The amount of time it takes a radioactive isotope to decay into a stable isotope is different for each radioactive isotope, and is characterized by its “half-life”. An isotope’s half-life is the amount of time it takes for half the number of atoms of that isotope to decay to another isotope. Since radioactivity is itself a random process it is impossible to know when any given radioactive atom will decay; however, because we know the probability of decay for each isotope it is possible to say with great certainty when half of any given collection will disappear – the “half-life”. Here is an example:
If you had 1,000 grams (or 1 kg) of strontium-89 in a container and its half-life is 50.5 days, at the end of 50.5 days, only 500 grams of the material would still be strontium-89. The rest would have decayed into other isotopes. After 101 days, there would only be 250 grams of strontium-89 remaining, and after 151.5 days, there would be 125 grams of radioactive strontium-89 and so on.
Some radioactive isotopes have half-lives that are only seconds long while others have half lives of billions of years. For example the half-life of polonium-216 is only 0.145 seconds while the half-life of uranium-238, is 4.5 billion years. Considering the average Canadian lives about 76 years, and the dinosaurs died off about 66 million years ago, a half-life of 4.5 billion years is a very, very long time.
The Earth itself is about 4.5 billion years old. The half-life of uranium-238, the dominant isotope in natural uranium, is also 4.5 billion years. When the earth was young, there was twice as much uranium-238 as exists today. Moreover, there was more than 64 times as much uranium-235 at that time than exists today (the half-life of uranium-235 is 704 million years).