Nuclear Equations
Learn Nuclear Equations for GCSE Physics with this free worksheet and full mark scheme — Foundation and Higher exam-style questions with worked answers for AQA, Edexcel and OCR. Nuclear equations represent decay, with alpha decay reducing the mass number by 4 and beta decay increasing the atomic number by 1.
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These worksheets and mark schemes are original, written for Virtus Academy and checked against the current AQA, Edexcel and OCR specifications. Every worksheet comes with a full mark scheme.
Topic overview
Nuclear equations show radioactive decay using symbols, and they must balance. The total mass number and the total atomic number must be the same on both sides.
Alpha decay reduces the mass number by 4 and the atomic number by 2, because an alpha particle carries away two protons and two neutrons. The element therefore changes.
Beta decay leaves the mass number unchanged but increases the atomic number by 1. This is because a neutron in the nucleus turns into a proton, and the electron produced is emitted as the beta particle. Gamma emission changes neither number, because gamma radiation carries no mass or charge.
Revision notes
Alpha decay
Mass number decreases by 4; atomic number decreases by 2.
The alpha particle is written as a helium nucleus with mass number 4 and atomic number 2. Because the atomic number changes, a different element is formed.
Beta decay
Mass number unchanged; atomic number increases by 1.
A neutron in the nucleus turns into a proton, and the electron produced is emitted. The beta particle is written with mass number 0 and atomic number −1.
Gamma emission
Neither the mass number nor the atomic number changes.
Gamma radiation carries no mass and no charge, so the nucleus becomes more stable without changing which element it is. Check both totals balance on each side of any nuclear equation.
Key points
- Nuclear equations must balance on both sides.
- Alpha decay reduces mass number by 4.
- Alpha decay reduces atomic number by 2.
- Beta decay leaves mass number unchanged.
- Beta decay increases atomic number by 1.
- Gamma emission changes neither number.
Worked examples
Example 1
A nucleus with mass number 226 and atomic number 88 emits an alpha particle. State the mass number and atomic number of the new nucleus. [2 marks]
Model answer
Mass number = 226 − 4 = 222 (1 mark). Atomic number = 88 − 2 = 86 (1 mark).
Example 2
Explain why beta decay increases the atomic number by one. [2 marks]
Model answer
A neutron in the nucleus turns into a proton (1 mark), so the number of protons increases by one while the total number of protons and neutrons stays the same (1 mark).
Example 3
State the effect of gamma emission on the mass number and atomic number of a nucleus. [2 marks]
Model answer
Neither changes (1 mark), because gamma radiation carries away no mass and no charge (1 mark).
Common mistakes
Changing the mass number in beta decay.
It stays the same; only the atomic number increases by one.
Decreasing the atomic number in beta decay.
It increases by one, because a neutron becomes a proton.
Saying gamma emission changes the element.
It changes neither number, so the element is unchanged.
Not checking that the equation balances.
Both totals must match on each side.
Exam tips
- Learn the three sets of changes as a table.
- Always check both totals balance.
- Explain beta decay through a neutron becoming a proton.
- Remember alpha decay changes the element; gamma does not.
Key terms
- Nuclear equation
- An equation showing a decay using symbols.
- Alpha decay
- Emission of a helium nucleus, reducing mass number by 4.
- Beta decay
- Emission of an electron as a neutron becomes a proton.
- Gamma emission
- Emission of electromagnetic radiation with no mass or charge.
Related topics
Written and reviewed against the current AQA, Edexcel and OCR specifications. Spotted an error? Let us know.