As the previous section showed, nuclei are held together by a strong binding energy, which counteracts the repulsive force of the protons contained within it. Despite this apparent equilibrium, it is not as convenient as it seems. Some isotopes are far more stable than others, and indeed, one natural isotope is so unstable that is spontaneously disintergrates. More on that later…

As it turns out, Iron is the most stable element, and the element Uranium is the most potentially unstable (that is, certain natural and artificial isotopes are highly fissionable). The way one roughly determines the fissionability of an isotope is to divide Z² by A . When this figure is around 45, the electrostatic repulsion of the protons is equal to the binding energy, and the atom no longer has anything holding it together.

So by dividing Z²/A by 45, one can find the fissionability parameter of a given isotope. When this is equal to one, the atom is totally unstable, when it is slightly less than one, the isotope is relatively unstable. For example, ²³⁵U has a fissionability parameter of about 0.8.