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Stable nuclides areisotopes of achemical element whosenucleons are in a configuration that does not permit them the surplus energy required to produce a radioactive emission. Thenuclei of such isotopes are not radioactive and unlikeradionuclides do not spontaneously undergoradioactive decay.[1] When these nuclides are referred to in relation to specific elements they are usually called that element'sstable isotopes.
The 80 elements with one or more stable isotopes comprise a total of 251 nuclides that have not been shown to decay using current equipment. Of these 80 elements, 26 have only one stable isotope and are calledmonoisotopic. The other 56 have more than one stable isotope.Tin has ten stable isotopes, the largest number of any element.
Most naturally occurringnuclides are stable (about 251; see list at the end of this article), and about 35 more (total of 286) are known to be radioactive with long enough half-lives (also known) to occur primordially. If the half-life of anuclide is comparable to, or greater than, the Earth's age (4.5 billion years), a significant amount will have survived since theformation of the Solar System, and then is said to beprimordial. It will then contribute in that way to the natural isotopic composition of a chemical element. Primordial radioisotopes are easily detected with half-lives as short as 700 million years (e.g.,235U). This is the present limit of detection,[citation needed] as shorter-lived nuclides have not yet been detected undisputedly in nature except when recently produced, such as decay products or cosmic ray spallation.
Many naturally occurring radioisotopes (another 53 or so, for a total of about 339) exhibit still shorter half-lives than 700 million years, but they are made freshly, as daughter products of decay processes of primordial nuclides (for example, radium from uranium), or from ongoing energetic reactions, such ascosmogenic nuclides produced by present bombardment of Earth bycosmic rays (for example,14C made from nitrogen).
Some isotopes that are classed as stable (i.e. no radioactivity has been observed for them) are predicted to have extremely long half-lives (sometimes 1018 years or more).[2] If the predicted half-life falls into an experimentally accessible range, such isotopes have a chance to move from the list of stable nuclides to the radioactive category, once their activity is observed. For example,209Bi and180W were formerly classed as stable, but were found to bealpha-active in 2003. However, such nuclides do not change their status as primordial when they are found to be radioactive.
Most stable isotopes on Earth are believed to have been formed in processes ofnucleosynthesis, either in theBig Bang, or in generations of stars that preceded theformation of the Solar System. However, some stable isotopes also show abundance variations in the earth as a result of decay from long-lived radioactive nuclides. These decay-products are termedradiogenic isotopes, in order to distinguish them from the much larger group of 'non-radiogenic' isotopes.
Of the known chemical elements, 80 elements have at least one stable nuclide. These comprise the first 82 elements fromhydrogen tolead, with the two exceptions,technetium (element 43) andpromethium (element 61), that do not have any stable nuclides. As of 2024, there are total of 251 known "stable" nuclides. In this definition, "stable" means a nuclide that has never been observed to decay against the natural background. Thus, these elements have half-lives too long to be measured by any means, direct or indirect.
Stable isotopes:
These last 26 are thus calledmonoisotopic elements.[3] The mean number of stable isotopes for elements which have at least one stable isotope is 251/80 = 3.1375.
Stability of isotopes is affected by the ratio of protons to neutrons, and also by presence of certainmagic numbers of neutrons or protons which represent closed and filled quantum shells. These quantum shells correspond to a set of energy levels within theshell model of the nucleus; filled shells, such as the filled shell of 50 protons for tin, confers unusual stability on the nuclide. As in the case of tin, a magic number forZ, the atomic number, tends to increase the number of stable isotopes for the element.
Just as in the case of electrons, which have the lowest energy state when they occur in pairs in a given orbital, nucleons (both protons and neutrons) exhibit a lower energy state when their number is even, rather than odd. This stability tends to prevent beta decay (in two steps) of many even–even nuclides into another even–even nuclide of the same mass number but lower energy (and of course with two more protons and two fewer neutrons), because decay proceeding one step at a time would have to pass through an odd–odd nuclide of higher energy. Such nuclei thus instead undergodouble beta decay (or are theorized to do so) with half-lives several orders of magnitude larger than theage of the universe. This makes for a larger number of stable even–even nuclides, which account for 150 of the 251 total. Stable even–even nuclides number as many as threeisobars for some mass numbers, and up to seven isotopes for some atomic numbers.
Conversely, of the 251 known stable nuclides, only five have both an odd number of protonsand odd number of neutrons: hydrogen-2 (deuterium),lithium-6,boron-10,nitrogen-14, andtantalum-180m. Also, only four naturally occurring, radioactive odd–odd nuclides have a half-life >109 years:potassium-40,vanadium-50,lanthanum-138, andlutetium-176. Odd–oddprimordial nuclides are rare because most odd–odd nucleibeta-decay, because the decay products are even–even, and are therefore more strongly bound, due tonuclear pairing effects.[4]
Yet another effect of the instability of an odd number of either type of nucleon is that odd-numbered elements tend to have fewer stable isotopes. Of the 26monoisotopic elements (those with only one stable isotope), all but one have an odd atomic number, and all but one has an even number of neutrons: the single exception to both rules isberyllium.
The end of the stable elements occurs afterlead, largely because nuclei with 128 neutrons—two neutrons above themagic number 126—are extraordinarily unstable and almost immediately alpha-decay.[5] This contributes to the very short half-lives ofastatine,radon, andfrancium. A similar phenomenon occurs to a much lesser extent with 84 neutrons—two neutrons above the magic number 82—where various isotopes oflanthanide elements alpha-decay.
The 251 known stable nuclides include tantalum-180m, since even though its decay is automatically implied by it being "metastable", this has not been observed. All "stable" isotopes (stable by observation, not theory) are the ground states of nuclei, except for tantalum-180m, which is anuclear isomer or excited state. The ground state, tantalum-180, is radioactive with half-life 8 hours; in contrast, the decay of the nuclear isomer is extremely strongly forbidden by spin-parity selection rules. It has been reported by direct observation that the half-life of180mTa to gamma decay must be >1015 years. Other possible modes of180mTa decay (beta decay, electron capture, and alpha decay) have also never been observed.
It is expected that improvement of experimental sensitivity will allow discovery of very mild radioactivity of some isotopes now considered stable. For example, in 2003 it was reported thatbismuth-209 (the only primordial isotope of bismuth) is very mildly radioactive, with half-life (1.9 ± 0.2) × 1019 yr,[6][7] confirming earlier theoretical predictions[8] fromnuclear physics that bismuth-209 would very slowlyalpha decay.
Isotopes that are theoretically believed to be unstable but have not been observed to decay are termedobservationally stable. Currently there are 105 "stable" isotopes which are theoretically unstable, 40 of which have been observed in detail with no sign of decay, the lightest in any case being36Ar.[citation needed] Many "stable" nuclides are "metastable" in that they would release energy if they were to decay,[9] and are expected to undergo very rare kinds ofradioactive decay, includingdouble beta decay.
146 nuclides from 62 elements withatomic numbers from 1 (hydrogen) to 66 (dysprosium) except 43 (technetium), 61 (promethium), 62 (samarium), and 63 (europium) are theoretically stable to any kind of nuclear decay — except for the theoretical possibility ofproton decay, which has never been observed despite extensive searches for it; andspontaneous fission (SF), which is theoretically possible for the nuclides withatomic mass numbers ≥ 93,[10] that is all those withatomic numbers ≥ 41.
Besides SF, other theoretical decay routes for heavier elements include:[10]
These include all nuclides of mass 165 and greater.Argon-36 is the lightest known "stable" nuclide which is theoretically unstable.[10]
The positivity of energy release in these processes means they are allowed kinematically (they do not violate conservation of energy) and, thus, in principle, can occur.[10] They are not observed due to strong but not absolute suppression, by spin-parity selection rules (for beta decays and isomeric transitions) or by the thickness of the potential barrier (for alpha and cluster decays and spontaneous fission).
This is a summary table fromList of nuclides. Numbers are not exact and may change slightly in the future, as nuclides are observed to be radioactive, or new half-lives are determined to some precision.
| Type of nuclide by stability class | Number of nuclides in class | Running total of nuclides in all classes to this point | Notes |
|---|---|---|---|
| Theoretically stable according to known decay modes, includingalpha decay,beta decay,isomeric transition, anddouble beta decay | 146 | 146 | All the first 66 elements, excepttechnetium,promethium,samarium, andeuropium. Ifspontaneous fission is possible for the nuclides withmass numbers ≥ 93, then all such nuclides are unstable, so that only the first 40 elements would be stable. Ifprotons decay, then there are no stable nuclides. |
| Energetically unstable to one or more known decay modes, but no decay yet seen. Considered stable until radioactivity confirmed. | 105[2][11] | 251 | Total is the observationally stable nuclides. All elements up tolead (excepttechnetium andpromethium) are included. |
| Radioactiveprimordial nuclides. | 35 | 286 | Includesbismuth,thorium, anduranium |
| Radioactive nonprimordial, but occur naturally on Earth. | ~61 significant | ~347 significant | Cosmogenic nuclides from cosmic rays; daughters of radioactive primordials such asfrancium, etc. |
The primordial radionuclides are included for comparison; they are italicized and offset from the list of stable nuclides proper.
Abbreviations for predicted unobserved decay:[12][2][11]
α for alpha decay,B for beta decay,2B for double beta decay,E for electron capture,2E for double electron capture,IT for isomeric transition,SF for spontaneous fission,* for the nuclides whose half-lives have lower bound. Double beta decay has only been listed when beta decay is not also possible.
^ Tantalum-180m is a "metastable isotope", meaning it is an excitednuclear isomer of tantalum-180. Seeisotopes of tantalum. However, the half-life of this nuclear isomer is so long that it has never been observed to decay, and it thus is an "observationally stable"primordial nuclide, a rare isotope of tantalum. This is the only nuclear isomer with a half-life so long that it has never been observed to decay. It is thus included in this list.
^^Bismuth-209 was long believed to be stable, due to its half-life of 2.01×1019 years, which is more than a billion times the age of the universe.
§Europium-151 andsamarium-147 areprimordial nuclides with very long half-lives of 4.62×1018 years and 1.066×1011 years, respectively.
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