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Is there any reason why neutrons and protons can't combine in a P-N-N-N-P type chain rather than in a tightly packed sphere?

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    $\begingroup$ Why would they want to form a chain instead of a tightly packed sphere? $\endgroup$
    – JMac
    Feb 1 '17 at 11:39
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    $\begingroup$ Can you expand your question to give us some idea why you think the nucleons might arrange themselves as a chain? For mutually attracting particles a spherical arrangement obviously minimises the energy so you must have some specific mechanism in mind for there tio be a different arrangement. $\endgroup$ Feb 1 '17 at 11:57
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    $\begingroup$ The basic answer is that there's no Neutron-Neutron bond. There's a very strong Proton-Neutron bond. N-P-N-P-N would be closer to possible, but experiments show that 5-He is unstable. I would think the reason 5-He isn't stable is because 4-He is super-stable, so to speak. A complete shell or magic number, en.wikipedia.org/wiki/Magic_number_(physics) so the 3rd Neutron is a 5th wheel for Helium. $\endgroup$
    – userLTK
    Feb 1 '17 at 11:59
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    $\begingroup$ Non-spherical nuclei. $\endgroup$
    – rob
    Feb 2 '17 at 5:22
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    $\begingroup$ @userLTK I suggest you see Krane's chapter (Introductory Nuclear Physics) on "The Force Between Nucleons". It's true that there is not a stable N-N bound nucleus (of only 2 neutrons), but that doesn't mean there is no bonding force. $\endgroup$
    – Bill N
    Mar 9 '18 at 22:32
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A chain would be highly unstable, the Strong Nuclear Force would immediately pull the protons and neutrons together, to minimise their potential energy.

A chain would also distort the spherical symmetry of the electrostatic force, the orbital patterns and energy levels of electrons would be completely different than we observe.

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  • $\begingroup$ I think D Stringers question is a very good one and very reasonable. As thodic pointed out "... there's no Neutron-Neutron bond. There's a very strong Proton-Neutron bond." A macroscopic model of two charged and two isolated spheres - loosely chained in circle - would stretch themselves in a way that the charged spheres will be at the maximum possible distance from each other. This is what D Stringer just ask. Your second point should have to do with the electrons "pressure" on the nucleus. That could be a possible reason for spherical shape of the nucleus. $\endgroup$ Feb 1 '17 at 20:46
  • $\begingroup$ @HolgerFiedler Hi Holger, firstly my answer is way too short. I was also thinking only of a chain that would "automatically" pull together, :) I am also particularly lacking in knowledge, except the basic idea that the SNF will overcome electrostatic charge, but that assumes the particles are very close together, which could be wrong.......In other words I will simply have to learn more about the subject, seriously. But I had never seen evidence of anything other than the "all clumped/bound together" model. But it's a very good question and a fascinating subject. $\endgroup$
    – user140606
    Feb 1 '17 at 22:33
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The existence of highly stable shells in nuclei have given rise to observations of molecule-like arrangements in nuclei. In particular in 12-Be (8 neutrons, 4 protons) the arrangement of the nucleons is often better described as two highly stable alpha particles (two protons, two neutrons) bound by the four extra neutrons.

However, although nuclei are able to form exotic structures like this at neutron, proton numbers far from the closed shells, this is not due to the single inter nucleon forces (as your "P-N-N-N-P type chain" might allude to) but the collective effects of grouped nucleons and the potentials they create.

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  • $\begingroup$ Your responses are much appreciated. $\endgroup$
    – D Stringer
    Feb 3 '17 at 10:45
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A rapidly spinning nucleus can form into a linear chain of several small clusters of neutrons and protons. See http://physics.aps.org/story/v28/st10. This is not a P-N-N-N-P type chain as per your question, but still very far from a sphere.

See also https://arxiv.org/abs/1410.3986. It seems these extreme nucleus configurations are important in stellar nucleosynthesis processes.

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