We know that the nucleus occupies the central position in an atom. Nearly all the mass of an atom is concentrated in the nucleus. Since nucleus has a radius of only about 10-15m, the density of nucleus is enormous, more than 1012 times that of lead.

Although about 20 different particles have been detected as the product of different nuclear reactions, it does not mean that all these particles are present in the nucleus. Actually nucleus is believed to consist of two building bocks, protons and neutrons, which are collectively called as nucleons. Other particles (fundamental particles of nucleus are electrons, antiproton, positron, neutrino, photon, graviton, meson and gamma particles) are considered as created by stresses in which energy is converted into mass or vice versa, e.g. an electron (beta-particle) from a radioactive nucleus may be regarded as derived from a neutron in the following way:-

Neutron ---? Proton + Electron

Similarly, photons are produced from internal stresses within the nucleus. Since the radius of nucleus is very small (? 10-15m), two protons lying in the nucleus are found to repel each other with an electrostatic force of about 6 tonnes. Now since the radius of the nucleus is of the order of 10-15m, the question arises how such a large number of protons are present in such a tiny space without repulsion.

It is postulated that stronger proton-neutron, neutron-neutron and even proton-proton attractive forces exist in the nucleus. These attractive forces are called nuclear forces. Nuclear forces are nearly 1021 times stronger than electrostatic forces. Unlike electrostatic forces which operate over long ranges also, the nuclear forces operate only within small distance of about 1 X 10-15m (1 fermi) and drop rapidly to zero at a distance of 1.4 X 10-12m. Hence these are referred to short range forces. Unlike electrostatic forces, nuclear forces do not obey the inverse square law. When we speak of stability of a nucleus it may be explained in many different ways, but the most accepted theory about this stability is based upon the fact that the observed atomic mass of all known isotopes (except hydrogen) is always less from the sum of the weights of protons and neutrons (nucleons) and electrons present in it. The difference between the expected mass and the actual mass is called as mass defect. The natural question here is that where this mass has gone? It has been suggested that this mass is converted into energy which is released in the formation of the given nucleus from individual protons and neutrons. The release of energy results in the stability of the nucleus and helps in binding the nucleons together. So this binding energy of the nucleus is very strong and is called as its binding energy.