The half-life of a radioactive substance is the amount of time it takes for half of the atoms in a sample to decay into more stable forms.
In other words, if you start with 100 radioactive atoms, after one half-life, only 50 will remain undecayed; after two half-lives, 25 will remain, and so on.
Key points:
- It’s a statistical measure — we can’t predict exactly when a single atom will decay, but we can predict how long it takes for half a large group of atoms to decay.
- Different isotopes have different half-lives:
- Uranium-238 → ~4.5 billion years
- Carbon-14 → ~5,730 years (used in radiocarbon dating)
- Iodine-131 → ~8 days (used in medicine)
- The decay rate is exponential, not linear.
Example: If a sample has a half-life of 10 years and starts with 80 grams:
- After 10 years → 40 g remain
- After 20 years → 20 g remain
- After 30 years → 10 g remain