Nuclear Fusion and Fission

When a heavy nucleus undergoes nuclear fission , it splits to two smaller, more stable nuclei and produces heat, similarly, when two light nuclei fuse, to make a heavier, more stable nucleus and produces energy.


The produced energy, is due to the mass defect between the reactants and the products, which is transformed to energy, according the mass-energy equivalence.



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> But if the products of each process(fission, and fusion) are more stable, that means that they have more binding energy, thus, shouldn't the mass defect be transformed to nucleus binding energy, rather than emitted as heat or another sort of energy ?
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Note that binding energy of nucleons does not mean contained energy, but the energy needed to break it. 


Therefore kernels with lower binding energy per nucleon contain more nuclear energy per nucleon and have higher mass per nucleon.


It means forming kernels with higher binding energy per nucleon releases energy and vice versa.


It is the same as for chemical bonds, just some 6-7 orders higher.


E.g. nitroglycerine has lower binding energy per atom than reaction products formed by its explosion.


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> When a heavy nucleus undergoes nuclear fission , it splits to two smaller, more stable nuclei and produces heat, similarly, when two light nuclei fuse, to make a heavier, more stable nucleus and produces energy.
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That is correct. But note that there is a distinction between "light nuclei" and "heavy nuclei." 


In other words "light nuclei" are light because when they fuse they release energy. But this doesn't say that you can keep fusing the "light products" ad infinitum and that you will keep getting another "light" nucleus. (Think about two mass 4 atoms fusing to make a mass 8 nucleus. Then two mass 8 atoms fusing to make a mass 16 nucleus, and so on....) 


And "heavy nuclei" are heavy because when they fuse they release energy. But this doesn't say that you can keep splitting the "less heavy products" ad infinitum and that you will keep getting another "heavy" nucleus. (Think about a mass 256 atoms splitting to make two mass 128 nucleus. Then a mass 128 atoms splitting to make two mass 64 nucleus, and so on....) 


The [nuclear binding energy](https://en.wikipedia.org/wiki/Nuclear_binding_energy) curve is shown below. 


[![enter image description here](https://i.stack.imgur.com/lWb5A.png)](https://i.stack.imgur.com/lWb5A.png)


So in a star fusing light elements to form heavier isotopes releases energy until $\ce{^{56}Fe}$. To make heavier elements requires energy. Thus elements heavier than $\ce{^{56}Fe}$ are not made by stellar fusion, but from super nova explosions and the collision of neutron starts. 


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