The constituents of a nucleus are neutrons and protons, collectively known as neucleons . Mass of a nucleus is always less than the sum of the masses of its constituent nucleons, the difference between the two is called the mass defect and the equivalent energy the binding energy of the nucleus. Binding energy of a nucleus is the amount of energy required to separate the nucleus into its constituent nucleons.
For a nucleus X of mass with atomic number and mass number , the binding energy is given by
The adjoining figure gives a plot of the binding energy per nucleon of the elements. It may be noted that the nucleus Fe is the most tightly bound nucleus with a binding energy of about 8.8 MeV per nucleon. This is the reason why the iron group of nuclei are the most stable of the nuclei.
X of mass 
with atomic number 
and mass number 
, the binding energy is given by ![$\displaystyle E_B = \left[ Zm_p + (A-Z)m_n-M\right]c^2$](img/img389.png){kind=small}
Fe is the most tightly bound nucleus with a binding energy of about 8.8 MeV per nucleon. This is the reason why the iron group of nuclei are the most stable of the nuclei. 
