text: Spontaneous fission arises as a result of competition between the attractive properties of the strong nuclear force and the mutual coulombic repulsion of the constituent protons. Nuclear binding energy increases in proportion to atomic mass number (A), however coulombic repulsion increases with proton number (Z) squared. Thus, at high mass and proton numbers, coulombic repulsion overpowers the nuclear binding forces, and the nucleus is energetically more stable as two separate fragments than as a single bound system.
  Spontaneous fission is usually a slow process, as the nucleus cannot simply jump to the lower energy (divided) state. Instead it must tunnel through a potential barrier, with a probability determined by the height of the barrier. Such a barrier is energetically possible for all A ≥ 93, though its height generally decreases with increasing Z, and fission is only practically observed for A ≥ 232.
  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: 
  with  and . 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.
  Shell effects and nucleon pairing effects may further affect observed half-lives. Decays of odd-A nuclides are hindered by 3–5 orders of magnitude compared to even–even nuclides. The barrier to fission is expected to be zero around A = 300, though an island of stability may exist centred around Z = 114, N = 184.
  To date, true ab initio models describing the complete fission process are not possible. Computational theories based on Hartree-Fock or density-functional theory approaches have been developed, however computational complexity makes it difficult to reproduce the full behaviour. The semi-classical liquid drop model provides a primarily qualitative description of the phenomenology by treating the nucleus as a classical drop of liquid to which quantum corrections can be applied, which provides a useful conceptual picture that matches in part with experimental data, but ignores much of the quantum nature of the system and fails at more rigorous predictions.
  In this model, as with a classical liquid drop, a "surface tension" term is introduced which promotes the spherical shape of the nucleus. Acting in opposition is coulombic repulsion term, which acts to increase the distance between repelling proton pairs and thus promotes elongation of the nucleus into an oval shape. As the deformation of the nucleus increases, and particularly for large nuclei due to their stronger coulombic repulsion, the nucleus may find itself in a state where a thin neck develops, forming a bridge between two clusters of nuclear matter which may exceed the ability of the surface tension to restore the undeformed shape, eventually breaking into two fragments at the "scission point". Introducing the effects of quantum tunnelling, the nucleus always has a chance to scission which increases with increasing deformation, and may do so even if the deformation is insufficient to trigger rupture of the neck. After separation, both fragments are highly positively charged and therefore gain significant kinetic energy via their mutual repulsion as they accelerate away from each other.
  Shape isomers (also called fission isomers) are excited nuclear states existing before scission which may deviate from the spherical geometry, increasing nuclear deformation compared to the ground state without undergoing full fission. These states are metastable – a nucleus is this state may, on timescales between nanoseconds and microseconds, either decay back to the ground state via gamma-emission, or tunnel through the scission barrier and break apart. Should the nucleus find itself in this state, either through quantum tunnelling or via random statistical fluctuation, the barrier for fission is much reduced, as shape isomers are always at a higher energy level than the ground sate and therefore are no longer required to tunnel through the entire barrier. The resulting increased probability for fission reduces the effective half-life of the nuclide. Triple-humped barriers have been suggested for some nuclear species such as , further reducing its observed half-life.