<P> When a nucleon such as a proton or neutron is added to a nucleus, the nuclear force attracts it to all the other nucleons of the nucleus (if the atom is small enough), but primarily to its immediate neighbours due to the short range of the force . The nucleons in the interior of a nucleus have more neighboring nucleons than those on the surface . Since smaller nuclei have a larger surface area - to - volume ratio, the binding energy per nucleon due to the nuclear force generally increases with the size of the nucleus but approaches a limiting value corresponding to that of a nucleus with a diameter of about four nucleons . It is important to keep in mind that nucleons are quantum objects . So, for example, since two neutrons in a nucleus are identical to each other, the goal of distinguishing one from the other, such as which one is in the interior and which is on the surface, is in fact meaningless, and the inclusion of quantum mechanics is therefore necessary for proper calculations . </P> <P> The electrostatic force, on the other hand, is an inverse - square force, so a proton added to a nucleus will feel an electrostatic repulsion from all the other protons in the nucleus . The electrostatic energy per nucleon due to the electrostatic force thus increases without limit as nuclei atomic number grows . </P> <P> The net result of the opposing electrostatic and strong nuclear forces is that the binding energy per nucleon generally increases with increasing size, up to the elements iron and nickel, and then decreases for heavier nuclei . Eventually, the binding energy becomes negative and very heavy nuclei (all with more than 208 nucleons, corresponding to a diameter of about 6 nucleons) are not stable . The four most tightly bound nuclei, in decreasing order of binding energy per nucleon, are 62 Ni, 58 Fe, 56 Fe, and 60 Ni . Even though the nickel isotope, 62 Ni, is more stable, the iron isotope 56 Fe is an order of magnitude more common . This is due to the fact that there is no easy way for stars to create 62 Ni through the alpha process . </P> <P> An exception to this general trend is the helium - 4 nucleus, whose binding energy is higher than that of lithium, the next heaviest element . This is because protons and neutrons are fermions, which according to the Pauli exclusion principle cannot exist in the same nucleus in exactly the same state . Each proton or neutron's energy state in a nucleus can accommodate both a spin up particle and a spin down particle . Helium - 4 has an anomalously large binding energy because its nucleus consists of two protons and two neutrons, so all four of its nucleons can be in the ground state . Any additional nucleons would have to go into higher energy states . Indeed, the helium - 4 nucleus is so tightly bound that it is commonly treated as a single particle in nuclear physics, namely, the alpha particle . </P>

When did nuclear fusion began in the sun