Some nuclides are stable while others are not. Unstable nuclides are radioactive. A sample of radioactive nuclei will decay over time, emitting radiation in the process. The number of decays per unit time is called the activity, A.It has the SI unit of Becquerel (Bq). 1 Bq = 1 decay per second.
Unlike nuclear fusion and fission, radioactive decay is spontaneous and random.
By spontaneous, we mean that the decay is not triggered by anything. It just decays when it does. This means that the rate of decay of a radioactive sample is not affected by external environmental factors such as temperature, pressure, etc.
By random, we mean that each decay is a probabilistic event. So there is absolutely no way one can predict when an individual nucleus is going to decay. However, if every nucleus in the sample has the same probability of decay, the rate of decay for the population as a whole can be predicted accurately.
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