Question

Why is Iron the most stable element?

Answer 1

Stability means that the given atom or molecule is stable and exists like this only in nature. The stability may be the chemical stability, structural stability or the nuclear stability. In a periodic table the noble gases are regarded as the most stable in terms of chemical stability and not in terms of structural or nuclear stability.

Complete answer:
Iron is the most stable element in terms of structural stability and in terms of nuclear stability but not in terms of the chemical stability. However iron- $56$ is the most stable isotope and not all forms of iron. The reason could be found in the binding energy and mass per nucleon.
The binding energy per nucleon is the energy required to break the atom into its sub atomic particles i.e., breaking an atom to its subatomic parts like electrons, protons, neutrons, neutrino, muons, leptons, hardrons, mesons, baryons, hyperons, alpha particles, and many more. The binding energy calculated for iron- $56$ was found to be $8.8$ MeV. This is the highest of all the atoms present till date. Much higher than helium which is a noble gas.
Another reason for the stability could be the mass defect or mass per nucleon. Iron- $56$ isotope has the lowest mass defect till date found to be.

Note:
The binding energy per nucleon and the mass defect are two other criteria of stability other than the reactive stability. Noble gases are said to be stable as they do not react normally. But iron does. But if we take into account the above mentioned two criteria, iron- $56$ is found to be more stable than the noble gases.