At the peak of binding energy, nickel-62 is the most tightly-bound nucleus, followed by iron-58 and iron-56.  (This is the basic reason why iron and nickel are very common metals in planetary cores, since they are produced abundantly as end products in supernovae and in the final stages of silicon burning in Stars). Iron-56 is more common than nickel isotopes because its unstable progenitor nickel-56 is copiously made by staged build-up of 14 helium nuclei inside supernovas, and it then decays to iron-56 within a few weeks. (http://wapedia.mobi/en/Binding_energy?t=3.)
The most tightly bound of the nuclei is 62Ni, a case made convincingly by M. P. Fewell in an article in the American Journal of Physics. Though the championship of nuclear binding energy is often attributed to 56Fe, it actually comes in a close third. The four most tightly bound nuclides are listed in the table below with a tabulation of the binding energy B divided by the mass number A. The curve adapted from Fewell shows those nuclides that are close to the peak.
Data from Wapstra and Bos.
The absolute nuclear binding energy is the hypothetical energy release if a given nuclide were synthesized from Z separate hydrogen atoms and N (equal to A − Z) separate neutrons. An example is the calculation giving the absolute binding energy of the stablest of all nuclei, iron-56:
Remember how I said that the greater the binding energy per nucleon of an atom, the greater it's stability? Well, above is a graph of the relative binding energy per nucleon vs. mass number (total number of nucleons composing an atom). Notice that the nuclei of the light elements are generally less stable than the heavier nuclei up to those with a mass number around 56. The nuclei of the heaviest elements are less stable than the nuclei that have a mass number of around 56. From this, you can see that the nuclei around iron are the most stable. This information implies two methods towards the converting of mass into useful amounts of energy: fusion and fission.
The isotope with the shortest average internal nuclear length, and hence the weakest binding energy per nucleon is Hydrogen 5. The isotopes with the longest internal nuclear lengths are Nickel 62, Iron 56, and Iron 58, and thus have the strongest binding energy per nucleon. The shells of the nucleus fill according to 2, 8, 20, 28, 50, 84, 126. Iron 58 appears to be a point where all four inner shells are completely full (2+8+20+28=58). Iron 56 appears to be a point where the second, third and fourth shells are completely full (8+20+28=56). Nickel 62 appears to be a situation where all four of the inner shells are full and an alpha particle (helium nucleus) may be formed in the 5th shell. (2+8+20+28+4=62). From these observations it may be correct to say that the internal nuclear length equation is dependent on the shell structure and fill pattern of the nucleus.
From this graph we can see the following:
· The vast majority of nuclides have a binding energy of 8 MeV per nucleon.
· Helium has a particularly high value of binding energy per nucleon, much higher than the light isotopes of hydrogen.
· There is a trend for nuclides of nucleon numbers in multiples of 4 to be particularly stable (i.e. have a high binding energy).
· Fe is the most stable nuclide.
· The largest nuclides tend to be less stable, with slightly lower binding energies per nucleon.
Iron has the highest binding energy per nucleon so is the most stable nucleus. If we look at large nuclei (greater than iron), we find that the further to the right (greater nucleon number) the less stable the nuclei. This is because the binding energy per nucleon is getting less. The explanation for this observation lies in that the strong nuclear force that binds the nucleus together has a very limited range, and there is a limit to the number of nucleons that can be crammed into a particular space.
Notice that iron has the highest binding energy per nucleon and that this binding energy drops off as the mass of atoms increases after iron. Recall that stars can only fuse atoms together up to iron, which is the most stable nucleus. Heavy atoms such as uranium have relatively low binding energies per nucleon and if they split apart the binding energy of their daughter atoms is actually higher. The difference in energy shows up as heat, which is how we get power out of the fission of uranium.
iron peak nuclei
This curve indicates how stable atomic nuclei are; the higher the curve the more stable the nucleus. Notice the characteristic shape, with a peak near A=60. These nuclei (which are near iron in the periodic table and are called the iron peak nuclei) are the most stable in the Universe. The shape of this curve suggests two possibilites for converting significant amounts of mass into energy. (http://csep10.phys.utk.edu/astr162/lect/energy/bindingE.html)
How does the average binding energy per nucleon change as Z increases? We find that the average binding energy per nucleon first increases up to iron-56, , then decreases gradually as shown:
The curve shows that nucleons can fall deeper into the nuclear ``well'' in two circumstances:
If a nucleus to the right of breaks into smaller pieces, the nucleons inside the pieces are deeper inside the ``well'' than they had been in the original nucleus. - fission
If two nuclei to the left of combine into a nucleus smaller than , the nucleons inside will find themselves deeper inside the ``well.'' - fusion
The nuclides of intermediate mass number have the largest values for the binding energy per nucleon and the most stable is has a value 8.8 MeV it’s the most stable since it need the most energy to disintegrate.
The smaller values of binding energy per nucleon for higher and lower mass numbers imply that potential source of nuclear energy.
The figures show that nuclides with low mass numbers can produce energy by fusion, when two light nuclei have fused to produce a heavier nucleus.
In contrast heavier nuclides (i.e. higher mass numbers) can produce energy by fission (disintegration) into their nuclei into two lighter nuclei.
In both cases, nuclei are produced having great binding energy per nucleon and therefore
1. more stable nuclei
2 there is consequently a mass transfer during their formation
BINDING ENERGY CURVE
In heavier stars the temperature is higher. In stars of about 10 solar masses, the iron isotope 56Fe is reached. This is the heaviest nucleus that can be formed in the core of stars by nuclear fusion.
After high-mass stars have nothing but sulfur and silicon in their cores, they further contract until their cores reach in the range of 2.7–3.5 GK; silicon burning starts at this point. Silicon burning entails the alpha process which creates new elements by adding the equivalent of one helium nucleus (two protons plus two neutrons) per step
The entire silicon-burning sequence lasts about one day and stops when nickel–56 has been produced. Nickel–56 (which has 28 protons) has a half-life of 6.02 days and decays via beta radiation (in this case, "beta-plus" decay, which is the emission of a positron) to cobalt–56 (27 protons), which in turn has a half-life of 77.3 days as it decays to iron–56 (26 protons). However, only minutes are available for the nickel–56 to decay within the core of a massive star. At the end of the day-long silicon-burning sequence, the star can no longer release energy via nuclear fusion because a nucleus with 56 nucleons has the lowest mass per nucleon (proton and neutron) of all the elements in the alpha process sequence.
In stars, rapid nucleosynthesis proceeds by adding helium nuclei (alpha particles) to heavier nuclei. Although nuclei with 58 and 62 nucleons have the very lowest binding energy, fusing a helium nucleus into nickel–56 (14 alphas) to produce the next element — zinc–60 (15 alphas) — actually requires energy in its production rather than releases any. Accordingly, nickel–56 is the last fusion product produced in the core of a high-mass star. Decay of nickel-56 explains the large amount of iron-56 seen in metallic meteorites and the cores of rocky planets
In stars of about 10 solar masses, the iron isotope 56Fe is reached. This is the heaviest nucleus that can be formed in the core of stars by nuclear fusion.
The star has run out of nuclear fuel and within minutes begins to contract. The potential energy of gravitational contraction heats the interior to 5 GK and this opposes and delays the contraction. However, since no additional heat energy can be generated via new fusion reactions, the contraction rapidly accelerates into a collapse lasting only a few seconds. The central portion of the star gets crushed into either a neutron star or, if the star is massive enough, a black hole. The outer layers of the star are blown off in an explosion known as a Type II supernova that lasts days to months. The supernova explosion releases a large burst of neutrons, which synthesizes in about one second roughly half the elements heavier than iron, via a rapid neutron-capture mechanism.