I have heard many times that when there is the “right balance” of protons and neutrons the atom will be stable, and therefore it will decay to achieve this stability. But what about this supposed balance of protons and neutrons makes it stable? Does it have to do with quarks? Density? I am quite curious as to what causes this.

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    $\begingroup$ To be honest, no matter what we come up with, in the end it is just the way the universe works:-) $\endgroup$ – user79161 May 4 '19 at 20:01
  • $\begingroup$ Useful reading: en.wikipedia.org/wiki/Valley_of_stability $\endgroup$ – Mathew Mahindaratne May 4 '19 at 22:17
  • $\begingroup$ Simply put, stable isotopes are particular combinations of neutrons and protons which minimise the total energy of the system, which is the sum of kinetic energy, electric potential energy, and strong nuclear potential energy. Unfortunately, that last term is fiendishly difficult to calculate/understand theoretically. Furthermore, local energy minima can lead to stable isotopes, depending on the kinetics of weak nuclear force interactions. We understand the weak nuclear force quite well, but kinetics is naturally a harder topic. $\endgroup$ – Nicolau Saker Neto May 4 '19 at 23:47

There is a shell structure within the nucleus also. So the neutron/proton ratio varies from 1:1 for the light elements to 1.5 for the heavy elements.

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Note: I ripped off the image from another website. Not sure where they got it from...

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    $\begingroup$ Looks like it's Figure 21.2 Belt of Stability that's been replicated with pgfplots from Brown's Chemistry: The Central Science 8th Ed. (2000). A more recent 14th Ed. (2018) has an updated plot (for the reference it's Figure 21.1 on p. 950). $\endgroup$ – andselisk May 4 '19 at 14:08

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