4.2.7Atomic Structure

Random Nature of Radioactive Decay — AQA GCSE Combined Science Trilogy 8464 - Physics Revision Notes & Topical Questions

5 min read 6 parts Lesson 18 of 23

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Part 1 of 4What 'random' means here

Unpredictable individual nuclei

Unpredictable individual nuclei It is impossible to say when one particular unstable nucleus will decay.
It is also impossible to say which nucleus in a sample will decay next.

Equal chance

Equal chance Every unstable nucleus of the same isotope has the same probability of decaying in any given time interval.

Not affected by surroundings

Not affected by surroundings Temperature, pressure and chemical reactions do not change the chance of a nucleus decaying.

Only probabilities

Only probabilities What can be estimated is the probability that a nucleus decays in a given period of time.

Place a Geiger-Muller tube near a source, or simply measure background radiation, and set the counter to zero.

Record the number of counts after a fixed time, for example one minute.

Reset the counter and repeat the measurement several times.

Look at the results: for example 32, 11, 25, 16 and 28 show no steady trend or pattern.

Conclude that each count comes from a decay, and since the counts fluctuate unpredictably, decay must be a random process.

Rolling a dice for a six

Cannot predict one throw

Cannot predict one throw On any single roll you cannot know whether a six will appear, just as you cannot tell when one nucleus will decay.

Known probability

Known probability Each roll has a 1 in 6 chance of being a six, like a fixed probability of decay per unit time.

Large numbers behave predictably

Large numbers behave predictably Over a thousand rolls you can predict roughly how many sixes will occur, just as a large sample of nuclei decays at a predictable overall rate.

Situation	Can it be predicted?	Explanation
When one named nucleus decays	No	It is a matter of chance
Which nucleus decays next	No	All have the same probability
Roughly how many decay in a time	Yes, approximately	Large numbers average out the randomness
Effect of heating the sample	No change in decay rate	Decay depends on the nucleus, not the conditions

Term	Meaning
Random	Happening by chance, so that no individual event can be predicted.
Probability	A measure of how likely an event is to occur in a given time or trial.
Geiger-Muller tube	A detector that produces a count each time radiation enters it.
Background radiation	Low-level radiation that is always present from natural and man-made sources.
Count rate	The number of detections per unit time recorded by a detector.

Saying a nucleus decays after a set time - it can decay at any moment, and only the probability can be given.

Believing that heating or chemically reacting a radioactive substance speeds up its decay; the rate is unaffected.

Describing varying counts as a faulty instrument rather than as evidence of random decay.

Thinking that the nucleus that has waited longest is 'due' to decay; each nucleus has the same chance at all times.

Last updated: 20 October 2026