Fission of nuclei is possible because the binding energy per...

The binding energy per nucleon for the middle nuclides (from $A=20$ to $A=56$ ) is maximum. Hence, these are more stable. 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. So, they can be fissioned easily.