Here's an example of how half-life works: Let's say you have 80 ants, and these
ants have a half-life of ten days. In ten days, then, 40 of your ants would
still be alive. Ten days after that, 20 ants would still be alive. Ten more
days, and ten would be alive. You get the picture.
If you took just one of these ants, though, there would be no way to predict
when it would die. It might die right away or it might live for a long time.
It's only when you have a lot of them that this half-life thing works.
In real life, you can't use the half-life method to determine ant death rates.
You can, however, use the method to determine when radioactive atoms will decay
into some other form.
Half-lives vary greatly from one radioactive atom to another. The reason is
that a highly unstable atom wants to change quickly, whereas a slightly
unstable atom is only slightly uncomfortable with its condition. The half-life
of highly unstable radon-222 is less than four days. The half-life of the
slightly unstable uranium-238 is 4.5 billion years. Carbon-14 has a half-life
of 5,730 years.