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Usually, stable isotopes have highest abundances, often much higher than radioactive ones. Are there any elements having most commonly occurring isotope different from the most stable isotope?

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    $\begingroup$ What is "most stable"? $\endgroup$ Feb 24 at 1:07
  • $\begingroup$ Fe-56 is the most stable iron isotope and the most abundant. Ni-62 has the highest binding energy of all but is not the most abundant Ni isotope. Nuclide formation in stars is complex as is radioactive decay and artificial element manufacture. There is a lot in Wikipedia. $\endgroup$
    – jimchmst
    Feb 24 at 2:40
  • $\begingroup$ When isotopes are stable (resistence to spontaneous decay) enough , observationally stable or assumed as stable, their relative abundance is not determined by their stability, but by probability of the path of the nucleogenesis, leading to the considered isotope. If there is formed about 10 times more isotope A than isotope B, that there is about 10 times more of isotope A than B, even if B is more stable. $\endgroup$
    – Poutnik
    Mar 2 at 18:36
  • $\begingroup$ How about $\ce{^{79}Br}$ and $\ce{^{81}Br}$ $(1:1)$? $\endgroup$ Mar 3 at 22:35

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Yes.

You're probably thinking about comparing nuclear stability by comparing radioactive decay half-lives, which is a reasonable and intuitive (though somewhat limited) criterion. Already here there are multiple examples which fit the requirement.

For instance, rhenium has two isotopes naturally found on Earth. 62.6% of all rhenium is rhenium-187, which beta-decays to osmium-187 with a measured half-life of 41.2 billion years (about four times the current age of the universe). Meanwhile, the remaining 37.4% of rhenium is "observationally stable" rhenium-185, which is to say it is theoretically expected to undergo spontaneous nuclear decay, but no such decay has yet been measured, suggesting its half-life is likely far higher.

An even more lopsided case is indium. 95.7% of all indium is radioactive indium-115 with a double beta-decays with a half-life of 441 trillion years, compared to 4.3% of observationally-stable indium-113.

Another interesting example is tellurium, which has eight naturally occurring isotopes. Two of these, tellurium-128 and tellurium-130, compose 65.8% of all tellurium, and both have staggeringly huge measured half-lives, greater than a billion times the age of the universe.

There are also a number of other close cases such as rubidium, neodymium, samarium and cadmium, where radioactive isotopes compose >10% of natural abundance, but not enough to beat the predominant observationally-stable isotope.

It's possible to adopt other definitions of stability, such as comparing the nuclear binding energies per nucleon. Under this criterion, then it turns out that for the majority of elements the most stable isotope is not the most naturally abundant. You can conveniently check using this Wolfram applet. If you scroll though the elements, you'll notice that the peaks in the two charts very often don't match perfectly, though they're usually close.

One may ask an interesting question - why do these mismatches exist in the first place? I suspect the answer ultimately comes down to the fact that most naturally-occurring isotopes are generated in violent astrophysical processes which are far from equilibrium, combined with the fact that nuclear processes only allow a limited variety of discrete steps up/down in proton and neutron numbers to be made, and sometimes isotopes get trapped because there is no good combination of steps to change them.

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    $\begingroup$ Just as in Chemistry thermodynamics says what can happen, but kinetics controls which of those possibilities actually does. $\endgroup$
    – Ian Bush
    Mar 3 at 6:21
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Tantalum-180 famously has an isomer, $_{73}^{180m}\ce{Ta}$, which isn't the lowest energy (nuclear) isomer having that combination of mass number and atomic number. But it has a long half-life ($\ge 4.5×10^{16}$ years). This isomer is only about $\sim 0.01$% of naturally occurring tantalum, the rest being tantalum-181; but it far exceeds the ground-state tantalum-180 isomer which is known only synthetically.

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