Understanding Nuclear physics: OxfordAQA IGCSE Physics Explained

Nuclear Physics for OxfordAQA IGCSE Physics: A Friendly Walkthrough

If nuclear physics feels like the topic where you either "get it" straight away or find yourself staring at the page, you're not alone, and I promise it's more forgiving than it looks. oxfordaqa igcse physics nuclear physics covers the structure of the atom, how unstable nuclei decay, and two very different ways of releasing nuclear energy: fission and fusion. Once you see how those pieces connect, the whole topic becomes a lot less intimidating.

These igcse revision notes take you through nuclear physics oxfordaqa igcse step by step, with the kind of worked examples and common mistakes I'd talk through with a student who's just starting to feel confident with this material. Take it slowly the first time through and you'll find the calculations are genuinely manageable.

Atomic Structure: Building the Model

Start with the picture in your head, because almost every question in this section rewards you for having it clearly fixed. An atom is tiny, with a radius of around 10^-10 metres. At its centre sits a small, positively charged nucleus made of protons and neutrons, with electrons arranged around it at various distances. The nucleus is far smaller than the atom as a whole, and it's where almost all of the atom's mass is concentrated.

You should know how we found this out: the scattering of alpha particles by thin metal foil gave scientists evidence for how mass is distributed inside the atom. Most alpha particles passed straight through, a few deflected at large angles, and that pattern told physicists the atom is mostly empty space with a dense, positively charged centre.

In a neutral atom, the number of electrons equals the number of protons, so the atom carries no overall charge. If an atom loses or gains electrons, it becomes a charged particle called an ion. Every atom of a given element has the same number of protons, called the atomic number (or proton number). Atoms of the same element can have different numbers of neutrons; these are called isotopes. The total number of protons and neutrons together is the mass number, and you'll see atoms written with both numbers, like the sodium example in your textbook, with the mass number above and the atomic number below the element symbol.

Worked example. An atom of chlorine has 17 protons and 18 neutrons. Write down its mass number and atomic number, and state how many electrons it has.
Atomic number = number of protons = 17.
Mass number = protons + neutrons = 17 + 18 = 35.
Since the atom is neutral, the number of electrons equals the number of protons, so it has 17 electrons.

Isotopes of the same element behave almost identically in chemical reactions, because chemistry is governed by the arrangement of electrons, not by the number of neutrons in the nucleus. Where isotopes differ is in nuclear stability: some isotopes of an element are perfectly stable, while others of the same element are radioactive.

Ionising Radiation from the Nucleus

Some nuclei are unstable, and they become more stable by emitting particles or radiation, a process called radioactive decay. This is genuinely random: you cannot predict when any individual unstable atom will decay, even though, with a large enough sample, the overall pattern is very predictable.

Background radiation is around us all the time, so it's not something exotic that only appears near a nuclear reactor. It comes from a mix of sources: radioactive substances naturally present in rocks and soil, radiation arriving from space, and human-made sources such as X-ray machines in hospitals. When an experiment measures the activity of a radioactive source, the background count should always be subtracted first, otherwise the result overstates how radioactive the source actually is.

There are three main types of radiation to know, each with its own identity and behaviour:

• Alpha particles: two protons and two neutrons bound together, essentially a helium nucleus. Strongly ionising, but stopped by a sheet of paper or a few centimetres of air.
• Beta particles: high-speed electrons ejected from the nucleus when a neutron turns into a proton. Moderately ionising, stopped by a few millimetres of aluminium.
• Gamma radiation: electromagnetic radiation emitted from the nucleus. Weakly ionising, but highly penetrating, needing thick lead or a long distance to reduce it substantially.

You'll be asked to balance nuclear equations for single alpha and beta decay, completing the mass number and atomic number so both sides balance. You are not expected to name the resulting daughter element from memory, just to work out its numbers correctly.

Worked example. A nucleus of radon-222 (atomic number 86) undergoes alpha decay. Find the mass number and atomic number of the resulting nucleus.
An alpha particle carries away a mass number of 4 and an atomic number of 2.
New mass number = 222 - 4 = 218.
New atomic number = 86 - 2 = 84.
The resulting nucleus has a mass number of 218 and an atomic number of 84.

It's worth being precise about two terms that examiners often test together: contamination and irradiation. Contamination is the unwanted presence of radioactive atoms on another material; the hazard continues for as long as those atoms remain and keep decaying. Irradiation is the process of exposing an object to a radiation source; once removed from the source, an irradiated object does not itself become radioactive. Mixing these two up is one of the most common ways to lose marks in this part of the topic.

Half-life ties this all together. The half-life of a radioactive isotope is the average time it takes for the number of nuclei of that isotope in a sample to halve, or equivalently, the time it takes for the count rate from a sample to fall to half its initial level. Isotopes with short half-lives decay quickly and release a lot of radiation in a short time; isotopes with long half-lives release radiation slowly but for a very long time. Both patterns carry their own hazards, and you should be ready to weigh those hazards against each other in an extended answer.

Worked example. A sample of a radioactive isotope has an initial count rate of 800 counts per minute. Its half-life is 6 hours. What is the count rate after 18 hours?
18 hours is 3 half-lives (18 ÷ 6 = 3).
800 → 400 (after 1 half-life) → 200 (after 2) → 100 (after 3).
After 18 hours, the count rate is 100 counts per minute.

Notice that the count rate never mathematically reaches zero; it just keeps halving. That's worth stating explicitly if a question asks you to comment on a graph that seems to flatten out near the axis rather than touch it.

Nuclear Fission

Nuclear fission is the splitting of a large, unstable nucleus, releasing energy as it happens. The two fissionable substances you need to know are uranium-235 and plutonium-239, and you should be aware that most nuclear reactors in use are based on uranium-235.

For fission to occur, a uranium-235 or plutonium-239 nucleus must first absorb a neutron, which makes the nucleus unstable enough to split. It splits into two smaller nuclei, releasing two or three neutrons plus energy, and the amount of energy released is far greater than you'd get from a chemical reaction involving a similar mass of material.

Those released neutrons can go on to trigger fission in further nuclei, and if enough of them do, you get a chain reaction. Inside a nuclear reactor, control rods absorb some of these neutrons so that, on average, only one neutron from each fission event goes on to cause another fission. That balance is what keeps the reactor generating a steady, controlled output of energy rather than running away.

Common exam task. You may be asked to sketch or complete a labelled diagram showing how a chain reaction develops from a single fission event. Practise drawing one uranium-235 nucleus absorbing a neutron, splitting into two smaller nuclei plus neutrons, and those neutrons each triggering further fission events in a branching pattern.

Nuclear reactions also produce waste, and that waste can stay dangerous for a very long time depending on its half-life and the products formed. Careful disposal is a genuine factor in decisions about whether to use nuclear power to generate electricity, and it's a completely fair thing for an extended-response question to ask you to evaluate.

It's worth remembering that radioactive sources aren't only a hazard to be managed; they also have genuine uses that examiners like to bring into application questions. Medical tracers use short half-life isotopes so the radiation clears the body quickly after a scan. Smoke detectors use a weak alpha source, because alpha radiation is easily absorbed by smoke particles, which changes the current in a small ionisation chamber and triggers the alarm. Sterilising medical equipment relies on gamma radiation's ability to penetrate packaging and kill bacteria without heating the contents. Each of these applications is chosen for a specific property of that type of radiation, its range, its ionising power or its penetration, so link your answer back to the correct property rather than giving a generic description.

Nuclear Fusion

Where fission splits a large nucleus apart, nuclear fusion is the joining of two light nuclei to form a heavier one. In this process, some of the mass of the original smaller nuclei is converted into energy. Because the two nuclei are both positively charged, they repel each other strongly, and overcoming that repulsion to get them close enough to fuse only happens at extremely high temperatures and pressures. That's precisely why fusion is the process by which energy is released in stars, where gravity produces exactly those extreme conditions.

It helps to hold fission and fusion side by side rather than learning them in isolation, because exam questions frequently ask you to compare them directly.

Common Mistakes in Nuclear Physics

• Confusing mass number and atomic number when balancing decay equations, especially forgetting that an alpha particle removes both 4 from the mass number and 2 from the atomic number.
• Mixing up contamination (unwanted radioactive atoms present on a material) with irradiation (exposure to a source, which does not make the object radioactive).
• Describing half-life as "the time for all the radioactivity to disappear" rather than the time for the count rate, or number of nuclei, to fall to half.
• Writing that fission and fusion are "the same but opposite" without explaining the actual mechanism: fission splits one large nucleus after neutron absorption; fusion joins two small nuclei under extreme temperature and pressure.
• Forgetting that a chain reaction in a reactor needs control rods to keep the neutron count balanced, not to stop fission entirely.

Self-Check Questions

1. Describe the evidence that alpha-particle scattering provided for the structure of the atom.
2. State the relative mass and relative charge of a proton, a neutron and an electron.
3. A nucleus of bismuth-214 (atomic number 83) undergoes beta decay. Find the mass number and atomic number of the resulting nucleus.
4. Explain the difference between contamination and irradiation.
5. Explain, in terms of neutrons, how a chain reaction is controlled inside a nuclear reactor.
6. Explain why nuclear fusion requires extremely high temperatures and pressures to occur.
7. A radioactive sample has a count rate of 640 counts per minute. Its half-life is 4 hours. Calculate the count rate after 12 hours.
8. Give two sources of background radiation.
9. Compare fission and fusion in terms of what triggers each process and where each one naturally occurs.

Work through those questions from memory before checking back against the sections above; that's the fastest way to find out which idea still needs another pass. Keep these oxfordaqa igcse physics notes nearby while you work through oxfordaqa igcse physics practice questions on decay equations and chain reactions, because seeing the same idea tested in a slightly different way is what makes it stick. With a bit of repetition, nuclear physics stops being the topic you dread and becomes one you can walk into the exam hall confident about. This is oxfordaqa igcse physics explained the way igcse 9203 nuclear physics deserves: clearly, patiently, and with every calculation shown in full, so these oxfordaqa igcse physics revision notes genuinely help rather than just repeating the specification.