According to Wikipedia, a lot of the elements that have higher atomic numbers than dysprosium have isotopes that say "Observationally stable" instead of stable, for example in Isotopes of holmium. Why does this happen and what are observationally stable elements anyway?
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5$\begingroup$ The site expects that you write explicit compact summary of your prior effort to answer the question, based on your knowledge and on searching for existing related info or answers. It would prevent others to tell you what you already know or what you could easily find yourself. $\endgroup$– PoutnikJun 12, 2022 at 18:58
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$\begingroup$ See en.m.wikipedia.org/wiki/Stable_nuclide#Still-unobserved_decay $\endgroup$– Jon CusterJun 12, 2022 at 19:29
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$\begingroup$ Theoretical calculations predicts some stable isotopes like calcium-40 to be radioactive. But nobody has ever measured that this isotope emits a radiation. It may be assumed that the half-life of this isotope is so large that not even one Ca-40 atom is destroyed in any sample of this element during one life (~100 y). So the Ca-40 isotope is considered as "observationally stable". Another possibility is that the calculation of the nuclear stability has to be re-examined. Who knows ? $\endgroup$– MauriceJun 12, 2022 at 20:24
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1$\begingroup$ Isotopes that are theoretically believed to be unstable but have not been observed to decay are termed as observationally stable. Currently there are 162 theoretically unstable isotopes, 45 of which have been observed in detail with no sign of decay, the lightest in any case being $\ce{^{36}Ar}$ $\endgroup$– Nilay GhoshJun 13, 2022 at 2:14
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4$\begingroup$ I'll take this opportunity to shamelessly plug a closely related question on Physics.SE that I answered a couple of years ago. $\endgroup$– Michael SeifertJun 13, 2022 at 15:31
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2 Answers
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"Observationally stable" does not refer to an element. Rather, it refers to isotopes of a particular element.
Let us consider a hypothetical example. The intelligent beings on Planet Cartorze have measured nuclear masses, done energetic calculations, and concluded that carbon-14 should decay to nitrogen-14; but their experimental techniques to detect this decay are fairly crude and they can detect the decay only if half a typical laboratory specimen decays within 1000 years (the years on Cartorze are the same as on Earth). Of course it actually takes several times that long for carbon-14 to half-decay, so on Cartorze carbon-14 is observationally stable.
On Earth we are better at detecting radioactive decay, and we can render the decay of carbon-14 with reasonable precision. But when a calculated decay takes a half-life of billions of billions of years or maybe even more, we may not have experimentally verified it, and we have observational stability. Wikipedia reports that (at least to Earthlings) argon-36 is the lightest observationally stable isotope.
answered Jun 12, 2022 at 20:48
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1$\begingroup$ An element can be observationally stable, if all its "stable" isotopes are observationally stable. Bismuth with the only bismuth-209 was such a case until recently when the predicted decay has been observed. Holmium still is. $\endgroup$– PoutnikJun 13, 2022 at 5:58
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$\begingroup$ Or rather, .... if some of its natural "stable" isotopes are observationally stable. ..... $\endgroup$– PoutnikJul 11, 2022 at 8:01
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Very generally, anything "observationally stable" is unstable thermodynamically, but stable kinetically ("frozen"), without observable change (or below change threshold).
For example, diamonds at room conditions. Or a car parking on a slope, but with the activated parking break. Or a brick standing on the smallest side. Diamonds are not observed turning to graphite, nor the car is observed going downhill, nor a standing brick spontaneously tips over and falls down.
In the context of radioactive isotopes, observationally stable are those with a predicted decay, but with the decay half-time above detection limits. Note that some detections of the slowest decays have been done indirectly, finding evidence of consequences of long term decaying. Typically, it is finding otherwise unexpected elements formed by the decay or disturbed isotope ratios. Typical reasons for slow decay rates is too high activation energy, or low probability conditions due quantum physics.
Some of such cases belong to the class of double beta decays, where the decay path goes via less stable in-the-middle nuclide (with higher energy) by quantum tunneling. An example of double beta decay is tellurium-128 with the longest known half-time $\pu{2.2E24 y}$. Having 1 mol of tellurium-128, the typical decay rate would be about 1 nucleus per almost 4 years.
Strictly speaking, TD stable states are literally "observationally stable" too, but the term is not used for these conditions.
