OCR GCSE Physics A (J249) — Radioactivity
You can balance a normal chemical equation by counting atoms, but in a nuclear equation, the atoms themselves actually change into entirely new elements. Every time a radioactive atom decays, it acts like a perfectly balanced set of mathematical scales.
When an unstable parent nucleus undergoes radioactive decay, it transforms into a new daughter nucleus and emits radiation. To track these changes, we rely on a fundamental conservation law: the total mass and total charge must remain exactly the same before and after the decay.
In OCR Physics, we track this using two key numbers:
An alpha particle is identical to a helium nucleus, consisting of 2 protons and 2 neutrons. Its symbol in nuclear equations is .
When a nucleus emits an alpha particle, it loses 4 nucleons and 2 protons. Therefore, the nucleon number of the daughter nucleus decreases by 4, and the proton number decreases by 2.
Calculate the nucleon and proton numbers of the daughter nucleus when Plutonium-239 undergoes alpha decay. (Plutonium has a proton number of 94)
Step 1: Set up the nuclear equation.
Step 2: Balance the nucleon numbers (the top row).
Step 3: Balance the proton numbers (the bottom row).
Step 4: State the final balanced equation.
A beta particle is a high-speed electron emitted directly from the nucleus. This happens when a nucleus has too many neutrons to be stable, causing a neutron to suddenly transform into a proton and an electron. The electron is ejected, and its symbol is .
Because a neutron (mass 1) has turned into a proton (mass 1), the overall nucleon number remains completely unchanged. However, because there is now one extra proton in the nucleus, the proton number increases by 1.
Calculate the nucleon and proton numbers of the daughter nucleus when Strontium-90 decays by emitting a beta particle. (Strontium has a proton number of 38)
Step 1: Set up the nuclear equation.
Step 2: Balance the nucleon numbers.
Step 3: Balance the proton numbers.
Step 4: State the final balanced equation.
Unlike alpha and beta decay, a gamma ray is not a particle — it is a high-frequency electromagnetic wave. It is emitted when a nucleus is in an excited state (often after a previous decay) and needs to release excess energy to "cool down".
Because a gamma ray has no mass and no charge, its symbol is . Emitting a gamma ray causes absolutely no change to the nucleon number or the proton number. The element remains exactly the same.
Write the balanced nuclear equation for the gamma decay of an excited Cobalt-60 nucleus. (Cobalt has a proton number of 27)
Step 1: Identify the properties of gamma radiation.
Step 2: Set up and balance the equation.
In exam calculations, you are never expected to memorise the periodic table to figure out the letter symbols for the daughter nucleus. Examiners will either provide a short extract of the periodic table, give you the names in the question text, or simply accept a placeholder letter like or as long as your calculated numbers are correct.
Students frequently subtract 1 from the proton number in beta decay. Remember that the proton number actually INCREASES by 1, because Z = (Z+1) + (-1).
In 'Calculate' questions, always write out your step-by-step subtraction or addition for the nucleon and proton numbers; marks are often awarded for this working even if your final element symbol is incorrect.
Multiple-choice distractor options often show equations where only the mass is balanced or only the charge is balanced — check both the top and bottom rows thoroughly before choosing your answer.
OCR mark schemes strongly prefer the symbol 4/2He for an alpha particle to emphasise that it is a helium nucleus, rather than using an alpha symbol.
Nucleon number
The total number of protons and neutrons in a nucleus, also known as the mass number.
Proton number
The number of protons in a nucleus, which determines the element. Also known as the atomic number.
Parent nucleus
The original unstable nucleus before it undergoes radioactive decay.
Daughter nucleus
The new nucleus formed after a parent nucleus has emitted radiation.
Alpha particle
A particle consisting of two protons and two neutrons emitted from an unstable nucleus, identical to a helium nucleus.
Beta particle
A high-speed electron emitted from the nucleus when a neutron transforms into a proton.
Excited state
An unstable energy state where a nucleus holds excess energy, often released subsequently as a gamma ray.
Put your knowledge into practice — try past paper questions for Physics A
Nucleon number
The total number of protons and neutrons in a nucleus, also known as the mass number.
Proton number
The number of protons in a nucleus, which determines the element. Also known as the atomic number.
Parent nucleus
The original unstable nucleus before it undergoes radioactive decay.
Daughter nucleus
The new nucleus formed after a parent nucleus has emitted radiation.
Alpha particle
A particle consisting of two protons and two neutrons emitted from an unstable nucleus, identical to a helium nucleus.
Beta particle
A high-speed electron emitted from the nucleus when a neutron transforms into a proton.
Excited state
An unstable energy state where a nucleus holds excess energy, often released subsequently as a gamma ray.
