My chemistry teacher told me that $\ce{^{56}Fe}$ is the most stable nucleus because of the highest binding energy per nucleon. But if we consider a protium nucleus, it has only 1 proton so there is no chance of separation. so why isn't protium considered the most stable nucleus?
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4$\begingroup$ Will answer properly later if nobody else does, but firstly the proton is theorised to decay en.wikipedia.org/wiki/Proton_decay, and secondly read en.wikipedia.org/wiki/Nuclear_binding_energy - nuclear particles bind together to form lower energy states in a not totally dissimilar way to how atoms bind together to make molecules. There's probably a duplicate here as well $\endgroup$– Ian BushCommented Apr 14, 2020 at 7:38
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1$\begingroup$ When he made that statement he was certaintly thinking of nuclei with A > 1, making the interesting point that binding energy per nucleon peaks at Fe-56 (which is important for stellar evolution). It's obviously not meaningful to talk about binding energy/nucleon for a proton. As far as Fe56's vs. a proton's comparative stability, Ian's link shows the proton's estimated half-life is 10^34 years! I don't know if the nuclear binding energy in Fe56 adds to the stability of the proton, nor whether there are any estimates of the half life of the Fe56 nucleus as a whole. $\endgroup$– theoristCommented Apr 14, 2020 at 7:52
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1$\begingroup$ The stability is meant not as lack of self decay here, but disability to reach lower energy. Analogy: Nuclei are like pits and nucleons are like tiny balls in a children game. The 56Fe is the deepest pit for the balls/nucleons, while a proton/ball lays on the flat surface. It is conditionally stable, until it falls into a pit/nuclei, where it is more stable ( not in all cases ), as it cannot jump back up. $\endgroup$– PoutnikCommented Apr 14, 2020 at 7:54
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1$\begingroup$ Don't forget that "binding energy" is relative. Say the binding energy per nucleon of an isolated proton is 0, then the binding energy per nucleon of an iron nucleus is lower than zero (hence nuclear processes in stars can release energy by banging protons together to make heavier nuclei like iron (it is way more complex than this in reality but this view captures the essential point). $\endgroup$– matt_blackCommented Apr 14, 2020 at 16:32
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For the same reason why, in chemistry, a collection of identical separated atoms is unstable with respect to formation of chemical bonds, at low temperature.
In nuclear physics, the "low temperatures" can be several orders of magnitude higher, but the direct reaction between nuclei is suppressed because their electric charges keep them apart. Inside stars, high enough pressure and temperature provides the activation energy to turn protium (and many other nuclides) into more stable nuclides.
So, there are two process to consider stability relative to: Decay and fusion. Protium is not the most stable nucleus against fusion.
