### Half-life

Radioactive decay is a spontaneous, “chancy” process. According to the current theories of particle physics, nothing causes a radioactive atom to decay at a particular time. It just happens at random, or by chance. Nevertheless, there is a certain probability that a radioactive atom will decay over any given period of time. For example, an atom of tritium (hydrogen-3) has a 50% chance of decaying during any 12-year period. So, if you have a large collection of tritium atoms, then 12 years from now you’ll have about half as many, because approximately 50% of the tritium will have decayed by then. And in 12 more years, about half of the remaining atoms will decay, so in 24 years you’ll have about a ¼ as many tritium atoms as you have today. And 36 years from now you’ll have ⅛ as many, and so on.

You may be wondering why physicists care about the time it takes for half of the atoms to decay, rather than the time it takes for all of them to decay. The reason is that the time it takes for all of them to decay depends on how many atoms there were to begin with. But the half-life is more or less the same regardless of how many atoms there are.

The average time it takes for half of a large collection of atoms (of any given isotope) to decay is called the half-life of that isotope. The half-life of tritium (hydrogen-3) is 12 years, as explained above. The half-lives of different isotopes vary dramatically. For example, uranium-238 has a half-life of about 4.5 billion years, lithium-8 has a half-life of about 1 second, and hydrogen-4 has a half-life less than a billionth of a trillionth (1/1022) of a second.