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I have been reading about isotopes and their abundance on Wikipedia. It states that lithium has 2 stable isotopes, beryllium has 1 stable isotope (monoisotopic and mononuclidic) and boron has 2 stable isotopes. Although the total number of radioisotopes is much larger than the stable ones in each of these three, that is not the point of the question.

The actual question is if lithium and boron have 2 stable isotopes each, why does beryllium, which comes between lithium and boron, has only one stable isotope? What factor is making this happen here?

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    $\begingroup$ Beryllium would have had 2 stable isotopes and beryllium-8 would have been the ideal candidate if two helium-4 nuclei had not been even better energetical choice. Note that there exists no stable nucleus with 5 or 8 nucleons. It creates a little trouble for massive stars how to fuse helium-4 in heavier elements. $\endgroup$
    – Poutnik
    Commented May 11, 2023 at 8:07
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    $\begingroup$ Note that there is multiple monoisotopic elements, but near all are odd-proton count cases, that have max 2 stable isotopes (that is the rule), and one option is claimed by the lower energy isotope of its greedy even-proton count neighbors. Tc and Pm have double bad luck - both their stable candidates are stolen. $\endgroup$
    – Poutnik
    Commented May 11, 2023 at 13:22
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    $\begingroup$ Related: Why is tin-112 stable, but indium-112 radioactive? $\endgroup$
    – Loong
    Commented May 11, 2023 at 17:36

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If you actually label the stable isotopes of the elements involved ($\ce{^6Li,^7Li,^9Be,^{10}B,^{11}B}$) it becomes evident that $\ce{^8Be}$ is the missing stable isotope. What happens, as alluded to in the comments, is that pairs of alpha particles do not bond very well, much like pairs of noble gas atoms in ordinary chemistry except perhaps worse.

We often hear about "magic numbers" involving "filled nuclear shells" from physicists seeking stable superheavy elements, but these concepts extend down to lighter atoms and $\ce{^4He}$, an alpha particle, is actually the first of the highly stabilized, "doubly magic" nuclei. The stabilization of $\ce{^4He}$ by this property turns out especially effective in preventing pairing by nuclear forces unless you bring in either a three-way interaction (forming another relatively strongly stabilized $\ce{^{12}C}$ nucleus), or other particles such as an additional neutron.

So the two alpha-particle components of $\ce{^8Be}$ don't hold together; you need at least an additional neutron to hold the nucleus together against fission (a neutron works better than a proton because it avoids additional electrostatic repulsion). The stable isotopes of beryllium and boron (and heavier elements) have the additional particles they need to remain bound. Meanwhile for lithium the double-alpha splitting doesn't arise because you can't split off two alpha particles with only three protons.

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