Another type of radioactive decay results in products that are not defined, but appear in a range of "pieces" of the original nucleus.This decay, called spontaneous fission, happens when a large unstable nucleus spontaneously splits into two (and occasionally three) smaller daughter nuclei, and generally leads to the emission of gamma rays, neutrons, or other particles from those products.For a summary table showing the number of stable and radioactive nuclides in each category, see radionuclide.
All of these processes result in a well-defined nuclear transmutation.
A material that spontaneously emits such radiation is considered radioactive. random) process at the level of single atoms, in that, according to quantum theory, it is impossible to predict when a particular atom will decay, regardless of how long the atom has existed.
For a collection of atoms however, the collection's decay rate can be calculated from their measured decay constants or half-lives. The half-lives of radioactive atoms have no known lower or upper limit, spanning a time range of over 55 orders of magnitude, from nearly instantaneous to far longer than the age of the universe.
Rutherford was the first to realize that all such elements decay in accordance with the same mathematical exponential formula.
Rutherford and his student Frederick Soddy were the first to realize that many decay processes resulted in the transmutation of one element to another.
At first, it seemed as though the new radiation was similar to the then recently discovered X-rays.