A given radioactive atom has some probability of decaying in the next five minutes, say between 12:00-12:05. If the half-life of an isotope is h, then the probability, p, that a particular atom will still be around after some amount of time, t, is . What this is saying (in mathspeak) is that after every half-life, the probability that an atom will remain is halved. For example, if h=5 minutes, then after t=7 minutes, the probability that an atom won’t have decayed is . For example, uranium 238 (which makes up more than 99% of natural uranium) has a half-life of about 4.5 billion years.