Radioactive half-life
The time for half of a radioactive sample to decay. It does not depend on how much you start with.
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Analogy
Like popcorn popping at random: half are gone after the same time each round.
Example
100 -> 50 -> 25 -> 12.5 gFraction = (1/2)^3
In practice
After 3 half-lives, 100 g of a sample leaves 12.5 g.
Practice
Flashcards
- How much remains after 2 half-lives?One quarter.
Questions people ask
Can we predict when one atom will decay?
No. Decay of a single nucleus is random. We can only give the chance per second. For a large sample, the law of large numbers makes the half-life very exact. It is like a coin: unpredictable once, but half heads over a million tosses.
How does carbon dating work?
Living things take in carbon-14, which decays with a half-life of about 5730 years. After death, no new carbon-14 is added. Measuring what fraction of carbon-14 is left tells you how many half-lives have passed. If a quarter is left, the sample is about 11,500 years old.