AQA · A-Level · Physics
Course version: 7408 · Version from Sept 2016
− Equations for alpha decay, β decay including the need for the neutrino.
Full specification wording
Stable and unstable nuclei: − Equations for alpha decay, β decay including the need for the neutrino.
Nuclear decay equations track changes in proton and nucleon numbers.
Checked against the AQA 7408 specification (opens in a new tab) on .
- Point 3 of 4
- 3 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
Nuclear decay equations track changes in proton and nucleon numbers. In alpha decay, a nucleus emits an alpha particle (a helium nucleus, 2 protons and 2 neutrons), so its proton number decreases by 2 and its nucleon number by 4. For example, uranium-238 becomes thorium-234: ²³⁸₉₂U → ²³⁴₉₀Th + ⁴₂He. In beta-minus decay, a neutron converts to a proton, emitting an electron and an antineutrino: ¹⁴₆C → ¹⁴₇N + ⁰₋₁e + ν̄ₑ. The antineutrino is needed because the electron's kinetic energy varies continuously; without a third particle, energy and momentum would not be conserved. Beta-plus decay emits a positron and neutrino. Balancing proton and nucleon numbers on both sides confirms the equation.
What earns the marks
- Alpha decay: nucleon number decreases by 4 and proton number decreases by 2, with an alpha particle (⁴₂He) emitted.
- Beta-minus decay: a neutron changes into a proton, so proton number increases by 1 while nucleon number stays the same, with an electron (⁰₋₁e) emitted.
- Beta-plus decay: proton number decreases by 1 and a positron (⁰₊₁e) is emitted.
- The neutrino or antineutrino carries away energy and momentum, explaining the continuous beta energy spectrum.
- Equations must balance nucleon number and proton number (or charge) on both sides.
- Correct symbols and notation, such as ⁴₂He, ⁰₋₁e, ν̄ₑ and νₑ, should be used.
Common mistakes and how to put them right
Writing beta decay as if the electron came from the nucleus without changing a neutron.
Put it right: A neutron transforms into a proton, increasing proton number by 1.
Omitting the antineutrino in beta-minus decay.
Put it right: Include it to conserve energy and momentum.
Forgetting to balance nucleon numbers in alpha decay.
Put it right: Check that the total nucleon number is the same on both sides.
Exam tips
- Write the full nuclear equation and check both nucleon and proton numbers balance before moving on.
- State clearly that the neutrino is needed to account for conservation of energy and momentum in beta decay.
- Use correct symbols for alpha, beta-minus, beta-plus, neutrino and antineutrino to avoid losing communication marks.
Try the lesson
The start of the Stable and unstable nuclei: − Equations for alpha decay, β decay including the need for the neutrino. lesson, made from checked teaching notes for this specification point.
Have a go first
A nucleus of radium-226 (²²⁶₈₈Ra) decays by emitting an alpha particle. Which equation correctly represents this decay?
Listen
Nuclear decay equations track changes in proton and nucleon numbers. In alpha decay, a nucleus emits an alpha particle (a helium nucleus, 2 protons and 2 neutrons), so its proton number decreases by 2 and its nucleon number by 4. For example, uranium-238 becomes thorium-234: ²³⁸₉₂U → ²³⁴₉₀Th + ⁴₂He. In beta-minus decay, a neutron converts to a proton, emitting an electron and an antineutrino: ¹⁴₆C → ¹⁴₇N + ⁰₋₁e + ν̄ₑ. The antineutrino is needed because the electron's kinetic energy varies continuously; without a third particle, energy and momentum would not be conserved. Beta-plus decay emits a positron and neutrino. Balancing proton and nucleon numbers on both sides confirms the equation. A strong answer earns marks for these points. Alpha decay: nucleon number decreases by 4 and proton number decreases by 2, with an alpha particle (⁴₂He) emitted. Beta-minus decay: a neutron changes into a proton, so proton number increases by 1 while nucleon number stays the same, with an electron (⁰₋₁e) emitted. Beta-plus decay: proton number decreases by 1 and a positron (⁰₊₁e) is emitted. The neutrino or antineutrino carries away energy and momentum, explaining the continuous beta energy spectrum. A common slip: Writing beta decay as if the electron came from the nucleus without changing a neutron. Instead, a neutron transforms into a proton, increasing proton number by 1. Exam tip: Write the full nuclear equation and check both nucleon and proton numbers balance before moving on.
See it
Key words
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Conservation of nucleon number
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Conservation of charge
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Particle emission and the neutrino hypothesis
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See every definition
- Conservation of nucleon number
- The total nucleon number must be the same on both sides of a decay equation, so equations must balance nucleon number.
- Conservation of charge
- Proton number, or charge, must balance on both sides of a decay equation, and equations must balance proton number.
- Particle emission and the neutrino hypothesis
- The neutrino or antineutrino carries away energy and momentum, explaining the continuous beta energy spectrum, so it must be emitted in beta decay.
Label the diagram
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1Box 1
2Box 2
3Box 3
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