Spontaneous fission is like alpha and cluster decay in that a nuclide splits in two. The difference is that with spontaneous fission, the nuclide splits roughly in half. It is only observed for the heavier nuclides and even then is only a primary decay mode for a handful of nuclides. Spontaneous fission can be generically represented by:
Z AX L M n
2→ + +ν
where L and M are fission products (or fragments), and νn indicates the release of a small whole number (typically 0-4) neutrons. A wide variety of fission products are formed by the decay of a parent, but products with magic numbers (such as 50 and 82) of neutrons and/or protons have increased odds of formation.