Different half-lives of radioactive isotopes — AQA GCSE Physics
Test yourself on Different half-lives of radioactive isotopes with AQA GCSE practice questions.
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Different half-lives of radioactive isotopes explained
Half-life values span an extraordinary range, from tiny fractions of a second to billions of years.
Read the full explanation
Uranium-238 has a half-life of about 4.5 billion years, carbon-14 about 5730 years, technetium-99m about 6 hours and some isotopes decay in microseconds. The consequence matters more than the numbers: a short half-life means each nucleus is very likely to decay soon, so for the same number of nuclei the activity is high and the decay curve is steepest at the start. A long half-life means a low activity that persists for a very long time. In a sample holding equal numbers of polonium-209, half-life 125 years, and polonium-210, half-life 138 days, the polonium-209 has the lower activity precisely because its half-life is longer.
Your focus
- Compare the activities of two isotopes present in equal numbers, using their half-lives.
- Convert half-lives given in different units so that two isotopes can be ranked.
- Match decay curves to isotopes by relating initial steepness to half-life.
Different half-lives of radioactive isotopes exam tips
Marking Points
- one mark for the range running from fractions of a second to millions or billions of years
- one mark for a short half-life giving a high activity from the same number of nuclei
- one mark for the isotope with the shortest half-life having the steepest curve at the start
- one mark for a comparison in the correct direction, such as polonium-209 having a lower activity than polonium-210 in the same sample
Examiner Tips
- 💡Convert to a common unit before comparing two half-lives; days against years catches people out.
- 💡State the activity comparison first and then the half-life reason, matching the order the mark scheme expects.
- 💡Remember that comparisons of activity only work directly when the numbers of atoms are equal, which the question will usually tell you.
Common Mistakes
- assuming a long half-life automatically means a more dangerous source
- comparing half-lives given in different units, such as 125 years and 138 days, without converting
- reading a steeper decay curve as a longer half-life
- saying an isotope with more nuclei remaining later must be decaying faster
- stating the comparison of activities without giving the half-life as the reason, when the reason mark depends on it