The half-life of a radioactive isotope represents the average
time it would take half of a collection of this type of nucleus to
decay. For example, you start with a sample of 1000 Oxygen-15
(15O) nuclei, which has a half-life of 122 seconds.
After 122 seconds, half of the 15O nuclei will have
decayed into Nitrogen-15 (15N) nuclei. After another
122s, half of the remaining Oxygen nuclei will have also decayed,
and so on.
Suppose you start with 3.71×10315O nuclei...