Radioactive decay is an exponential function. Different radioactive
     elements decay at differing rates. We call it "half-life". Say we have
     100 kilograms of element 109, Unnilennium (Une for short). It has a
     half life of 5 milliseconds (ms for short). So, after 5 ms we have 50
     kilograms of Une, after 5 more ms, 25 kg, 5 ms later, 12.5 kg, etc.
     This is the concept; half of a half of a half etc.... The Une will
     become simpler elements. (The particles don't just disappear, they
     rearrange into more stable configurations).

     The general form of an exponential function is:

          x
     Y = 2

     in other words Y equals 2 to the Xth power. I had to rearrange the
     equation a bit to produce DECAY.EXE, but the principle is the same. The
     yellow line will never touch the horizontal white line. This makes that
     white line something called an asymptote. (Of course we're limited by
     the low resolutions available to our computers; the picture isn't
     perfect). Now, some radioactive elements have half-lives measured in
     millions of years, others in seconds, or hours, or minutes, you name
     it. But theoretically, the number can never reach zero.