Edexcel GCSE Physics Topic 6 Radioactivity: a complete overview of the nuclear atom, isotopes, types of radiation, decay equations, half-life, background radiation and fission and fusion

What Topic 6 actually demands

Radioactivity is the largest Paper 1 topic, combining atomic structure, nuclear equations, half-life calculations and the uses and dangers of radiation. It rewards precise recall (relative masses and charges, the radiation types), accurate calculation (balancing equations and halving for half-life), and clear explanation (irradiation versus contamination, fission versus fusion).

This guide walks through all eight dot points of the topic, then sets out the exam patterns Pearson repeats. Each dot point has a matching page with practice questions; this overview ties them together.

The nuclear model of the atom

An atom has a tiny, positive nucleus of protons and neutrons surrounded by electrons in energy levels. The nucleus holds almost all the mass but is about $10\,000$ times smaller than the atom, so the atom is mostly empty space. Relative charges: proton $+1$, neutron $0$, electron $-1$; relative masses: proton $1$, neutron $1$, electron negligible.

Isotopes and ions

The atomic number is the proton count; the mass number is protons plus neutrons. Isotopes are atoms of the same element (same protons) with different neutrons. A neutral atom has equal protons and electrons; it becomes a positive ion by losing electrons and a negative ion by gaining them.

Types of nuclear radiation

An alpha particle is a helium nucleus, a beta-minus particle is an electron from the nucleus, and a gamma ray is electromagnetic radiation; beta-plus is a positron and neutron radiation is a free neutron. Penetration increases alpha to gamma (paper, aluminium, lead); ionisation is the reverse, with alpha the most ionising.

Nuclear decay equations

In alpha decay the mass number falls by 4 and the atomic number by 2. In beta-minus decay a neutron becomes a proton, so the mass number is unchanged and the atomic number rises by 1. In beta-plus decay the atomic number falls by 1. Gamma changes neither. Equations balance when the top and bottom numbers each balance across the arrow.

Half-life

The half-life is the time for half the undecayed nuclei (or the activity) to halve. Decay is random. Activity is measured in becquerels. To find what remains, find the number of half-lives and halve the amount that many times; after $n$ half-lives a fraction $\left(\frac{1}{2}\right)^{n}$ is left.

Background radiation and detection

Background radiation is ever-present low-level ionising radiation from the Earth (radon, rocks), space (cosmic rays) and our bodies, plus smaller artificial sources. It is detected with photographic film (fogging) and a Geiger-Muller tube (counting particles). The background count must be subtracted to find a source's true count rate.

Uses, dangers, fission and fusion

Radiation is used in medical tracers (gamma, short half-life), treating cancer, sterilising and industrial thickness gauges (beta). Irradiation is external exposure; contamination is radioactive material on or in the body. Fission splits a large nucleus in a chain reaction (reactors); fusion joins small nuclei in stars and needs extreme temperature and pressure.

How Topic 6 is examined

A typical Edexcel profile for Radioactivity:

• Recall. Atomic structure, the radiation types, and their penetration and ionisation.
• Calculations. Balancing alpha and beta decay equations and half-life problems.
• Explanations. Irradiation versus contamination, the random nature of decay, and fission versus fusion.
• Applications. Choosing a radiation type for a medical or industrial use with the reason.

Check your knowledge

A mix of recall, calculation and explanation questions covering Topic 6. Attempt them under timed conditions, then check against the solutions.

1. State the relative charge of a proton, a neutron and an electron. (2 marks)
2. Define what is meant by isotopes. (2 marks)
3. State what an alpha particle is and what stops it. (2 marks)
4. In alpha decay, state how the mass number and atomic number change. (2 marks)
5. A sample has an activity of $400\,\text{Bq}$ and a half-life of 2 days. Calculate its activity after 6 days. (2 marks)
6. Name one source of background radiation from the Earth. (1 mark)
7. State the difference between irradiation and contamination. (2 marks)
8. State the difference between nuclear fission and fusion. (2 marks)