In fission, an atom is split into two or more smaller, lighter atoms. Fusion, in contrast, occurs when two or more smaller atoms fuse together, creating a larger, heavier atom. For the former, the energy released by fission is a million times greater than that released in chemical reactions, but lower than the energy released by nuclear fusion. On the other hand, fusion process is three to four times greater than the energy released by fission. Hope this would help. Have a nice day.