Why not more fusion with a net release of energy?

[pivoxa15]

why does the energy-liberating fusion process ends with the formation of iron and not heavier elements such as uranium?

 

[Pengwuino]

I believe you mean fission?

 

[pivoxa15]

I don't think so. I was thinking how stars can fuse 3 helium nuclei to form a carbon-12 and get a net release of energy. And combine some carbon-12 to form an iron and get a net release of energy. But it is not possible to fuse some irons to form Uranium and get a net release of energy. To do the latter, the star must put energy in. Why is that? I am guess it is because atoms with more protons and neutrons than iron are unstable because the nuclei is too large for the strong force to be effective, so the coulomb repulsion force comes into play. But there must be a better explanation.

 

[ZapperZ]

Look at the nuclear binding energy. There's a maximum around iron.

Zz.

 

[Entropy]

Look at the nuclear binding energy. There's a maximum around iron.

I think he understands that, he wants to know why. I personally don't know enough about nuclear physics to answer your question. I'm not even sure if nuclear physicists know.

 

[Grogs]

According to the the 'hyperphysics' site, Nickel-62 is actually the most tightly bound nucleus, but it's a close race with Fe-56. Great site, I learn something new every time I visit.

I think he understands that, he wants to know why. I personally don't know enough about nuclear physics to answer your question. I'm not even sure if nuclear physicists know.

The answer seems to lie in the difference between the Electromagnetic force and the Strong Force. The EM force has an infinite range and works to push all of the protrons in the nucleus apart. The Strong Force is ~20 times stronger and tries to pull all of the protons and neutrons together, but it's range is extremely short, ~10^-15 meters.
In small nuclei, the nucleus is smaller than the range of the Strong Force and every nucleon (proton or neutron) interacts with every other nucleon in the nucleus. In large nuclei, the nucleus is actually larger than the range of the Strong Force, so not every nucleon reacts with every other one, but every proton is still interacting (via EM) with every other proton, resulting in a less tightly bound (on a per-nucleon basis) nucleus.