The stability of a nuclide against fission is expressed as the ratio of the Coulomb energy to the surface energy, which can be empirically estimated as the fissility parameter, x: x ≈ ≈ Z 2 50.88 A ( 1 − − η η I 2 ) with I = N − − Z A and η η ≈ ≈ 1.78 . For light nuclei, x is small and a sizeable fission barrier exists. As nuclear mass increases, so too does the fissility parameter, eventually approaching and exceeding unity, where stability against fission is lost altogether.