✓ Revise Right course
In alpha decay the nucleus loses an alpha particle (mass number 4, atomic number 2), so the mass number falls by 4 and the atomic number by 2. Free notes.
Understanding Nuclear Equations is part of Atoms and nuclear radiation in GCSE AQA Higher Physics.
4 min
Reading time
23
Lessons in this topic
GCSE AQA Higher
Pathway
Reviewed by Revise Right
Content review
In alpha decay the nucleus loses an alpha particle (mass number 4, atomic number 2), so the mass number falls by 4 and the atomic number by 2. Free notes.
AQA GCSE Physics mock draft, 26–31 Oct. Draft coverage: Physics Paper 1 (Higher tier). Reviewed for an original-practice sitting; check student availability before registering. Marking and results will follow later, after separate approval. Eligible new accounts receive a five-day Pro trial when signup email verification completes. Previous trials keep their original expiry. After expiry, new papers need Pro unless you have an explicitly confirmed free sitting offer for this event. That offer adds no Pro time or AI learning. Saved attempts and released results remain readable. No card or automatic charge.
Use this lesson to stay inside Atoms and nuclear radiation while connecting the detail back to the wider Atomic structure topic.
Read the core explanations from Understanding Nuclear Equations before testing yourself from memory.
When radioactive atoms decay, they don't just disappear - they transform into different atoms. Nuclear equations are the scientific way of showing exactly what happens during this transformation.
Think of a nuclear equation like a recipe. On the left side, you have your starting ingredient (the radioactive nucleus). On the right side, you have what you end up with (the new nucleus plus whatever particle was emitted). The equation must balance - you can't create or destroy protons and neutrons, only rearrange them.
Every nucleus in a nuclear equation is written with two numbers: the mass number (top) shows the total protons plus neutrons, and the atomic number (bottom) shows just the protons. This notation tells us exactly which element we're dealing with and which isotope.
Nuclear decay involves specific particles being emitted, and each has its own symbol. You need to recognise these symbols instantly in exams because they appear in every nuclear equation.
The alpha particle symbol makes sense when you think about it - it's literally a helium nucleus, so it has mass number 4 (2 protons + 2 neutrons) and atomic number 2 (2 protons). The beta particle is trickier - it's an electron, so it has virtually no mass (mass number 0) but carries negative charge (atomic number -1).
**Trap**: Many students think beta particles are positrons or that they come from electron shells. Actually, beta particles are electrons created inside the nucleus when a neutron transforms into a proton plus an electron.
Here's where nuclear equations become powerful - they show you exactly how the nucleus changes during decay. Different types of decay affect mass and charge in predictable ways.
When an alpha particle shoots out, it takes 4 units of mass (2 protons + 2 neutrons) and 2 units of positive charge with it. This means the remaining nucleus becomes lighter and less positively charged. The element actually changes because it now has fewer protons.
Worked example: Alpha Decay Analysis
Radium-226 undergoes alpha decay. Write the nuclear equation and explain the changes.
Answer: $^{226}_{88}Ra \rightarrow ^4_2\alpha + ^{222}_{86}Rn$
Beta decay works differently. When a neutron in the nucleus converts to a proton plus an electron, the electron (beta particle) gets ejected. The mass stays the same because you still have the same total number of particles, but the charge increases by 1 because you now have one more proton.
Worked example: Beta Decay Analysis
Carbon-14 undergoes beta decay. Write the nuclear equation and explain the changes.
Answer: $^{14}_6C \rightarrow ^0_{-1}\beta + ^{14}_7N$
Gamma rays are different from alpha and beta particles. They're electromagnetic radiation - pure energy with no mass and no charge. When a nucleus emits a gamma ray, it's getting rid of excess energy but not changing its composition.
This means gamma emission doesn't change the mass number or atomic number of the nucleus. The element stays the same, but the nucleus moves from an excited state to a lower energy state. Think of it like a light bulb giving off light - the bulb doesn't change, it just releases energy.
In nuclear equations, gamma rays are often written as $\gamma$ or sometimes omitted entirely because they don't affect the mass or charge balance. The key point for exams is knowing that gamma emission leaves the nucleus chemically unchanged.
Nuclear equations follow strict conservation laws - the total mass number and total atomic number must be the same on both sides. This is your key to solving any nuclear equation problem, even when you don't know all the details.
Worked example: Balancing an Unknown Decay
Complete this nuclear equation: $^{238}_{92}U \rightarrow ^4_2\alpha + ^A_ZX$
Answer: $^{238}_{92}U \rightarrow ^4_2\alpha + ^{234}_{90}Th$
The conservation approach works for any type of decay. For beta decay, remember that the beta particle contributes 0 to mass but -1 to charge. For gamma decay, the gamma ray contributes nothing to either mass or charge totals.
Nuclear equation questions are mark-winners if you know the method. Examiners love testing whether you can balance equations and identify the changes that occur during decay.
**Common exam mistakes**: Writing beta particles as $^0_1e$ instead of $^0_{-1}e$ (wrong charge), forgetting to balance both mass and atomic numbers, and confusing which number represents what in nuclear notation.
The key to success is systematic checking. After writing any nuclear equation, verify that the mass numbers add up correctly on both sides, then check the atomic numbers. This two-step verification catches most errors before you submit your answer.
Published by Revise Right. Use the official specification reference to check the requirements for your course. Content updated: 2026-03-20.
Move to the related lessons or back to the topic page when you need the wider sequence before exam-style practice.
In alpha decay the nucleus loses an alpha particle (mass number 4, atomic number 2), so the mass number falls by 4 and the atomic number by 2. Radium-226 (atomic number 88) decays to an alpha particle plus radon-222 (atomic number 86).
In beta decay a neutron converts to a proton plus an electron, so the mass number stays the same and the atomic number increases by 1. Carbon-14 (atomic number 6) decays to a beta particle plus nitrogen-14 (atomic number 7).
A nuclear equation is a symbolic representation of radioactive decay showing the parent nucleus, the decay particle and the daughter nucleus, with mass and atomic numbers balanced. Protons and neutrons cannot be created or destroyed, only rearranged.
To balance a nuclear equation, make the total mass number and the total atomic number the same on both sides. For U-238 (atomic number 92) emitting an alpha particle, the daughter has mass number 238 − 4 = 234 and atomic number 92 − 2 = 90, which is thorium.
Radium-226 undergoes alpha decay to become radon-222. Radium has atomic number 88 and mass number 226; losing an alpha particle gives mass 226 − 4 = 222 and atomic number 88 − 2 = 86, which is radon.
U-238, with atomic number 92, undergoes alpha decay to become thorium-234. Balancing the equation, the mass number is 238 − 4 = 234 and the atomic number is 92 − 2 = 90, which is thorium.
An alpha particle is written as helium with mass number 4 and atomic number 2. A beta particle is written as an electron with mass number 0 and atomic number -1; a common mistake is writing +1 instead of -1.
Gamma emission does not change the mass number or atomic number of the nucleus, so the element stays the same. The nucleus only gets rid of excess energy, moving from an excited state to a lower energy state.
In alpha decay, the mass number falls by 4 and the atomic number falls by 2, because the alpha particle carries away 2 protons and 2 neutrons. The element changes because the nucleus now has fewer protons.
In beta decay, the atomic number increases by 1 while the mass number stays the same. A neutron converts to a proton plus an electron, and the electron is ejected as the beta particle, so carbon-14 becomes nitrogen-14.
Common mistakes in nuclear equations are writing the beta particle with atomic number 1 instead of −1, forgetting to balance both mass and atomic numbers, and confusing which number is which. The mass number goes on top and the atomic number at the bottom.
A nuclear equation is checked in two steps: make sure the mass numbers add up to the same total on both sides, then check that the atomic numbers do too. This two-step check catches most errors.
Beta decay does not change the mass number because the total number of particles in the nucleus stays the same: a neutron becomes a proton. The beta particle is an electron with mass number 0, so it carries away no mass.
The element changes in alpha decay because the nucleus loses 2 protons, so its atomic number falls by 2. Radium-226, element 88, becomes radon, element 86.
A daughter nucleus is the new nucleus left after radioactive decay, shown on the right of a nuclear equation alongside the emitted particle. The starting nucleus on the left is the parent nucleus.