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In most atomic nuclei there are both protons and neutrons.

Having a poor understanding of the strong interaction, it puzzles me why a nucleus must consist of both. Namely, why can there not be nucleus-like clusters of neutron, assuming both nucleons interact in similar ways in terms of strong interaction. This confusion is worsened by the fact that such a cluster would appear to be more stable than real nuclei, since there is no need to overcome the electrostatic repulsion between protons.

Hence the question: why do naturally occurring, stable nuclei necessarily contain both protons and neutrons? (with the exception of hydrogen nucleus)


marked as duplicate by user289661, ACuriousMind, user36790, John Rennie, Floris Sep 26 '16 at 11:42

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  • 3
    $\begingroup$ Not generally true: the most common form of hydrogen is 1H, which contains no neutron. $\endgroup$ – valerio Sep 24 '16 at 17:10
  • $\begingroup$ See Wikipedia on neutronium $\endgroup$ – David Z Sep 24 '16 at 19:27
  • $\begingroup$ Not dupe... the first dupe flagger probably didn't even read both questions... no comment. $\endgroup$ – peterh Sep 25 '16 at 0:06
  • $\begingroup$ @peterh while the title is different, the detailed question actually seems to ask for exactly the information given in he other question's answer: can a small cluster of just neutrons be stable? Dupe. $\endgroup$ – Floris Sep 26 '16 at 11:42

There are "nuclei" that consist (mostly) of neutrons, called neutron stars. A nucleus (in the sense you are talking about) that consists of neutrons wouldn't have any electrons. As well, you have to take into account of the Pauli Exclusion Principle. This is (mostly) the reason why you don't have a nucleus consisting of two protons, the strong nuclear force is stronger than the Coulomb repulsion between two protons but you don't see such a nucleus: "Its instability is due to spin–spin interactions in the nuclear force, and the Pauli exclusion principle, which forces the two protons to have anti-aligned spins and gives the diproton a negative binding energy").

  • $\begingroup$ I feel like you are on to something, would you be bothered to rephrase the answer a bit? $\endgroup$ – user289661 Sep 24 '16 at 20:21
  • $\begingroup$ So why are there no neutrons only nuclei except for neutron stars? Is there a rule that they have to decay like a free neutron? $\endgroup$ – freecharly Sep 24 '16 at 22:49
  • $\begingroup$ Note: Neutron stars are bound by gravity, and (mainly) not by strong interaction. If gravity somehow "disappeared", they would burst like a supernova. $\endgroup$ – peterh Sep 25 '16 at 0:22
  • $\begingroup$ Most isotopes consist mostly of neutrons. Neutron stars contain huge numbers of protons and electrons, and they are not considered atomic nuclei. $\endgroup$ – Jirka Hanika Mar 24 '17 at 21:52

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