Topic 8: Nuclear and Particle Physics — Edexcel A-Level Physics
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Topic 8: Nuclear and Particle Physics explained
The proton number Z (atomic number) is the number of protons in a nucleus and fixes the element's identity.
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The nucleon number A (mass number) is the total number of protons plus neutrons. The neutron number is therefore A − Z. Nuclide notation places A as a superscript and Z as a subscript before the symbol, for example ²³⁸₉₂U has Z = 92 protons and A = 238 nucleons, so 238 − 92 = 146 neutrons. In nuclear equations both A and Z are conserved overall. Isotopes share the same Z but differ in A because they have different neutron numbers. Distinguish these from the relative atomic mass, which is a weighted average and not a whole-number count.
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Interactive Diagrams
1 interactive diagram to visualise key concepts
Conceptual Flow Outline
Classification tree of fundamental particles in the Standard Model.
Your focus
- State what proton number and nucleon number represent in a nucleus.
- Calculate the number of neutrons from A and Z.
- Interpret nuclide notation and distinguish isotopes by their A and Z values.
Topic 8: Nuclear and Particle Physics exam tips
Marking Points
- Defines proton number Z as the number of protons in the nucleus, determining the element.
- Defines nucleon number A as the total number of protons plus neutrons.
- Derives the neutron number as A − Z.
- Interprets nuclide notation, reading the superscript as A and the subscript as Z.
- Recognises that isotopes have the same Z but different A (different neutron numbers).
- Applies conservation of A and Z in simple nuclear equations.
Examiner Tips
- 💡Write A and Z explicitly for each nuclide before balancing a nuclear equation.
- 💡Use neutrons = A − Z whenever a question asks for the neutron count.
- 💡Check that both A and Z totals match on each side of a nuclear equation.
Common Mistakes
- Confusing nucleon number with neutron number: the correction is that nucleon number counts protons plus neutrons, so neutrons = A − Z.
- Thinking proton number changes between isotopes: the correction is that isotopes share the same proton number but differ in nucleon number.
- Reading the subscript as the mass number: the correction is that the superscript is A and the subscript is Z.
- Treating relative atomic mass as a whole-number nucleon count: the correction is that it is a weighted average of isotope masses.