text: From the description of the beginning of the fission process to the "scission point," it is apparent that the change of the shape of the nucleus is associated with a change of energy of some kind. In fact, it is the change of two types of energies: (1) the macroscopic energy related to the nuclear bulk properties as given by the liquid drop model and (2) the quantum mechanical energy associated with filling the shell model orbitals. For the nuclear bulk properties with small distortions, the surface, , and Coulomb, , energies are given by:
  where  and  are the surface and Coulomb energies of the undistorted spherical drops, respectively, and  is the quadrupole distortion parameter. When the changes in the Coulomb and surface energies (, ) are equal, the nucleus becomes unstable with respect to fission. At that point, the relationship between the undistorted surface and Coulomb energies becomes:
  where  is called the fissionability parameter. If , the liquid drop energy decreases with increasing , which leads to fission. If , then the liquid drop energy decreases with decreasing , which leads to spherical shapes of the nucleus.
  The Coulomb and surface energies of a uniformly charged sphere can be approximated by the following expressions:
  where  is the atomic number of the nucleus,  is the mass number of the nucleus,  is the charge of an electron,  is the radius of the undistorted spherical nucleus,  is the surface tension per unit area of the nucleus,  and . The equation for the fissionability parameter then becomes:
  where the ratio of the constant  is referred to as . The fissionability of a given nucleus can then be categorized relative to . As an example, plutonium-239 has a  value of 36.97, while less fissionable nuclei like bismuth-209 have a  value of 32.96.
  For all stable nuclei,  must be less than 1. In that case, the total deformation energy of nuclei undergoing fission will increase by an amount , as the nucleus deforms towards fission. This increase in potential energy can be thought of as the activation energy barrier for the fission reaction. However, modern calculations of the potential energy of deformation for the liquid drop model involve many deformation coordinates aside from  and represent major computational tasks.