Relate nuclear charge and relative mass to proton and nucleon numbers
Use positive nuclear charge proportional to Z and approximate relative nuclear mass proportional to A.
Exam scope and assessment notes
Extended only (Supplement) · Physics 0972, 2026–2028. Use labelled nuclides, supplied count intervals and stated background conditions. Model drawings are not to scale. Written practice is self-checked. Human subject-teacher review, supervised practical competence and cumulative assessment remain pending. Cambridge states that IGCSE 0972 has the same subject content as IGCSE 0625, with 9–1 grading. This StudyForA pathway maps the 2026–2028 syllabus only. This 9–1 graded qualification is available only in a limited number of administrative zones. Confirm that your centre can enter you for this code and exam series with Cambridge International. Cambridge has published a separate syllabus for 2029. StudyForA has not reconciled that version yet; this page supports exams in 2026, 2027 and 2028 only.
Before you begin
- Know proton and neutron charges.
- Read A and Z in nuclide notation.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Which gives the relative charge on a nucleus?
What you will learn
- Use positive nuclear charge proportional to Z and approximate relative nuclear mass proportional to A.
- Apply +Z nuclear charge and approximate A-based relative mass while preserving their limits.
Count the charge-carrying constituents
Each nuclear proton contributes +1 relative charge and each neutron contributes zero, so a nucleus with proton number Z has relative charge +Z. This is nuclear charge: a neutral atom can still have this positive nucleus because surrounding electrons balance it.
A ¹⁴₆C nucleus has relative charge +6. A supplied ¹⁶₈O nucleus has relative charge +8. Their charge ratio is 6:8 = 3:4. The neutron counts affect nuclear mass but do not add electric charge.
Use the mass approximation explicitly
Protons and neutrons each have approximately one unit of relative mass; electron mass is very small by comparison. Therefore the relative mass of a nucleus is approximately its nucleon number A. Carbon-14 and carbon-12 have an approximate nuclear mass ratio 14:12 = 7:6.
The approximation is not an exact rest-mass formula. Nuclear binding and the small constituent mass differences matter for exact masses, so an exact fission or fusion energy cannot be obtained by subtracting integer A labels. Ionisation changes the whole atom’s electron inventory, not the nucleus’s Z or A.
Pause and explain
Can exact fusion energy be obtained by treating integer A values as exact rest masses?
Worked example
State relative nuclear charge and approximate relative nuclear mass for ¹⁴₆C.
Show the worked solution
- Six protons give relative charge +6.
- Fourteen nucleons give approximate relative mass 14.
- Use positive sign for nuclear charge.
Answer Charge +6; approximate relative mass 14.
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Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Find nuclear charge and approximate relative mass for ¹⁶₈O.
Give me a hint
Assign a different role to each nuclide number.
Compare my reasoning
- Z = 8 counts protons.
- Charge is +8.
- A = 16 gives approximate relative nuclear mass 16.
+8; approximately 16.
Look for these in your work
- Keep the positive sign.
- State the mass approximation.
Compare approximate nuclear masses of carbon-14 and carbon-12 as a simplest ratio.
Give me a hint
Use A for both isotopes.
Compare my reasoning
- The approximate masses are 14 and 12.
- Divide both by two.
- The ratio is seven to six.
7:6 approximately.
Look for these in your work
- Use nuclear rather than electron counts.
- Do not claim exact mass equality to A.
A neutral carbon-14 atom is reported to have a zero-charge nucleus. Correct the account.
Give me a hint
Balance nucleus and electrons separately.
Compare my reasoning
- Its nucleus has six protons.
- That nucleus has relative charge +6.
- Six surrounding electrons balance the whole neutral atom.
The nucleus is +6; the complete neutral atom totals zero.
Look for these in your work
- Distinguish subsystem and whole atom.
- Retain the supplied isotope identity.
Common mistakes
- Use Z for charge and A for mass.
- Nuclear charge is not ion charge.
- Relative mass is approximate.
What can you explain now?
Which nuclear constituent adds charge zero but contributes mass?
Compare with the explanation
A neutron
A neutron is uncharged but is a nucleon with approximately one relative mass unit.
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Make it stick
Apply +Z nuclear charge and approximate A-based relative mass while preserving their limits.
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