# Radioactive half-life

The time for half of a radioactive sample to decay. It does not depend on how much you start with.

Physics (university) · Part 10: Relativity and quantum physics · Card 11 of 11 · Last updated 2026-10-19

## Analogy

Like popcorn popping at random: half are gone after the same time each round.

## Example

**Three half-lives**

100 -> 50 -> 25 -> 12.5 g
Fraction = (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.

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