1. Mass defect `(Delta M)` : The difference in mass of a nucleus and its constituents is called the mass defect. The nuclear mass M is always less than the total mass, `Sigma m`, of its constituents.
Mass defect, `(Delta M)=[Z m_(P)+(A-Z)m_(n)-M]`
2. Binding energy : The energy required to break the nucleus into its constituent nucleons is called the binding energy.
Binding Energy, `(E_(b))=Delta MC^(2)=[Zm_(P)+(A-Z)m_(n)-M]931.5 MeV`.
Nuclear binding energy is an indication of the stability of the nucleus.
Nuclear binding energy per nucleon `E_(bn)=(E_(b))/(A)`.
3. The following graph represents how the binding energy per nucleon varies with the mass number A.
4. From the graph that the binding energy is highest in the range 28 lt A lt 138. The binding energy of these nuclei is very close to 8.7 MeV.
5. With the increase in the mass number the binding energy per nucleon decreases and consequently for the heavy nuclei like Uranium it is 7.6 MeV.
6. In the region of smaller mass numbers, the binding energy per nucleon curve shows the characteristic minima and maxima.
7. Minima are associated with nuclei cotaining an odd number of protons and neutrons such as `._(3)^(6)Li, ._(5)^(10)B, ._(7)^(14)N` and the maxima are associated with nuclei having an even number of protons and neutrons such as `._(2)^(4)He, ._(6)^(12)C, ._(8)^(16)O`.
Significance :
8. The curve explains the relationship between binding energy per nucleon and stability of the nuclei.
9. Uranium has a relatively low binding energy per nucleon as 7.6 MeV. Hence to attain greater stability Uranium breaks up into intermediate mass nuclei resulting in a phenomenon called fission.
10. On the other hand light nuclei such as hydrogen combine to form heavy nucleus to form helium for greater stability, resulting in a phenomenon called fusion.
11. Iron is the most stable having binding energy per nucleon 8.7 MeV, and it neither undergoes fission per fussion.
