Yes, the binding energy curve is the correct reply.

You can overshoot iron, but you'll decay back to it.

Only that this is false, or more precisely intentionally misleading. The total binding energy increases all the way up. See carefully the description: It's binding energy per nucleon. It's the amount of energy you get by fusing one more proton with the nuclide, which peaks with iron, but it never goes to negative, ever.

Any system capable of fusion is going to yield more energy from turning into Iron then turning into anything else.

If you fused into something bigger, it would still be more efficient to split that atom and fuse it's nucleons into more Iron.

Atoms are mostly empty space - chemical lattices are mostly empty space - so in something like a star you can make super-dense iron and turn anything else kicking around into more iron.

EDIT: I feel like the problem here if you're assuming that the size of an atomic nucleus is anything other then negligible compared to it's electron shells. There's no situation under which a non-neutron star will "fit" more of a heavier nucleus in, compared to just densifying iron.

>If you fused into something bigger, it would still be more efficient to split that atom and fuse it's nucleons into more Iron.

Of course, but that doesn't easily happen, as the nucleons are already bound together into heavier nuclides. In fact I guess that might happen in supernovas: The main sequence burns into lead, which is then reconstituted into the iron-nickel mixture that we know from iron meteorites, which also releases energy due to the higher average binding energy per nucleon, but needs extreme conditions to occur.

Except in a star anytime you get two particles interacting enough to potentially fuse, that's potentially a fission event as well. There's no rule which says any given nuclear interaction is fission or fusion: that's what we conclude after it happens.

So in a star creating regular fusion events, even an errant heavier nuclei is going to eventually be decayed down to Fe (or head off to become a heavy element in the upper crust).

That just doesn't happen. We know that elements are stable. It doesn't normally happen that e.g. gold spontaneously explodes into iron, you'd need the extreme conditions inside a supernova to make that happen.

It seems others have already answered you, but just in case: something can't "increase all the way up" and "peak" in the middle.

Below iron fusing nucleons gives you energy; above iron it takes energy. If binding energy increased all the way up you wouldn't be able to have nuclear fission.

>If binding energy increased all the way up you wouldn't be able to have nuclear fission.

You would, because the smaller nuclides have higher total binding energy than the large nuclides. That doesn't contradict every step inbetween being favorable. You are adding hydrogen with zero binding energy.

> Smaller nuclides have higher total binding energy than the large nuclides

i.e. total binding energy literally goes down as you get to larger nuclides.

> That doesn't contradict every step inbetween being favorable.

Yes, it does. Energy is strictly additive.

> You are adding hydrogen with zero binding energy.

Exactly. You are taking a bare nucleon, with zero binding energy, adding it to a system, and reducing the total binding energy (i.e. increasing the potential energy of the system). By conservation of mass-energy, this requires you to put in energy, and increases the total mass of the system.

>total binding energy literally goes down as you get to larger nuclides.

No, it goes up. Iron-56: 492.26 MeV Uranium-238: 1801.7 MeV

So, if you have a great bunch of nucleons, is it more energetically favourable to have them as iron or as uranium?

Iron. That doesn't contradict that fusing more hydrogen with iron is energeticaly favorable, as the binding energy of hydrogen is zero. So you still get energy from fusing it into iron.

Fusing into iron is different from fusing with iron.

Binding energy is a negative energy value; iron having a high binding energy means it is a low energy state, so adding zero-binding-energy hydrogen to that system and decreasing the binding energy increases the potential energy of the system from the low value of iron.