The structure of the atom and isotopes, radioactive decay as a random process, the nature and properties of alpha, beta and gamma radiation, their penetrating and ionising power, and nuclear equations for alpha and beta decay.

A focused answer to the OCR Gateway GCSE Combined Science A topic P4 on radioactivity, covering atomic structure and isotopes, radioactive decay as a random process, the nature and properties of alpha, beta and gamma radiation, their penetrating and ionising power, and nuclear equations.

Generated by Claude Opus 4.89 min answerUpdated 2026-06-02

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What this topic is asking
Atomic structure and unstable nuclei
Alpha, beta and gamma radiation
Nuclear equations

What this topic is asking

OCR wants you to describe atomic structure and isotopes, explain radioactive decay as a random process, describe the nature and properties of alpha, beta and gamma radiation, compare their penetrating and ionising power, and complete nuclear equations.

Atomic structure and unstable nuclei

A radioactive isotope has an unstable nucleus that becomes more stable by emitting radiation, a process called radioactive decay. This decay is a random process: it is impossible to predict which nucleus in a sample will decay next, or exactly when a given nucleus will decay, and the decay of an individual nucleus is not affected by external conditions such as temperature. We can only state the probability that a nucleus will decay in a given time, which leads to the idea of half-life.

Alpha, beta and gamma radiation

The three radiations differ in how strongly they ionise (knock electrons off atoms) and how far they penetrate materials, and the two are inversely related: alpha is highly ionising but easily stopped, while gamma is weakly ionising but very penetrating. This explains both their dangers and their uses: a highly ionising alpha source is very dangerous if it gets inside the body but harmless outside (skin stops it), whereas a penetrating gamma source is hazardous even from a distance.

Nuclear equations

When a nucleus decays, the atomic and mass numbers change in a way that conserves the totals. In alpha decay, the nucleus loses an alpha particle (2 protons and 2 neutrons), so the mass number falls by 4 and the atomic number falls by 2, forming a different element. In beta decay, a neutron in the nucleus changes into a proton and emits a fast electron (the beta particle), so the mass number is unchanged and the atomic number rises by 1, again forming a different element. Gamma emission carries away energy only, so it changes neither the mass number nor the atomic number. In a nuclear equation the total mass numbers and the total atomic numbers must each balance on both sides.

Tracking the changes in alpha then beta decay

A nucleus has an atomic number of $84$ and a mass number of $212$ . It emits an alpha particle, and the product then emits a beta particle. Find the atomic number and mass number after each step.

step 1: Apply alpha decay

Alpha decay removes 2 protons and 2 neutrons, so the atomic number falls by 2 ( $84 - 2 = 82$ ) and the mass number falls by 4 ( $212 - 4 = 208$ ).

step 2: State the result after alpha decay

After the alpha decay the nucleus has atomic number $82$ and mass number $208$ (a different element).

step 3: Apply beta decay

Beta decay turns a neutron into a proton, so the atomic number rises by 1 ( $82 + 1 = 83$ ) and the mass number is unchanged ( $208$ ).

step 4: State the final result

After the beta decay the nucleus has atomic number $83$ and mass number $208$ . Checking that mass numbers and atomic numbers balance at each step is the key to nuclear equation questions.

Exam-style practice questions

Practice questions written in the style of OCR exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.

OCR 20194 marksDescribe the nature of alpha, beta and gamma radiation, and compare their penetrating power and how each can be stopped.

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A Physics Paper 6 structured question on the three radiations. Reward: an alpha particle is a helium nucleus (two protons and two neutrons), is the most ionising, and is the least penetrating, stopped by a sheet of paper (or a few centimetres of air). A beta particle is a fast-moving electron (emitted from the nucleus when a neutron changes to a proton), is moderately ionising and moderately penetrating, stopped by a few millimetres of aluminium. Gamma is a high-energy electromagnetic wave, is the least ionising and the most penetrating, stopped (reduced) by thick lead or concrete. Markers credit the correct identity of each radiation, and the order of penetrating power (alpha least, gamma most) with what stops each.

OCR 20214 marksExplain what is meant by saying that radioactive decay is a random process, and state what happens to the atomic number when a nucleus emits an alpha particle.

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A P4 question on decay. Reward: radioactive decay is random because it is impossible to predict which nucleus in a sample will decay next, or exactly when a particular nucleus will decay; we can only give the probability of decay in a given time. The decay of an individual nucleus is not affected by external conditions. When a nucleus emits an alpha particle it loses two protons (and two neutrons), so its atomic number decreases by two (and its mass number decreases by four), forming a different element. Markers credit the idea that decay is unpredictable for an individual nucleus (only probability is known), and that alpha emission reduces the atomic number by two.

Related dot points

Sources & how we know this

OCR GCSE Gateway Science Suite Combined Science A (J250) specification — OCR (2016)