Describe quark changes in beta decay and identify leptons
Track down-to-up or up-to-down changes and recognise electrons and neutrinos as leptons.
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 quark changes in beta-minus decay?
What you will learn
- Describe the quark-composition change in beta-minus and beta-plus decay
- Recall that electrons and neutrinos are leptons
- Relate the emitted beta particle and neutrino to the quark change
Beta decay changes one quark flavour
In beta-minus decay, a down quark changes to an up quark. In a neutron udd, this produces a proton uud; an electron and electron antineutrino are also emitted. The nuclear proton number rises by one while nucleon number stays fixed.
In beta-plus decay, an up quark changes to a down quark. In a proton uud, this produces a neutron udd; a positron and electron neutrino are emitted. The nuclear proton number decreases by one while nucleon number stays fixed.
Leptons are fundamental particles in this model
Electrons and neutrinos are fundamental particles called leptons. The positron and antineutrinos are their corresponding antiparticles.
The quark change occurs inside the nucleon, while the electron or positron and neutrino partner are emitted. Do not treat the emitted beta particle as a constituent quark.
Pause and explain
Which listed particles are leptons?
Worked example
A neutron changes into a proton in beta-minus decay. State the quark change and emitted leptons.
Show the worked solution
- A neutron is udd and a proton is uud.
- One d changes to u.
- An electron and electron antineutrino are emitted.
Answer d → u; e⁻ and electron antineutrino are emitted.
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.
A neutron undergoes beta-minus decay. Show how its quark composition changes and state the two emitted leptons.
Give me a hint
Compare udd and uud, then recall the beta-minus products.
Compare my reasoning
- One down quark becomes an up quark: udd → uud.
- The neutron becomes a proton.
- An electron and electron antineutrino are emitted.
d → u; n(udd) becomes p(uud) with e⁻ and ν̄ₑ emitted.
Look for these in your work
- Quark change correct
- Nucleon identity updated
- Both leptons named
In beta-plus decay, state the quark change, resulting nucleon and emitted neutral lepton.
Give me a hint
Reverse the beta-minus quark change and use beta-plus products.
Compare my reasoning
- One up quark changes to down: uud → udd.
- A proton becomes a neutron.
- An electron neutrino is emitted with the positron.
u → d; proton becomes neutron and an electron neutrino is emitted with the positron.
Look for these in your work
- Quark change correct
- Nucleon change correct
- Neutrino type correct
A learner labels an emitted electron as a quark because it appears in a nuclear equation. Explain the distinction and classify it.
Give me a hint
Separate a nucleon constituent from a decay product in the particle model.
Compare my reasoning
- Protons and neutrons contain quarks; an electron is not one of their listed quark constituents.
- Electrons and neutrinos are fundamental leptons.
- In beta-minus decay the electron is emitted alongside the changed nucleon.
The electron is a lepton, not a quark constituent; it is emitted in beta-minus decay when a down quark changes to up inside the nucleon.
Look for these in your work
- Constituent and product separated
- Lepton classification correct
- Decay context explained
Common mistakes
- Reversing the quark changes for beta-minus and beta-plus
- Calling the electron a quark
- Pairing beta-plus with an antineutrino instead of neutrino
What can you explain now?
Which quark changes in beta-minus decay?
Compare with the explanation
A down quark changes to an up quark
This converts neutron udd to proton uud.
After trying it yourself, choose your next review. This is your self-assessment.
Your review choice appears on Today. Sign in to sync it across devices.
Make it stick
How can one changed quark turn a neutron into a proton without changing nucleon number?
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.