Atomic Binding Energy

Iron

Iron has the least binding energy of all the atoms. Fusion of iron with other atoms requires an energy input. Fission of iron to lighter elements requires an energy input.

Iron is the end point of stellar fusion in heavier stars. Fusion ceases once the lighter atoms have fused to iron. The stellar core then collapses under gravity. The energy of the collapse provides the energy that allows iron to fuse and form heavier elements.