Classify hadrons and use proton and neutron quark models
Distinguish baryons and mesons and calculate the charges of proton and neutron quark compositions.
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
What is the quark composition of a proton?
What you will learn
- Describe protons and neutrons as non-fundamental quark combinations
- Classify baryons as three quarks and mesons as quark-antiquark pairs
- Add constituent charges to check a hadron model
Protons and neutrons are composite baryons
A proton is composed of two up quarks and one down quark (uud). Its charge is 2(+2e/3) + (−e/3) = +e. A neutron is udd, with charge (+2e/3) + 2(−e/3) = 0.
Protons and neutrons are not fundamental particles because they have constituent quarks. The syllabus model uses these valence-quark combinations to describe their charge and classification.
Hadrons are baryons or mesons
A hadron is either a baryon, consisting of three quarks, or a meson, consisting of one quark and one antiquark. Both are composite particles containing quarks.
When classifying a proposed combination, count the constituents and identify whether one is an antiquark. A neutral total charge does not by itself determine whether the particle is a neutron or a meson.
Pause and explain
What defines a meson in this syllabus?
Worked example
Check the charge of uud and udd using up +2e/3 and down −e/3.
Show the worked solution
- uud: 2(+2e/3) + (−e/3) = +e.
- udd: (+2e/3) + 2(−e/3) = 0.
- These match proton and neutron charges.
Answer uud is a proton with +e; udd is a neutron with zero charge.
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.
Calculate the charge of uud and identify the particle.
Give me a hint
Add the three signed quark charges.
Compare my reasoning
- Each u contributes +2e/3; d contributes −e/3.
- Total = 4e/3 − e/3 = +e.
- This is the proton composition.
+e; a proton.
Look for these in your work
- Charges added
- Net charge correct
- Particle identified
A hadron consists of u, d and s quarks. Classify it by its constituent count and find charge.
Give me a hint
Three quarks make a baryon; then add the three charges.
Compare my reasoning
- There are three quarks, so it is a baryon.
- Charge = +2e/3 − e/3 − e/3.
- Total charge is zero.
A neutral baryon in this quark model.
Look for these in your work
- Constituents counted
- Baryon classified
- Charge calculated
A proposed hadron contains one charm quark and one anti-strange quark. Classify it and find its charge.
Give me a hint
A quark plus antiquark is a meson; use their signed charges.
Compare my reasoning
- One quark and one antiquark makes a meson.
- c has +2e/3; anti-s has +e/3.
- Total charge = +e.
A meson with charge +e.
Look for these in your work
- Meson criterion applied
- Antiquark charge correct
- Total charge found
Common mistakes
- Calling protons fundamental particles in this quark model
- Classifying a meson as three quarks
- Using the constituent count but forgetting the antiquark charge sign
What can you explain now?
What is the quark composition of a proton?
Compare with the explanation
uud
Two up quarks and one down quark sum to +e.
After trying it yourself, choose your next review. This is your self-assessment.
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Make it stick
Why are protons and neutrons hadrons but not fundamental particles in this model?
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.