Fission of nuclei is possible because the binding energy per nucleon in them:

Step-by-step solution

proportional to the atomic mass of the nucleus. 5. As the mass number increases, the binding energy per nucleon gradually decreases and ultimately binding energy per nucleon of heavy nuclides (such as uranium, etc) is comparatively low. Hence, these nuclides are relatively unstable. 6. Fission occurs generally for elements with high atomic mass. 7. Therefore, we can say that fission of nuclei is possible because the binding energy per nucleon in them decreases with mass number at high mass numbers.