Calculate proton nucleon and neutron numbers
Use Z for proton number and A for total nucleons, then find neutrons as A minus Z.
Section 5.1.2 · The nucleus
Exam scope and assessment notes
Core and Extended · 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
- Identify protons and neutrons.
- Subtract whole numbers.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Which expression gives neutron count?
What you will learn
- Use Z for proton number and A for total nucleons, then find neutrons as A minus Z.
- Define Z and A and recover a physically consistent neutron count.
Define both numbers
Proton number, also called atomic number, Z is the number of protons in the nucleus. Nucleon number, also called mass number, A is the number of protons plus neutrons. Therefore neutron number is A − Z. Neither number includes atomic electrons.
A supplied nucleus with A = 23 and Z = 11 has 11 protons and 12 neutrons. Do the subtraction in that order. A valid ordinary nuclide has whole-number counts and A at least as large as Z. An A smaller than Z would imply a negative neutron count and flags an inconsistent entry.
Use identity and charge only where given
Z identifies the element because element identity is defined by proton count. A distinguishes isotopes once Z is fixed. An electron count follows from neutrality or a stated ion charge; A and Z alone do not assert that the surrounding atom is neutral.
Mass number is a particle count, not a mass in grams or a decimal periodic-table average. Keep the integer A for the specified nuclide separate from relative atomic mass averaged over a mixture of isotopes.
Pause and explain
An entry has A = 8 and Z = 10. Is it a consistent ordinary nuclide count?
Worked example
For a supplied nuclide with A = 23 and Z = 11, find protons and neutrons.
Show the worked solution
- Z directly gives 11 protons.
- A counts all nucleons.
- Neutrons are 23 − 11 = 12.
Answer 11 protons; 12 neutrons.
Try a topic test
Choose Easy, Normal or Difficult. Save your answers and open worked review after finishing.
Test this topic →From guided practice to a new situation
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 neutrons when A = 35 and Z = 17.
Give me a hint
Use the nucleon total minus proton count.
Compare my reasoning
- A is 35.
- Z is 17.
- 35 − 17 gives 18.
18 neutrons.
Look for these in your work
- State the count type.
- Use the correct subtraction.
A nucleus has eight protons and ten neutrons. Find Z and A.
Give me a hint
One number counts protons and the other counts all nucleons.
Compare my reasoning
- Z is eight.
- A is eight plus ten.
- The total is eighteen.
Z = 8; A = 18.
Look for these in your work
- Define both symbols.
- Do not include electrons.
Only A and Z are supplied. A student declares the atom’s electron count must equal Z. Evaluate it.
Give me a hint
Check whether neutrality or ion charge has been stated.
Compare my reasoning
- Z fixes proton count.
- A − Z fixes neutron count.
- Electron count needs the atom or ion charge condition.
Electron count equals Z only if neutrality is specified.
Look for these in your work
- Separate nucleus from whole atom.
- Name the missing charge condition.
Common mistakes
- Subtract Z from A.
- A excludes electrons.
- Mass number is an integer count.
What can you explain now?
What two types contribute to nucleon number?
Compare with the explanation
Protons and neutrons
Nucleon number sums the two nuclear constituent counts.
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Make it stick
Define Z and A and recover a physically consistent neutron count.
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.