Why is iron 56 considered as the most stable nucleus?

There are a few key reasons for this:

Binding Energy per Nucleon: Iron-56 has the highest binding energy per nucleon (𝐵/A) among all naturally occurring elements. Binding energy is the energy required to completely separate a nucleus into its constituent protons and neutrons. The high binding energy per nucleon indicates that iron-56 is very tightly bound, making it energetically favorable and stable compared to other nuclei.

Nuclear Stability Curve: The concept of the nuclear stability curve, also known as the "binding energy curve" or "valley of stability," illustrates the relationship between the number of protons and neutrons in a nucleus and its stability. Iron-56 lies close to the peak of this curve, indicating optimal stability. Nuclei with fewer or more nucleons than iron-56 tend to undergo processes such as radioactive decay to approach the stability of iron-56.

Nuclear Shell Model: The stability of nuclei can also be explained by the nuclear shell model, which describes the arrangement of nucleons (protons and neutrons) in energy levels or "shells" within the nucleus, similar to electron shells in atoms. Iron-56 has a particularly favorable configuration of protons and neutrons that fills several nuclear shells, making it exceptionally stable according to this model.

Astrophysical Significance: Iron-56 plays a crucial role in stellar nucleosynthesis. Fusion processes in stars produce elements up to iron-56, but elements heavier than iron-56 typically require input energy rather than releasing energy through fusion. This means that iron-56 marks a boundary in stellar nucleosynthesis, beyond which elements are typically formed through processes like supernova nucleosynthesis or neutron capture.

Overall, the combination of its high binding energy per nucleon, its position on the nuclear stability curve, its favorable nuclear shell structure, and its astrophysical significance make iron-56 one of the most stable nuclei known.