Pearson Edexcel · GCSE · Physics
Course version: 1PH0 · Version from Sept 2016
Explain that the half-life of a radioactive isotope is the time taken for…
Full specification wording
Topic 6 – Radioactivity: 6.25 Explain that the half-life of a radioactive isotope is the time taken for half the undecayed nuclei to decay or the activity of a source to decay by half
Half-life is a fixed time interval for a given isotope.
Checked against the Pearson Edexcel 1PH0 specification (opens in a new tab) on .
- Point 24 of 42
Study notes
From the checked teaching notes for this specification point.
The idea
Half-life is a fixed time interval for a given isotope. It can be defined in two equivalent ways. First, it is the time taken for half of the undecayed nuclei present in a sample to decay. Second, it is the time taken for the activity of a source, measured in becquerels, to fall to half its starting value. Both descriptions track the same process: as undecayed nuclei are used up, the count rate and activity fall together. For example, a source with an activity of 800 Bq and a half-life of 5 days falls to 400 Bq after 5 days, then to 200 Bq after 10 days. Half-life does not mean half the sample has vanished in mass, and it is not the time for all activity to stop.
What earns the marks
- States that half-life is the time for half the undecayed nuclei to decay.
- States that half-life is the time for the activity of a source to halve.
- Recognises that the two definitions describe the same process for a given isotope.
- Applies successive halvings to find activity after a whole number of half-lives.
- Distinguishes half-life from the time for all activity to cease.
Common mistakes and how to put them right
Confuses half-life with the time for the whole sample to decay.
Put it right by stating that half-life halves the undecayed nuclei, not removes them all.
Treats half-life as half the mass disappearing.
Put it right by linking the definition to undecayed nuclei and to activity.
Halves the activity only once regardless of elapsed time.
Put it right by halving repeatedly for each half-life that passes.
Assumes half-life changes as the source ages.
Put it right by noting half-life is constant for a given isotope.
Exam tips
- Read the question to see whether it asks about undecayed nuclei or about activity, then use the matching definition.
- Count how many half-lives fit into the stated time before doing any halving.
- Check your answer by confirming the activity falls, never rises, with each half-life.
Try the lesson
The start of the Topic 6 – Radioactivity: 6.25 Explain that the half-life of a radioactive isotope is the time taken for half the undecayed nuclei to decay or the activity of a source to decay by half lesson, made from checked teaching notes for this specification point.
Have a go first
A radioactive isotope has a half-life of 8 days. What fraction of the original undecayed nuclei remains after 16 days?
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Test your understanding of Topic 6 – Radioactivity: 6.25 Explain that the half-life of a radioactive isotope is the time taken for half the undecayed nuclei to decay or the activity of a source to decay by half
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