Nuclear models
There are two models that attempt to explain the stability of nuclei. (a) In the liquid-drop model, nuclear matter is treated as if it does not have any structure, and the deformation of nuclei depends on whether the repulsive force of the protons can overcome the surface tension of the "drop". In this model, heavier nuclei are more likely to split into two via spontaneous fission than lighter nuclei. (b) The microscopic nuclear theory, on the other hand, describes nuclei in terms of proton and neutron shells, which can allow certain heavy nuclei to live much longer. The difference between the two models is particularly clear for the heavy element 108 (red curves). In the liquid-drop model the absence of a fission barrier means that 108 has a half-life of about 10-19 s, whereas in the microscopic model, shell effects increase the height of the fission barrier such that a neutron-rich isotope of element 108 (i.e. with N = 184) lives for at least 1015 s.