It is well known that in chemical reactions, and more specifically in processes used to obtain energy, as for example oil, gas and carbon combustion, only some electronVolts (eV) can be obtained for every couple of atoms involved. This depends on the fact that binding energies of external atomic electrons are in the eV range.
On the other hand, in nuclear transformations, the energy quantities that can be absorbed or released are of the order of mega-electronVolts (MeV) for every couple of nuclei involved in the process. As a consequence, for every given amount of energy obtained, the mass to be transformed by a nuclear process is about a millionth of that necessary for a combustion.
It is a general rule, valid for all stable compounds, that the mass for a compound is lower than the total mass of all constituents. In such conditions, the mass-energy conservation principle guarantees stability against the spontaneous disintegration into the components. As a consequence, for the nuclei, the mass
of every stable nucleus turns out to be lower than the sum of the masses of all its components (protons and neutrons).
If we denote by mp and mn the mass values of free protons and neutrons, and by np and nn the numbers of protons and neutrons belonging to a given (stable) nucleus N, the nuclear stability is insured by the always positive difference
Figure 1: Binding Energy versus number of nucleons
where mN represents the nucleus mass.
An important parameter, whose value is directly connected to the nuclear stability, is the binding energy for a nucleon B , defined as the ratio between Δ and the mass number, that is the total nucleon number np+ nn:
Fig.1 shows, for all stable nuclei, the binding energy B (expressed in Mev/cˆ2) versus the total number of nucleons (protons and neutrons) .
As is evident from the definition of B, nuclear stability is characterized by large values of the binding energy for nucleon. Nuclei having a mass number around 60 (as Fe, Co and Ni) are characterized as particularly stable.
Fig.1 shows clearly the two existing possibilities in order to obtain energy from nuclear transformations: they consist in producing more stable nuclei starting from low mass or from high mass nuclei. Such two processes are respectively referred to as fusion and fission.
Fusion processes occur naturally in the stars, where helium and other elements are produced, starting from hydrogen. Other similar phenomena, which lead to the production of heavier elements, occur in hydrogen rich stellar atmospheres, after supernovae collapse.
Artificial fission processes are obtained in nuclear reactors by means of neutron interactions with Uranium or Thorium which induce nuclear breaking and neutrons release. There exist no natural fission processes, with the only exception of a flooded Uranium mine in Gabon  which reproduced, about two millions years ago, physical conditions similar to the ones occurring in a nuclear reactor.