The three basic characterstics of nuclear forces are as follows:
1. Nuclear forces are very strong attractive forces.
2. Nuclear forces are short range forces.
3. Heavy nuclei for their stability must also have a repulsive component such that the nuclear forces are on the whole attractive.
We now proceed to discuss these points one by one.
Our studies of nuclear reactions shows that the energies involved in unclear reactions are tremendously large as compared to the corresponding energy changes involved in chemical reaction. This shows that the force which are responsible for holding of its nucleons must be very strong. Another clue about the fact that the nuclear forces are very strong is obtained from the enormously large value of nuclear densities. The fact that atomic nuclei are stable and contain protons within nuclear volumes provide another support to the strong nature of nuclear forces. The presence of protons within nucleus must involve enormously large electrostatic repulsive force. The only way one can explain the stability of the nucleus, is to assume the existance of very strong attractive forces between nucleons. These strong (and short range) nuclear forces must overcome the electrostatic repulsive forces. We have seen that the binding energy per nucleon (exception light nuclei) has the same value (~ 8 MeV) for all nuclei. Thus we can conclude that binding energy of a nucleus is proportional the number of nucleons constituting the nucleus. Assuming that every nucleon in a nucleus of mass number A interacts with every other nucleon, the number of interacting pairs would be A(A - 1)/2. Hence the total interaction energy and therefore the binding energy would the proportinoal to A2 . However this is not the case in the least.
A clear deduction is that every nucleon in the nucleus does not interact with every other nucleon. Hence a nucleon is only intteracting with its immediate nearest neighbour. Thus nuclear forces must have a short range. Nuclear forces falls of very rapidly with the distance between two nucleons, the decrease has to be much more rapid than 1/r2.
Experimental studies show that if the nuclear forces holding the nucleons were entirely attractive, heavy nuclei would collapse. This is so because in heavy nuclei, the nucleon’s are so closed logether so that each one is within the range of attractive forces of every other nulceon. This would ultimately mean that within the same nuclear dimensions the attractive energy acting on each particle would grow with the total number of nucleons. This contradicts the facts observed in reality. Thus we all are led to unescaple conclusion that nuclear force must also have a repulsive component that saves the nuclei from collapsing. Thus nuclear forces only on the balance are attractive.
