5
$\begingroup$

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?

$\endgroup$
5

2 Answers 2

10
$\begingroup$

"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.

$\endgroup$
2
  • 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. $\endgroup$
    – Poutnik
    Jun 13, 2022 at 5:58
  • $\begingroup$ Or rather, .... if some of its natural "stable" isotopes are observationally stable. ..... $\endgroup$
    – Poutnik
    Jul 11, 2022 at 8:01
8
$\begingroup$

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.

$\endgroup$
0

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.