Nuclear decay and half-life — Chemistry, 14–17
Unstable atomic nuclei can change into more stable ones and release radiation. Half-life describes how quickly a sample changes, not how long one particular atom will last.
Unstable nuclei change
Some nuclei have an arrangement of protons and neutrons that is not stable. Without being pushed by a chemical reaction, such a nucleus can change by itself and emit an alpha particle, a beta particle or gamma radiation. The new nucleus may be more stable, but it can also be another radioactive nucleus. Tu cannot predict the exact moment for one atom.
Why half-life was needed
Radioactive samples do not change at one steady rate: the number of undecayed nuclei keeps getting smaller. Scientists needed a measure that worked for any starting amount. The half-life is the time needed for half the original nuclei in a sample to decay, so it lets tu compare isotopes and plan safe storage or medical doses.
A sample after three half-lives
A sample starts with 80 mg of a radioactive isotope whose half-life is 6 days. After 6 days: 80 ÷ 2 = 40 mg. After 12 days: 40 ÷ 2 = 20 mg. After 18 days: 20 ÷ 2 = 10 mg. So after three half-lives, 10 mg remains, while 70 mg has decayed. Tu are halving the remainder each time, not subtracting 40 mg repeatedly.
Half-life is not an expiry date
It is reasonable to hear “half-life” and imagine that every atom lasts exactly that long before disappearing. Decay is random for individual nuclei: half-life describes the pattern of a huge collection. After one half-life, about half the sample has decayed, but you cannot identify which particular atoms did it. Tu should use probability for the sample, not a timetable for one atom.
Medicine, dating and detectors
Radioactive decay is useful when it is measured and controlled. Doctors use selected isotopes to image organs or target diseased tissue; archaeologists estimate the age of once-living material from carbon-14; smoke detectors use a tiny source to notice particles in the air. Tu still need shielding, distance and careful procedures because radiation can damage cells.
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