AQA GCSE Combined Science Foundation (8464), Foundation tier · Physics › Atomic structure › Atoms and nuclear radiation
Practise Nuclear equations. 8 exam-style questions plus unlimited generated ones on this subtopic, at up to four difficulty levels, with full mark schemes and a progress tracker. Free, no account needed.
How to write and balance nuclear equations for alpha and beta decay, and how alpha, beta and gamma emission change the mass and charge of a nucleus. Expect 'complete the equation' questions and questions asking you to identify the type of decay from the numbers.
Key facts
Alpha \({}^{4}_{2}\mathrm{He}\): mass number − 4, atomic number − 2
Beta \({}^{0}_{-1}\mathrm{e}\): mass number unchanged, atomic number + 1
Gamma \({}^{0}_{0}\gamma\): no change to mass number or atomic number
Neutron \({}^{1}_{0}\mathrm{n}\): mass number − 1, atomic number unchanged
Top numbers balance; bottom numbers balance
New atomic number = new element
Notes
Symbols in nuclear equations
Each nucleus is written as \({}^{A}_{Z}\mathrm{X}\): A is the mass number and Z is the atomic number (its charge).
Alpha particle: \({}^{4}_{2}\mathrm{He}\) (sometimes written \({}^{4}_{2}\alpha\)).
Beta particle: \({}^{0}_{-1}\mathrm{e}\) (sometimes written \({}^{0}_{-1}\beta\)). Mass number 0, charge −1.
The mass numbers (top numbers) add up to the same total on both sides of the arrow.
The atomic numbers (bottom numbers) add up to the same total on both sides.
If the atomic number changes, the element changes. Use the new atomic number to find the new element.
What each type of decay does
diagram
Alpha decay: mass number down by 4, atomic number down by 2. Both the mass and the charge of the nucleus decrease. Example: \({}^{226}_{88}\mathrm{Ra} \rightarrow {}^{222}_{86}\mathrm{Rn} + {}^{4}_{2}\mathrm{He}\)
Alpha decay: mass numbers 226 = 222 + 4 and atomic numbers 88 = 86 + 2.
Beta decay: mass number unchanged, atomic number up by 1. The mass does not change but the charge increases, because a neutron has become a proton. Example: \({}^{90}_{38}\mathrm{Sr} \rightarrow {}^{90}_{39}\mathrm{Y} + {}^{0}_{-1}\mathrm{e}\)
Beta decay: a neutron becomes a proton plus an electron. Mass number unchanged; atomic number up by 1.
Gamma emission: no change to the mass or the charge of the nucleus. The nucleus just gets rid of energy, often straight after an alpha or beta decay.
How to answer each type of question
Describe how a decay changes the nucleus
2 marksGrade 5
For each decay, say what happens to the mass (mass number).
Then say what happens to the charge (atomic number).
Example. Compare the effect on a nucleus of emitting an alpha particle and emitting a gamma ray.
Show the model answerHide the model answer
Alpha: the mass number decreases by 4 and the atomic number (charge) decreases by 2 (1). Gamma: there is no change to the mass or the charge of the nucleus (1).
Don’t lose marks
Writing the beta particle as \({}^{0}_{1}\mathrm{e}\) or \({}^{1}_{0}\mathrm{e}\). It is \({}^{0}_{-1}\mathrm{e}\)
Taking 2 off the mass number in alpha decay. It is 4 off the mass number and 2 off the atomic number.
Keeping the same element symbol after alpha or beta decay. The atomic number has changed, so the element has too.
Thinking beta decay lowers the atomic number. It raises it by 1.
More tips
Memory tricks
Alpha: 'minus 4, minus 2'. Beta: 'same, plus 1'. Gamma: 'no change'.
Check every equation with two quick sums: the top numbers on each side, then the bottom numbers.
The −1 on the beta particle is why the new nucleus goes UP by 1: the bottom numbers must still add up.
Exam technique
Write both numbers for every particle in the equation, even if the question only asks for one.
If a periodic table or list of elements is given, use the new atomic number to choose the symbol.
When asked to 'describe' the change, use words too: e.g. 'the mass stays the same and the charge increases'.
What each grade needs
What you need to be able to do, from the first marks up to the top grade.
Grade 4
Recall the symbols for alpha and betaAlpha is \({}^{4}_{2}\mathrm{He}\) and beta is \({}^{0}_{-1}\mathrm{e}\)
Grade 5
State how alpha decay changes a nucleusThe mass number falls by 4 and the atomic number falls by 2.
Grade 5
State how beta decay changes a nucleusThe mass number stays the same and the atomic number goes up by 1.
Grade 5
Explain why gamma emission changes neither numberGamma is electromagnetic radiation with no mass and no charge.
Quick recall
Cover the answers and test yourself. The app has these as flashcards that come back just before you'd forget them.
What happens to the mass number and to the atomic number of a nucleus when it emits an alpha particle?
Mass number falls by 4; atomic number falls by 2.
In a balanced nuclear equation, what must be true about the mass numbers on each side of the equation?
The total mass number is the same on both sides.
Sample questions
Written for this site in the style of AQA exam questions. They are not taken from real past papers.
Question 1Easy5 marks
(a) Which symbol represents an alpha particle? Tick (✓) one box.[1]
\({}^{4}_{2}\mathrm{He}\)
\({}^{0}_{-1}\mathrm{e}\)
\({}^{1}_{0}\mathrm{n}\)
\(\gamma\)
(b) Which symbol represents a beta particle? Tick (✓) one box.[1]
\({}^{4}_{2}\mathrm{He}\)
\({}^{0}_{-1}\mathrm{e}\)
\({}^{1}_{0}\mathrm{n}\)
\(\gamma\)
(c) What happens to the mass number and to the atomic number of a nucleus when it emits an alpha particle?[2]
(d) What happens to the mass number and to the atomic number of a nucleus when it emits a gamma ray?[1]
Show the answer and mark scheme
(a)Answer: \({}^{4}_{2}\mathrm{He}\)
(b)Answer: \({}^{0}_{-1}\mathrm{e}\)
(c)Answer: Mass number falls by 4; atomic number falls by 2.
the mass number decreases by 4
the atomic number decreases by 2
(d)Answer: Neither changes.
neither changes / both stay the same
Question 2Medium7 marks
Radium-226 decays by emitting an alpha particle. The equation for this decay is: \({}^{226}_{88}\mathrm{Ra} \rightarrow {}^{x}_{y}\mathrm{Rn} + {}^{4}_{2}\mathrm{He}\)
(a) Determine the values of x and y in the equation.[2]
(b) Strontium-90, \({}^{90}_{38}\mathrm{Sr}\), decays by emitting a beta particle. The nucleus formed is an isotope of yttrium (Y). Write a balanced nuclear equation for this decay.[3]
(c) Explain why the mass number does not change when a nucleus emits a beta particle.[2]