Radioactive decay is the process by which an atomic nucleus of an unstable atom loses energy by emitting ionizing particles (ionizing radiation). The emission is spontaneous, in that the atom decays without any physical interaction with another particle from outside the atom. Usually, radioactive decay happens due to a process confined to the nucleus of the unstable atom, but, on occasion (as with the different processes of electron capture and internal conversion), an inner electron of the radioactive atom is also necessary to the process.
Radioactive decay is a stochastic (i.e., random) process at the level of single atoms, in that, according to quantum theory, it is impossible to predict when a given atom will decay.[1] However, the chance that a given atom will decay is constant over time, so that given a large number of identical atoms (nuclides), the decay rate for the collection is predictable to the extent allowed by the law of large numbers.
The decay, or loss of energy, results when an atom with one type of nucleus, called the parent radionuclide, transforms to an atom with a nucleus in a different state, or a different nucleus, either of which is named the daughter nuclide. Often the parent and daughter are different chemical elements, and in such cases the decay process results in nuclear transmutation. In an example of this, a carbon-14 atom (the "parent") emits radiation (a beta particle, antineutrino, and a gamma ray) and transforms to a nitrogen-14 atom (the "daughter"). By contrast, there exist two types of radioactive decay processes (gamma decay and internal conversion decay) that do not result in transmutation, but only decrease the energy of an excited nucleus. This results in an atom of the same element as before but with a nucleus in a lower energy state. An example is the nuclear isomer technetium-99m decaying, by the emission of a gamma ray, to an atom of technetium-99.
Nuclides produced by radioactive decay are called radiogenic nuclides, whether they themselves are stable or not. There exist stable radiogenic nuclides that were formed from short-lived extinct radionuclides in the early solar system[citation needed]. The extra presence of stable radiogenic nuclides against the background of primordial stable nuclides can be inferred by various means. Presently-radioactive nuclides are from three sources: many naturally-occurring radionuclides are short-lived radiogenic nuclides that are the daughters of ongoing radioactive primordial nuclides (types of radioactive atoms that have been present since the beginning of the Earth and solar system). Other naturally-occurring radioactive nuclides are cosmogenic nuclides, formed by cosmic ray bombardment of material in the Earth's atmosphere or crust. Finally, some primordial nuclides are radioactive, but are so long-lived that they remain present from the primordial solar nebula. For a summary table showing the number of stable nuclides and of radioactive nuclides in each category, see radionuclide.
The early researchers also discovered that many other chemical elements besides uranium have radioactive isotopes. A systematic search for the total radioactivity in uranium ores also guided Marie Curie to isolate a new element polonium and to separate a new element radium from barium. The two elements' chemical similarity would otherwise have made them difficult to distinguish.
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