SEC G3 Combined Science Chemistry component K326/K328
C2: The Particulate Nature of Matter
Connect particle states and energy changes to atomic structure, nuclide notation, isotopes, and ion particle counts.
Reviewed for the 2027 cohort on 19 July 2026. The official syllabus remains authoritative for assessable wording and paper details.
Core notes
Particle models explain changes of state, while atomic models account for element identity, isotope differences, and ion charge.
States and interconversion
Solid particles are closely arranged and vibrate about fixed positions. Liquid particles remain close but rearrange, while gas particles are widely separated and move rapidly in random directions.
Melting and boiling require energy transfer into the substance. During a constant-temperature phase change, average kinetic energy does not fall; the transferred energy changes particle arrangement and interaction energy as attractions are overcome. Freezing and condensation transfer energy out while attractions bring particles into a closer, more ordered arrangement. The particles remain chemically the same during these physical changes.
Atomic structure and notation
Protons and neutrons are in the nucleus, while electrons occupy shells. Protons have relative charge and mass , neutrons charge and mass , and electrons charge with relative mass approximately . This is why electron mass is often treated as negligible in simple atomic-mass calculations.
Proton number counts protons and identifies the element. Nucleon number counts protons plus neutrons. In nuclide notation, place nucleon number above proton number beside the element symbol.
Isotopes, atoms, and ions
Isotopes are atoms of one element with equal proton numbers but different neutron numbers. They therefore have different nucleon numbers.
For a neutral atom, electron number equals proton number. A positive ion has lost electrons and a negative ion has gained them; nuclear particle numbers do not change when an ion forms.
Assessed representations
Atomic structure and nuclide notation
Formulae and relationships
| Relationship | Use |
|---|---|
| Find neutron number from nuclide notation. |
Worked examples
Example 1: A ion is given. State its proton, neutron, and electron numbers.
- Proton number gives 11 protons.
- Neutrons .
- A +1 ion has one fewer electron than the neutral atom, so it has 10.
Answer: 11 protons, 12 neutrons, and 10 electrons.
Chapter checkpoint
Use these three moves to organise the topic before attempting a mixed or practical question.
- Explain States and interconversion with the named terms, evidence, and causal links kept distinct.
- Use Atomic structure and notation to interpret the evidence given and justify each conclusion.
- Apply Isotopes, atoms, and ions to a new example, then check the conclusion against the information given.
Official outcome coverage
K326 C2: 7 mapped outcomes, references C2.1(a), C2.2(a), C2.2(b), C2.2(c), C2.2(d), C2.2(e), C2.2(f). Check the official K326 syllabus.
K328 C2: 7 mapped outcomes, references C2.1(a), C2.2(a), C2.2(b), C2.2(c), C2.2(d), C2.2(e), C2.2(f). Check the official K328 syllabus.
The outcome wording is not reproduced here. The relevant official syllabus remains authoritative for exact assessable scope.
Practical and data connection
Use observable state changes or diffusion evidence to test the particle model. Control temperature and sample amount, distinguish the observation from the particle-level explanation, and use atomic diagrams to infer proton, neutron and electron numbers rather than claiming that subatomic particles were observed.
Exam traps and retrieval check
Avoid these traps
- Changing particle size during a state change.
- Subtracting proton number from neutron number.
- Changing proton count when an ion forms.
Check from memory
Which number fixes element identity?
The proton number.
What differs between isotopes?
Neutron number.
What does a 2- ion have gained?
Two electrons.
Official Combined Science scope
This shared Combined Chemistry owner serves both K326 and K328. States and energy changes, atomic structure, proton and nucleon numbers, nuclide notation, isotopes, and particle counts. Orbitals and the s, p, d, and f notation are outside this scope.