Iron atom is the boundary of energy absorption or energy release in nuclear reaction. Any nuclear fusion lighter than iron can release energy, and any nuclear fusion heavier than iron needs to absorb energy.

No matter nuclear fission or nuclear fusion, iron nuclei will absorb energy, because iron nuclei are the most stable nuclei. The stability of nuclear structure is determined by two forces, namely, nuclear force (strong interaction gravity) and coulomb force (electromagnetic force). In the nucleus, protons have the same charge, so coulomb force will make protons repel each other. However, due to the nuclear force between protons and neutrons, the nuclear force firmly binds protons and neutrons together like glue, preventing protons from repelling each other and separating, thus leading to the disintegration of the nucleus, which is why there must be neutrons in heavy nuclei.

The nuclear force is many times stronger than Coulomb 100, so even if there is repulsion between protons, the nuclear force can firmly bind the nucleus. But nuclear power is not infinitely powerful. With the increase of the number of protons in the nucleus, the total coulomb repulsion will gradually exceed the nuclear force, and the nucleus will become more and more unstable. This is why the overweight elements are radioactive elements, because the nucleus hopes to release some protons by radiation, so that the Coulomb repulsion in the nucleus will not exceed the nuclear force and the nucleus will not collapse. When we talk about the most stable nuclear, we should consider both nuclear fusion and nuclear fission. Therefore, the most stable iron core is actually an "average score" result of its ability to resist nuclear fusion and nuclear fission.