Use proton number, nucleon number and nuclide notation
Identify isotopes and read or construct nuclide notation using proton, neutron and nucleon numbers.
AS Level; Cambridge International Physics 9702, 2025–2027 syllabus, Topic 11. These original particle models and equations are study aids; full timed assessment and human subject-teacher review remain outstanding.
Before you begin
- Use atomic number, nucleon number, electric charge and conservation with signed values.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Which number identifies the element?
What you will learn
- Distinguish proton number Z from nucleon number A
- Explain isotopes as same element with different neutron counts
- Use nuclide notation ᴬZX
Nucleon number counts protons and neutrons
Proton number Z is the number of protons in the nucleus and identifies the element. Nucleon number A is the total number of protons and neutrons. Neutron number is A − Z.
Nuclide notation places A as the upper-left number and Z as the lower-left number of the element symbol X: ᴬZX. For a neutral atom, electron number equals Z; for an ion, use the charge to determine electron number separately.
Isotopes differ in neutron number
Isotopes are forms of the same element with the same proton number but different neutron numbers. They therefore have the same Z and different A.
Changing the neutron count does not change the element's identity. Do not use the mass number alone to identify an element; use the proton number or symbol.
Pause and explain
How many neutrons are in ³⁵₁₇Cl?
Worked example
For ²³₁₁Na, find proton, neutron and electron numbers for a neutral atom.
Show the worked solution
- Z = 11, so there are 11 protons.
- Neutrons = A − Z = 23 − 11 = 12.
- A neutral atom has 11 electrons.
Answer 11 protons, 12 neutrons and 11 electrons.
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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.
For ⁴⁰₂₀Ca, find the neutron number.
Give me a hint
Subtract proton number from nucleon number.
Compare my reasoning
- A = 40 and Z = 20.
- N = A − Z = 40 − 20.
- State neutrons.
20 neutrons.
Look for these in your work
- A and Z identified
- Subtraction correct
- Particle count stated
Two nuclides have Z = 8 and A = 16 or 18. Explain why they are isotopes and find their neutron counts.
Give me a hint
Isotopes share proton number but differ in neutron number.
Compare my reasoning
- Both have Z = 8, so both are oxygen.
- For A = 16, N = 16 − 8 = 8.
- For A = 18, N = 18 − 8 = 10.
They are isotopes of oxygen: ¹⁶O has 8 neutrons and ¹⁸O has 10.
Look for these in your work
- Element identity established
- Both neutron numbers calculated
- Isotope definition applied
A neutral nuclide has 13 protons and 14 neutrons. Write its nuclide notation and state electron number.
Give me a hint
Find A by adding protons and neutrons; the element symbol follows Z.
Compare my reasoning
- A = 13 + 14 = 27 and Z = 13.
- Element 13 is aluminium, Al.
- A neutral atom has 13 electrons.
²⁷₁₃Al; 13 electrons.
Look for these in your work
- Nucleon number found
- Correct element identified
- Neutral electron count stated
Common mistakes
- Swapping A and Z in nuclide notation
- Using A as the proton number
- Calling atoms isotopes when their proton numbers differ
What can you explain now?
Which number identifies the element?
Compare with the explanation
Proton number Z
Elements have distinct proton numbers.
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Make it stick
Why can two nuclides have different nucleon numbers but still be the same element?
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.