Fission generally refers to the process by which a heavy atomic nucleus splits into smaller nuclei, along with the release of energy. It typically involves heavier nuclei, such as uranium-235 or plutonium-239. These heavier nuclei can become unstable and undergo fission when they absorb a neutron, leading to the release of a significant amount of energy and additional neutrons.
In contrast, lighter nuclei tend to undergo a different nuclear process called fusion, in which two light nuclei combine to form a heavier nucleus, again releasing energy in the process. Fusion is the process that powers stars, including our sun.
In summary, fission primarily involves heavier nuclei, while fusion involves lighter nuclei.