Key Stage 4 science · Physics – Atomic structure

GCSE radioactivity, half-life, fission and fusion: a guide

Some nuclei are unstable and change by giving out radiation, a random process that cannot be sped up or slowed down. At GCSE, students learn the different types of radiation and how each changes the nucleus, what half-life means and how to use it, the difference between irradiation and contamination, and how nuclear fission and fusion release energy, including in the Sun. It sounds alarming, but the topic is mostly careful reasoning and a few reliable rules.

Radioactivity, half-life, fission and fusion, explained

An unstable nucleus emits radiation to become more stable. An alpha particle is two protons and two neutrons, the same as a helium nucleus, so emitting one lowers the mass number by 4 and the atomic number by 2. In beta emission a neutron in the nucleus turns into a proton and a fast electron, which is thrown out; the mass number stays the same and the atomic number goes up by 1. Gamma rays are electromagnetic waves that carry away surplus energy and change neither the mass nor the charge of the nucleus. Some nuclei emit a neutron, which lowers the mass number by 1. Each type is shown in a nuclear equation in which the mass numbers and atomic numbers on each side must add up.

The types differ in how far they travel and how strongly they ionise. Alpha particles are strongly ionising but are stopped by a sheet of paper or a few centimetres of air. Beta particles penetrate further, and are stopped by a few millimetres of aluminium. Gamma rays are weakly ionising but very penetrating, and a thick block of lead or concrete is needed to reduce them. Because the process is random, nobody can say when one particular nucleus will decay. What can be predicted is the pattern for a large number of them, which is described by half-life: the time for the number of undecayed nuclei, or the activity (decays per second), to fall to half.

Radiation is hazardous because it ionises atoms and can damage cells and DNA. Irradiation is exposure to radiation from outside the body, which stops when the source is moved away or shielded. Contamination is having radioactive material on or inside the body, which keeps irradiating until it is removed or decays. This is why alpha emitters are most dangerous inside the body, where their strong ionisation acts directly on cells, while outside the body beta and gamma are the greater hazard, because they can reach the body’s cells from a distance and alpha cannot get through the skin. A material’s half-life tells us how long its waste stays hazardous, so nuclear waste is sealed and stored securely for very long periods.

Nuclear fission is when a large nucleus, such as uranium-235, absorbs a neutron and splits into two smaller nuclei and two or three neutrons, releasing energy. The neutrons can trigger more fissions, which makes a chain reaction. A power station controls this with control rods that absorb neutrons. Nuclear fusion is when small nuclei, such as hydrogen isotopes, are forced together to make a larger nucleus, such as helium, releasing even more energy per kilogram of fuel. It needs millions of degrees and extremely high pressure to overcome the repulsion between the positive nuclei. This is what happens in the core of the Sun and other stars, and why the Sun can shine steadily for billions of years.

Words your child will hear

Activity: How many nuclei in a sample decay each second, measured in becquerels (Bq).
Half-life: The time it takes for the activity, or the number of undecayed nuclei, to halve.
Irradiation: Being exposed to radiation from a source outside the body.
Contamination: Having radioactive material on or inside the body or an object.
Nuclear fission: A large nucleus splitting into two smaller ones, releasing energy and neutrons.
Nuclear fusion: Two light nuclei joining to make a heavier one, releasing energy.
Chain reaction: Neutrons from one fission causing further fissions, and so on.

What the National Curriculum says

The programme of study for Key Stage 4 says:

Students should be taught about radioactive nuclei: emission of alpha or beta particles, neutrons, or gamma-rays, related to changes in the nuclear mass and/or charge
Students should be taught about radioactive materials, half-life, irradiation, contamination and their associated hazardous effects, waste disposal
Students should be taught about nuclear fission, nuclear fusion and our Sun's energy

Objectives from the National Curriculum in England, © Crown copyright, used under the Open Government Licence v3.0.

A worked example

(a) Radium-226 (atomic number 88) decays by emitting an alpha particle. Write the nuclear equation, given that radon has atomic number 86. (b) A different, imaginary radioactive source has a half-life of 6 hours and an activity of 800 counts per second. What is its activity after 18 hours?

An alpha particle is a helium nucleus, mass number 4 and atomic number 2. Subtract from the radium to find the new nucleus.
```
mass:   226 − 4 = 222
atomic:  88 − 2 = 86
```
An atomic number of 86 is radon, so the equation is balanced on both sides. The top numbers add to 226 and the bottom ones to 88.
```
Ra-226 → Rn-222 + He-4
(88)      (86)      (2)
```
Now half-life. Count how many half-lives pass in 18 hours.
```
18 ÷ 6 = 3 half-lives
```
Halve the activity once for each half-life.
```
800 → 400 → 200 → 100
```

(a) Radium-226 → radon-222 + alpha particle. (b) After 3 half-lives the activity is 100 counts per second.

Common mistakes and how to help

After two half-lives, everything has decayed

“Half” is read as “half of the time” and two halves make a whole.

What helps: Each half-life halves what is left, so the sample goes 100%, 50%, 25%, 12.5%. It never reaches zero in a neat step. Sketching the curve or drawing the halving on paper makes it stick.

Being irradiated makes you radioactive

The words irradiated and radioactive sound the same, and films often show people glowing after exposure.

What helps: Irradiation is just exposure to radiation, like standing near a source, and it does not make the person radioactive. Contamination is different: radioactive material gets on or into the body or an object and keeps emitting radiation until it is removed or decays. Practise explaining the difference in one sentence each.

The beta particle is an electron from the outside of the atom

Beta particles are electrons, and students know electrons orbit the nucleus.

What helps: In beta decay, the electron is made in the nucleus when a neutron turns into a proton. That is why the atomic number goes up by one.

Fission and fusion are the same thing, or fusion is what power stations do now

Both release nuclear energy and the names differ by one letter. The Sun’s fusion is often confused with nuclear power.

What helps: Fission splits big nuclei and is used in power stations today. Fusion joins small nuclei and happens in stars; scientists are researching fusion power, but it does not yet supply electricity to homes. Say them as “fission splits, fusion fuses”.

How to help at home

Simulate half-life with coins. Start with 64 coins or small sweets. Throw them all, remove the ones landing heads, and count what is left. Repeat until there are none or very few. Plot the number against throws and the curve is a half-life graph, with a half-life of one throw.
Practise nuclear equations. Write a few in advance, such as alpha emission from uranium-238 or beta emission from carbon-14, and check that the top and bottom numbers balance. Use the sequence “mass, then atomic number” each time.
Compare the energy sources. Ask your teenager to explain how a fission power station differs from the Sun. Have them use the words fission, fusion, chain reaction and control rods, and add one advantage and one drawback of nuclear power, including waste.
Talk about everyday radiation sensibly. Radiation is natural: rocks, the air and space all contribute. Talk through why we shield, keep distance and limit time near sources, and why doctors use gamma sources and X-rays carefully. Do not buy or handle any radioactive source at home.