Iron peak

The iron peak is a local maximum in the vicinity of Fe (Cr, Mn, Fe, Co and Ni) on the graph of the abundances of chemical elements, as seen below.

For elements lighter than iron on the periodic table, nuclear fusion releases energy. For iron, and for all of the heavier elements, nuclear fusion consumes energy, but nuclear fission releases it. Chemical elements up to the iron peak are produced in ordinary stellar nucleosynthesis. Heavier elements are produced only during supernova nucleosynthesis. This is why we have more iron peak elements than in its neighbourhood.

Abundances of the chemical elements in the Solar System. Hydrogen and helium are most common, from the Big Bang. The next three elements (Li, Be, B) are rare because they are poorly synthesized in the Big Bang and also in stars. The two general trends in the remaining stellar-produced elements are: (1) an alternation of abundance in elements as they have even or odd atomic numbers, and (2) a general decrease in abundance, as elements become heavier. The "iron peak" may be seen in the elements near iron as a secondary effect, increasing relative abundances of elements with nuclei most strongly bound.

Binding energy

The graph below shows the nuclear binding energy per nucleon of various elements. Increasing values of binding energy can be thought as the energy released when a collection of nuclei is rearranged into another collection for which the sum of nuclear binding energies is higher. As can be seen, light elements such as hydrogen release large amounts of energy (a big increase in binding energy) when combined to form heavier nuclei—the process of fusion. Conversely, heavy elements such as uranium release energy when converted to lighter nuclei—processes of alpha decay and nuclear fission. 56
28Ni
is the most thermodynamically favorable in the cores of high-mass stars (see also Silicon burning process). Although iron-58 and nickel-62 have even higher (per nucleon) binding energy, their synthesis cannot be achieved in large quantities because the required number of neutrons is typically not available in the stellar nuclear material.

See also

• Abundances of the elements (data page)