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. Chemical elements up to the iron peak are produced in ordinary stellar nucleosynthesis, with the alpha elements being particular abundant. Some heavier elements are produced by less efficient processes such as the r-process and s-process. Elements with atomic numbers close to iron are produced in large quantities in supernova due to explosive oxygen and silicon fusion, followed by radioactive decay of nuclei such as Nickel-56. On average, heavier elements are less abundant in the universe, but some of those near iron are comparatively more abundant than would be expected from this trend.
The graph below shows the nuclear binding energy per nucleon (total average binding energy per nucleic subatomic particle (protons and neutrons) of a given element) of those 7 "key"(to fusion & fission study) elements denoted in the graph by their abbreviations (4 with more than 1 isotope referenced). 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
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.