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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.
The graph below shows the nuclear binding energy per nucleon of various elements.[clarification needed] 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.