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Alpha decay typically occurs in the heaviest nuclides. Theoretically, it can occur only in nuclei somewhat heavier than nickel (element 28), where the overall binding energy per nucleon is no longer a maximum and the nuclides are therefore unstable toward spontaneous fission-type processes.
Radioactive decay (also known as nuclear decay, radioactivity, radioactive disintegration, or nuclear disintegration) is the process by which an unstable atomic nucleus loses energy by radiation. A material containing unstable nuclei is considered radioactive. Three of the most common types of decay are alpha, beta, and gamma decay.
Many of these in theory could decay through spontaneous fission, alpha decay, double beta decay, etc. with a very long half-life, but no radioactive decay has yet been observed. Thus, the number of stable nuclides is subject to change if some of these 251 are determined to be very long-lived radioactive nuclides in the future.
Chart of nuclides by type of decay. Black squares are stable nuclides. Nuclides with excessive neutrons or protons are unstable to β − (light blue) or β + (green) decay, respectively. At high atomic number, alpha emission (orange) or spontaneous fission (dark blue) become common decay modes.
This is because the mass–energy is a convex function of atomic number, so all nuclides on an odd isobaric chain except one have a lower-energy neighbor to which they can decay by beta decay. See Mattauch isobar rule. (123 Te is expected to decay to 123 Sb, but the half-life appears to be so long that the decay has never been observed.)
[1] [2] Stable alpha nuclides, and stable decay products of radioactive alpha nuclides, are some of the most common metals in the universe. Alpha nuclide is also shorthand for alpha radionuclide, referring to those radioactive isotopes that undergo alpha decay and thereby emit alpha particles. [3]
Thus, alpha decay can be considered either a form of particle decay or, less frequently, as a special case of nuclear fission. The timescale for the nuclear strong force is much faster than that of the nuclear weak force or the electromagnetic force , so the lifetime of nuclei past the drip lines are typically on the order of nanoseconds or less.
For mass numbers of 147, 151, and 209+, the beta-stable isobar of that mass number has been observed to undergo alpha decay. (In theory, mass number 143 to 155, 160 to 162, and 165+ can also alpha decay.) This gives a total of 101 stable nuclides with odd mass numbers.