Question

why does fission of heavy nuclei tend to produce free neutrons?

Short answer

In the heavy nuclei, the Coulomb repulsion of the protons become important and thus the heavy nuclei tend to have more neutrons than protons

Step-by-step solution

Given data

Heavy nuclei fission is producing free neutrons

Concept of Nuclear fission

Nuclear fission, subdivision of a heavy atomic nucleus, such as that of uranium or plutonium, is into two fragments of roughly equal mass.

Explanation of nuclei fission producing free neutrons

In the heavy nuclei, the Coulomb repulsion of the protons become important and thus the heavy nuclei tend to have more neutrons than protons.

The fission products of the heavy nuclei are the intermediate-mass nuclei, which are much lighter and tend to have equal number of neutrons and protons.

The energy is lowered if the extra neutrons are simply freed.

Thus, the fission of the heavy nuclei will produce free neutrons