Half-life
Some atoms are unstable. In other words, left to themselves they decay, and become a different type of atom.
For instance:
- Fluorine-18 decays into oxygen-18
- Sodium-22 decays into neon-22
If you just have a single atom of fluorine-18, you can be certain that it will eventually decay into oxygen-18, but you can’t predict exactly when.
If you have a large number of fluorine-18 atoms, you still can’t predict when each one will decay, but you can predict that after 110 minutes, half of them will have decayed.
Twice the half-life isn’t the whole life
This doesn’t mean that after 220 minutes all your fluorine-18 atoms will be gone.
At the end of the second 110 minutes, only half of the remaining fluorine-18 atoms will have decayed into oxygen-18. You’ll be left with a quarter of the number of fluorine-18 atoms you started with. After another 110 minutes, you’ll have an eighth of your original quantity, and so on.
You still can’t predict which atom will decay when, but you can be certain that however many you have now, you’ll have half that number 110 minutes later.
We say that 110 minutes is the half-life of fluorine-18.
Heads you decay, tails you don’t
Eventually, of course, you’ll be left with a single atom of fluorine-18, and you can’t have half an atom.
You can’t ever predict when a single atom will decay, but the half life still applies. You don’t know when any individual atom will decay, but you do know there’s a fifty-fifty chance that it will decay during its half-life. If it doesn’t, there’s a fifty-fifty chance that it will decay during the next half-life.
For each individual atom it’s like tossing a coin once every half-life.
How long is a half-life?
Each type of unstable atom has its own half life, and there’s a huge range.
Uranium-227 has one of the shortest half-lives, at just a fraction of a second.
Uranium-238 has one of the longest half lives, which can be measured in billions of years.
