The mass loss or binding energy per nuclear particle (protons and neutrons) rises rapidly to a maximum at iron, then falls. Iron is the most stable nucleus of all. The mass losses or binding energies per nucleon are plotted above for all nuclei from helium through uranium. After some initial minor irregularities in the first- and second-row elements, the values settle down to a smooth curve, which rises to a maximum at iron, then begins a long descending slope through uranium and beyond.

This curve gives us information that was used in the discussion of stellar synthesis of elements in Chapter 8. Iron is the most stable nucleus of all. For elements with smaller atomic numbers than iron, fusion of nuclei to produce heavier elements releases energy, because the products are lighter and more stable on a per-nucleon basis than the reactants. In contrast, beyond iron, fusion absorbs energy because the products are heavier on a per-nucleon basis than the reactants.