2.5.2Forces, Space and Radioactivity

Radioactive Emissions — WJEC GCSE Science (Double Award) - Physics Revision Notes & Topical Questions

5 min read 6 parts Lesson 28 of 37

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Part 1 of 4Unstable nuclei

Why a nucleus is unstable

Why a nucleus is unstable A nucleus can be unstable if it has too many or too few neutrons compared with the number of protons.
Carbon-12 is stable, but carbon-14 has two extra neutrons and is unstable and radioactive.

Radioactive decay

Radioactive decay An unstable nucleus becomes more stable by emitting radiation, which can be a particle or an electromagnetic wave.
The radiation carries energy away from the nucleus. This process is called radioactive decay.

Radioactive decay

Spontaneous

Spontaneous Decay is not triggered by anything such as heat, pressure or chemical reactions.
- The nucleus simply decays when it does.

Random

Random It is impossible to predict when a particular nucleus will decay.
- Each nucleus has the same chance of decaying in any given time.

Predictable for large numbers

Predictable for large numbers Although one nucleus cannot be predicted, the pattern for a very large number of nuclei is predictable,
- which leads to the idea of half-life.

Radiation enters the Geiger-Müller tube and ionises the gas inside it.

Each time radiation is absorbed, the tube sends an electrical pulse to a counter.

The counter makes a click or displays a count rate, normally in counts per second or counts per minute.

A higher count rate means more radiation is reaching the tube, so the count rate falls as the tube is moved away from the source.

Action	Why it helps
Repeat readings and calculate an average	Random highs and lows tend to cancel out
Measure over a long time	Larger counts mean random fluctuations are a smaller percentage of the total
Subtract the background count rate	Removes counts from sources other than the one being studied
Keep the distance from source to tube fixed	Count rate depends on distance, so changing it adds a variable

Term	Meaning
Unstable nucleus	A nucleus that will emit radiation to become more stable, often because of an imbalance of neutrons and protons.
Radioactive decay	The process in which an unstable nucleus emits radiation.
Random	Cannot be predicted: it is impossible to know when a given nucleus will decay.
Spontaneous	Happens without any outside cause or trigger.
Count rate	The number of radiation counts detected per second or per minute.
Geiger-Müller tube	A radiation detector that produces an electrical pulse each time radiation is absorbed.

Saying scientists can predict when an individual nucleus will decay. They can only predict the behaviour of large numbers of nuclei.

Using 'radiation' and 'radioactive' as if they mean the same thing. A source is radioactive; the particles or waves it emits are radiation.

Forgetting to subtract background radiation from the measured count rate.

Stating that heating a sample makes it decay faster. Radioactive decay is not affected by temperature or chemical conditions.

Last updated: 20 October 2026