# Why plutonium and uranium are especially susceptible to do fission?

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

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.

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.

• @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 $U^{235}$. – peterh Aug 8 '17 at 19:53

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.

• why is this ratio important? or rather why is this specific value important? Why is a ratio $>1$ not enough? – ZeroTheHero Aug 8 '17 at 18:48
• 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. – rob Aug 8 '17 at 20:41