Binding energy per nucleon determines the stability of an atomic nucleus. The binding
energy is the total energy which is necessary to split the nucleus into constituent parts.
A nucleus can try to increase its stability (and thus the binding energy per nucleon) if it
undergoes nuclear fission or nuclear fusion. During these processes, the splitting (fission) or
merging (fusion) of the nuclei of the atoms releases nuclear energy.
Generally the fusion of two nuclei with masses which are lower than iron (which, together
with nickel, has the largest binding energy per nucleon) releases energy. However the fusion
of nuclei which are heavier than iron absorbs energy. One can see the opposite with regard
to the reverse process, nuclear fission. Generally this means that fusion occurs for lighter
elements only (e.g. hydrogen isotopes). Normally fission only occurs for heavier elements
(e.g. uranium and plutonium).