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
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.