Nuclear reactions

There are two reactions that involve an atom’s nucleus: fusion and fission.
The first starts from simple elements to produce complex ones; the second acts in the opposite way by splitting nuclei of heavy elements into nuclei of lighter elements.
In both cases there is a very high energy output. In the case of fusion for instance: the mass of the atomic nuclei that melt is greater than that of the new nucleus that will form. Since we know that mass and energy are equivalent as according to Einstein’s famous formula of E=mc2, the difference in mass is what is transformed into energy.
Likewise, also in fission the energy produced comes from the difference between the initial nucleus mass and that of the two resultant nuclei: in this case the first is greater than the sum of the latter.
To produce energy however, not all elements can melt just like not all of them can split. In nature in fact phenomena tend to be drawn spontaneously towards low “effort” states. Therefore if more energy is needed to split a nucleus than to keep it together, fission will not occur. This is the case in lighter elements; by melting and increasing the number of particles, the nucleus becomes more stable and excess energy will be released. On the other hand, if the nucleus of an element becomes too big, it will require a lot of energy to keep it together. Therefore it is easier to split it into two lighter and more stable nuclei by releasing, once again, the surplus energy.
When does fission become preferable to fusion in terms of energy? The limit is iron; its nucleus is too large to melt again and be able to produce another stable nucleus. Therefore the fusion of two iron nuclei requires more energy than it produces; so from here on the most convenient reaction is nuclear fission. In fact, it is no coincidence that during the evolution of massive stars, once iron is created in the nucleus no further fusions can take place.

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