Nuclear Stability :- Atomic nucleus is a densely packed collection of protons and neutrons. The very large electrostatic repulsive forces between protons should cause the nucleus to fly apart but nucleons are able to stay together because of strong Nuclear Force.
There are 118 different elements. All these elements have isotopes and as such there are 4000 different nuclides. Out of these, 252 are stable and the rest are unstable. Nuclear stability depends on many factors for example :
(i) The stability of a nucleus is determined by the value of its binding energy per nucleon. Higher the B.E./nucleon, more stable is the nucleus.
(ii) The stability of a nucleus is also determined by its neutron to proton ratio. A plot of neutron number (N) and proton number (atomic number Z) for stable nuclides is shown in figure below:
The solid line in the above plot shows the locations of nuclei that have an equal number of protons and neutrons (N = Z). We note that only light nuclei are on this line i.e. light nuclei are stable only if they contain about the same number of protons and neutrons. Heavy nuclei, on the other hand, are stable only when they have more neutrons than protons. Thus, heavy nuclei are neutron rich compared to lighter nuclei. This indicates that neutrons help to stabilize a nucleus. As the mass number increases, the number of protons in the nucleus increases and hence the electrical repulsive force between them. Therefore, more neutrons are needed to provide the strong attractive forces necessary to keep the nucleus stable. The long narrow region in the above figure containing the stable nuclei is referred to as the valley of stability or zone of stability.
(iii) Nuclear Stability is also determined by the consideration whether it contains an even or odd number of protons and neutrons. Studies show that stable nuclei often contain even number of protons or neutrons or both.
It is found that :
- An even-even nucleus (even Z and even N) is more stable (60% of stable nuclide have even Z and even N).
- An even-odd nucleus (even Z and odd N) or odd-even nuclide (odd Z and even N) is found to be lesser stable while the odd-odd nucleus is found to be less stable.
- Only five odd-odd nuclides are stable : 1H2, 3Li6, 5Be10, 7N14 and 75Ta180