has gained electrons; the nucleus is unchanged. Read nuclide notation before calculating protons, neutrons, and electrons.
Formulae and relationships
| Relationship | Use |
|---|---|
| Find neutron number from nucleon number A and proton number Z. | |
| Relate ion charge q to proton number Z and electron number e. |
Worked examples
Example 1: An ion has proton number 17, nucleon number 37, and 18 electrons. Find its protons, neutrons, and charge.
- Protons equal proton number: 17.
- Neutrons equal .
- There is one more electron than proton, so the ion charge is -1.
Answer: 17 protons, 20 neutrons, and charge -1.
Chapter checkpoint
Use these three moves to organise the topic before attempting a mixed or practical question.
- Use particle motion and spacing to explain states, diffusion, and changes of state.
- Distinguish elements, compounds, and mixtures through particles and separation behaviour.
- Use proton, neutron, electron, isotope, and electronic-structure ideas consistently.
Official outcome coverage
K324 C2: 6 mapped outcomes, references C2(a), C2(b), C2(c), C2(d), C2(e), C2(f). Check the official K324 syllabus.
The outcome wording is not reproduced here. The relevant official syllabus remains authoritative for exact assessable scope.
Practical and data connection
Connect diffusion or separation observations to a particle explanation without claiming that individual particles were directly seen.
Exam traps and retrieval check
Avoid these traps
- Saying particles expand during heating instead of describing increased motion and separation.
- Changing the proton number when an atom forms an ion.
- Defining isotopes as atoms with different proton numbers.
Check from memory
What changes during melting?
Particle arrangement and freedom of motion change; chemical identity does not.
Why does diffusion speed up when heated?
Particles have more kinetic energy and move faster.
How is neutron number found?
Subtract proton number from nucleon number.
Pure versus Combined scope
Combined Chemistry uses the same top-level topic label but a narrower outcome set. Use the Combined component checklist before removing or adding detail.
Shared explanation source
Eclat has a related explanation in its existing IP library. It can help with the shared concept, but its IP extensions and school-sensitive scope are not automatically part of K324. Open the related IP explanation.