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So, I've just been learning about chemistry recently and I came across the isotopes of an atom, which are obviously the different numbers of neutrons within atoms of the same element (i.e. same number of protons). Some of these are stable, and others are radioactive and can therefore spontaneously change (radioactively decay) into isotopes of the same element or can transmute (transmutate?) into different elements.

From what I used to understand and what I was taught about nuclear fission, it was that the neutron hit the atom's nucleus and split it into two parts, hence the fission.

However, from what I've seen on other diagrams such as wikipedia's (U235 --> U236 --> Kr92 Ba141 + 3 neutrons), it seems that the neutron is not itself splitting the atom, but is instead forming a new artificial isotope of the element which is then MUCH less stable and nearly instantly radioactively decays (which is to say emits energy through particles) into other smaller elements with other neutrons not having atoms to be in (this relates to another question about whether or not said neutrons turn into some form of energy like if the theory of relativity meant they could just swap into one or the other or that they are just particles without any atoms to latch onto -- if so, why wouldn't they find other atoms that are stable and add themselves onto their nucleus, especially if they could be stable in that isotope, but those are questions for aonther time).

So, from what I understand now it seems like nuclear fission caused by humans is more induced radioactive decay than it is intentionally splitting the atom into two parts, but I'm not sure. Any insight?

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    $\begingroup$ Yes, the probability of decay modes has changed with fission being much more likely. $\endgroup$
    – Farcher
    Commented Sep 21, 2023 at 7:16
  • $\begingroup$ Maybe it will be helpful if you consider how you would answer, if someone asked you how would you possibly experimentally differentiate between the two scenarios. $\endgroup$ Commented Sep 21, 2023 at 14:41

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The stability of a nucleus depends on the number of neutrons. There are basically two effects to consider. First, the asymmetry of protons and neutrons is based on the Pauli exclusion principle (identical fermions can't be in the same state). That's why a nucleus has lower energy states if the number of protons and neutrons is the same or similar. The other effect is that a nucleus with an even number of protons and an even number of neutrons is more stable than an odd-odd nucleus or an even-odd nucleus. This is caused by the alignment of the spins. If you are interested in more details, you can take a look at the Semi-empirical mass formula, especially at the asymmetry term and the paring term.

That is the reason, why changing the number of neutrons can cause instability. This can happen either by the $\beta$-decay or by shooting a neutron with not too much energy on the nucleus. For fission, it is necessary to overcome the fission barrier, this is the activation energy for this process. The neutron can bring this energy by exciting the nucleus. For very big nuclei the activation energy is small in comparison to the energy of the nucleus so fission can also happen without external energy from a neutron.

I hope that makes it a bit clearer

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