How do heavier nuclei maintain stability despite requiring energy for fusion?

[wasi-uz-zaman]

TL;DR Summary

hi, Fusion of nuclei till Fe or Ni is exothermic but after that it becomes endothermic, I want to ask answer in terms of two main rival forces of nucleus, coulomb force and strong nuclear force.

I understand overall decrease in potential energy of Lighter nuclei releases energy on Fusion- but for heavier nuclei - energy is required- means overall potential energy is not decreased - than how these heavier nuclei sustained their stability.

 

[dRic2]

You can see if a nuclear reaction is eso/endothermic by looking at the binding energy per nucleon graph

https://en.wikipedia.org/wiki/Nucle...le:Binding_energy_curve_-_common_isotopes.svg

In the graph the absolute value of the binding energy is plotted vs the number of atomic mass (A). The higher the binding energy, the strongest the attraction and you can see that Fe-56 has the highest binding energy per nucleon. Even for an heavy element the contribution of the binding energy is always attractive. The reason that fusion of heavier element requires to give energy is because you are working to the right of Fe-56:
if you increase A you move from a condition with higher binding energy (strong attraction) to a one with lower biding energy (lower attraction) so you have to supply energy in order to "weaken" the interaction between nucleons.

The trend of that graph is not easy to explain because nuclear forces are pretty complicated, but a simple and yet very good qualitative explanation comes from the liquid drop model (https://en.wikipedia.org/wiki/Semi-empirical_mass_formula)

 

[mfb]

Splitting heavy elements like e.g. gold would release energy. But the energy barrier to do so is so high that it doesn't happen naturally (unless you bombard the nucleus with something high-energetic). In principle it should be possible but with a half life so long that we'll never expect to see any decay.