Radioactive decay and half-life — Physical education, 14–17 years
Some atomic nuclei are unstable and change spontaneously, releasing radiation. Half-life gives a predictable pattern for a large sample, even though no one can know exactly when one particular nucleus will decay.
The idea
Radioactive decay is a random change inside an unstable atomic nucleus. Half-life is the time needed for half the nuclei in a sample to decay. It does not mean every nucleus waits that long; it describes how the number in the whole sample falls.
Why it matters
Scientists needed to compare radioactive materials and estimate how long they remain active. Half-life solves this problem with a simple repeated pattern: after one half-life, half remains; after two, one quarter remains. It turns an invisible process into something measurable.
A worked example
A sample contains 8000 unstable nuclei and has a half-life of 6 days. After 6 days, 4000 remain. After 12 days, half of those remain: 2000. After 18 days, 1000 remain. Each step divides the previous amount by two.
The common trap
Many people think that after two half-lives everything has disappeared, because “half, then half” sounds like zero. The second halving applies to what remains, not to the original whole: one half becomes one quarter. This is a reasonable slip when the changing reference amount is not stated.
Outside school
Hospitals use radioactive substances in imaging and some treatments, choosing their half-lives carefully. Archaeologists can estimate the age of some remains with radioactive dating. Radiation detectors also help monitor materials, but safe use depends on dose, exposure time and shielding, not half-life alone.
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