AQA · A-Level · Physics
Course version: 7408 · Version from Sept 2016
Changes in N and Z caused by radioactive decay and representation in…
Full specification wording
Nuclear instability (A-level only): Changes in N and Z caused by radioactive decay and representation in simple decay equations.
Radioactive decay changes the numbers of protons and neutrons in a nucleus, and these changes must be shown in balanced decay equations.
Checked against the AQA 7408 specification (opens in a new tab) on .
- Point 3 of 4
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
Radioactive decay changes the numbers of protons and neutrons in a nucleus, and these changes must be shown in balanced decay equations. For alpha emission, the parent nuclide loses ⁴₂He, so the daughter has A reduced by 4, Z reduced by 2 and N reduced by 2. For beta-minus decay, a neutron becomes a proton, so Z increases by 1, N decreases by 1 and A is unchanged; the equation includes an electron and an antineutrino. For beta-plus decay, a proton becomes a neutron, so Z decreases by 1, N increases by 1 and A is unchanged; the equation includes a positron and a neutrino. Electron capture combines an inner electron with a proton, so Z decreases by 1, N increases by 1 and A is unchanged, with a neutrino emitted. Balancing A and Z on both sides confirms the equation.
What earns the marks
- In alpha decay, the equation shows the parent nuclide producing a daughter with A decreased by 4 and Z decreased by 2, plus a ⁴₂He particle.
- In beta-minus decay, the equation shows the parent nuclide producing a daughter with Z increased by 1 and A unchanged, plus an electron and an antineutrino.
- In beta-plus decay, the equation shows the parent nuclide producing a daughter with Z decreased by 1 and A unchanged, plus a positron and a neutrino.
- In electron capture, the equation shows the parent nuclide combining with an electron to produce a daughter with Z decreased by 1 and A unchanged, plus a neutrino.
- Conservation of A and Z must hold on both sides of every decay equation.
Common mistakes and how to put them right
Writing a beta-minus decay equation without the antineutrino.
Put it right: Include the antineutrino so lepton number is conserved.
Changing A during beta decay.
Put it right: A remains constant because only a neutron-proton conversion occurs.
Omitting the electron in an electron capture equation.
Put it right: Show the captured electron on the left-hand side.
Exam tips
- Balance A and Z separately before checking the particle emitted.
- Use the periodic table or data sheet to confirm the chemical symbol of the daughter nuclide.
- Include the neutrino or antineutrino in beta and electron capture equations where required.
Try the lesson
The start of the Nuclear instability (A-level only): Changes in N and Z caused by radioactive decay and representation in simple decay equations. lesson, made from checked teaching notes for this specification point.
Have a go first
When a nucleus undergoes alpha decay, how does its proton number Z change?
Listen
Radioactive decay changes the numbers of protons and neutrons in a nucleus, and these changes must be shown in balanced decay equations. For alpha emission, the parent nuclide loses ⁴₂He, so the daughter has A reduced by 4, Z reduced by 2 and N reduced by 2. For beta-minus decay, a neutron becomes a proton, so Z increases by 1, N decreases by 1 and A is unchanged; the equation includes an electron and an antineutrino. For beta-plus decay, a proton becomes a neutron, so Z decreases by 1, N increases by 1 and A is unchanged; the equation includes a positron and a neutrino. Electron capture combines an inner electron with a proton, so Z decreases by 1, N increases by 1 and A is unchanged, with a neutrino emitted. Balancing A and Z on both sides confirms the equation. A strong answer earns marks for these points. In alpha decay, the equation shows the parent nuclide producing a daughter with A decreased by 4 and Z decreased by 2, plus a ⁴₂He particle. In beta-minus decay, the equation shows the parent nuclide producing a daughter with Z increased by 1 and A unchanged, plus an electron and an antineutrino. In beta-plus decay, the equation shows the parent nuclide producing a daughter with Z decreased by 1 and A unchanged, plus a positron and a neutrino. In electron capture, the equation shows the parent nuclide combining with an electron to produce a daughter with Z decreased by 1 and A unchanged, plus a neutrino. A common slip: Writing a beta-minus decay equation without the antineutrino. Instead, include the antineutrino so lepton number is conserved. Exam tip: Balance A and Z separately before checking the particle emitted.
Key words
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Nuclear equations
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Conservation laws
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Radioactive decay
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Nuclide notation
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See every definition
- Nuclear equations
- Balanced decay equations showing the parent nuclide producing a daughter plus the emitted particle, with A and Z balanced on both sides.
- Conservation laws
- Conservation of A and Z must hold on both sides of every decay equation.
- Radioactive decay
- Changes in N and Z in a nucleus, shown in simple decay equations for alpha, beta-minus, beta-plus and electron capture.
- Nuclide notation
- Representation of a parent nuclide and daughter with A and Z, as used in decay equations.
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