The radioactive decay law, that the number of decays per second is proportional to the number of particles present was discovered by Rutherford and Soddy in the course of their investigations into the decay of radioactive nuclei. The law, a consequence of quantum mechanics, also applies to the de-excitation of excited atoms and to the decay of unstable fundamental particles. The law is statistical in nature, that is, it does not predict when a given particle will decay but only gives the probability of its decay in a given time interval. For a collection of particles it predicts, subject to statistical fluctations, the number of decays in a given time interval.