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Why plutonium and uranium are especially susceptible to do fission? Are there any other elements that can also be subject to fission?

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It is not the elements but the isotopes U-235 and Pu-239. Those two have an odd number of neutrons, and can fission by absorbing a thermal neutron. It is the pairing energy (spin-up and spin-down) that is the cause of this.

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All of the nuclei - except the proton - are susceptible to fission.

The typical binding energy of a nucleon (particles of the core of the atoms, i.e. neutron or proton) is around $\mathrm{5MeV}$ in most atoms. Hitting a nucleus with anything sometimes of this energy, it will fissile.

However, not this is what you likely want. The ${}^{235}\mathrm{U}$ and the ${}^{239}\mathrm{Pu}$ are special, that we can do chain reaction with it: the neutrons leaving the fissiled nucleus can fissile other nuclei.

This chain reaction capability is a very special feature:

  • there are at least 1300 nuclei known (most of them is very short living)
  • roughly 130 of them are stable
  • there are capable to do around 10-12 nuclear reactions

Despite that, only these two are usable to make nuclear chain reaction from them.

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  • $\begingroup$ @john Ok. Btw, also heavier nuclei would be capable to this reaction. As far I know, roughly from Californium, all of the nuclei would be usable for fission. But they are very costly to produce (typically, from uranium, in nuclear reactors). As far I know, also $U^{233}$ is usable for fission, but it can't be mined (it decayed long ago), rather it has to be produced from $Th^{232}$ in a more complex process. And also $Np^{237}$ is usable in nuclear reactors, but far lesser useful as $U^{235}$. $\endgroup$ – peterh Aug 8 '17 at 19:53
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The n/p ratio, that is, neutron to proton ratio is much greater than 1.5.So they are highly fission able. Radioactivity is a nuclear phenomenon. No electrons are responsible for this.

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    $\begingroup$ why is this ratio important? or rather why is this specific value important? Why is a ratio $>1$ not enough? $\endgroup$ – ZeroTheHero Aug 8 '17 at 18:48
  • $\begingroup$ This does not explain why U-238, with more neutrons, has a lower fission cross-section than U-235, which has fewer neutrons. Neutron-to-proton ratio is a good predictor for stability against $\beta^-$ or $\beta^+$ decay, but fission has more details involved. $\endgroup$ – rob Aug 8 '17 at 20:41

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