The binding energy of a nucleus can be defined as the energy required
to separate a nucleus into individual nucleons. The mass of the nucleus
is less than the sum of the protons and neutrons of which it is
composed. The difference in mass between the nucleus and the sum of all
the protons and neutrons is known as the mass defect and it is this
which is transformed into the binding energy. The relationship between
mass and energy is given in Einstein's equation below where E is the
energy released, m is the mass defect and c is the speed of light.
E = mc^2
As the term c^2 is a large constant, a small loss of mass is equivalent to the loss of a large amount of energy. The Einstein equation explains why nuclear reactions are such an important source of energy. If the binding energy per nucleon was plotted against molar mass you would find that elements with a molar mass of around 60 are the most stable. Elements with nuclei heavier than this should be able to split up and form lighter, more stable nuclei with the release of energy. This process is known as fission.