When will a particular radioactive nucleus decay? It’s impossible to know. All that can be known is the average time between formation and decay. And we know that the number of decays will be proportional to the number of nuclei present. Now, suppose we start with a large number of nuclei. In a given time interval, say one second, a certain number will decay. At the end of that second the number of nuclei that haven’t decayed will be smaller, so in the next second there will be fewer decays. At some time there will be only half as many decays in a second as there were originally. That time is called the half life. One half life later there will be only ¼ as many decays as at the beginning. After another half life there will be half again as many, or 1/8 the number of initial decays.