GCSE Combined Science (AQA)

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Half-Life

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Atomic structure Atoms and nuclear radiation

Half-Life

7:05 Half-Life
Spec 4.4.2.3
Previous Random Nature of Radioactive Decay Next Measuring Half-Life
  • Radioactive sources contain millions of nuclei, allowing for accurate predictions of decay proportions over time.
  • Radioactive decay is a random process; while we can estimate decay likelihood, exact timing is unpredictable.
  • In large systems, such as radioactive atoms in a rock, decay proportions over time can be predicted.
  • Example: 50% of nuclei may decay in one hour, transitioning from unstable to stable nuclei.
  • Half-life is the time it takes for half of a radioactive isotope's nuclei to decay.
  • Half-life can also be seen as the time during which an individual nucleus has a 50% chance of decaying.
  • Rate of decay is measured in terms of half-life, not the number of decays, due to decreasing nuclei over time.
  • As more nuclei decay, fewer remain, leading to decreased decay numbers over time.
  • Half-life is independent of the number of remaining nuclei and is determined by the element and isotope.
  • External conditions like temperature do not affect half-life.
  • Different substances have varying half-lives, e.g., Carbon-11 (20 minutes), Uranium-232 (68.9 years), Uranium-235 (700 million years).
  • Short half-lives result in quick decay and high activity, while long half-lives result in slow decay and low activity.
  • A sample is considered stable once 99% of nuclei have decayed, typically after seven half-lives.
  • Understanding half-lives helps describe isotopes' decay behavior qualitatively.

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