Why do heavy nuclei tend to undergo fission?

Heavy nuclei tend to undergo fission due to their instability caused by the strong repulsive forces between protons.

In more detail, the nucleus of an atom is composed of protons and neutrons. Protons, being positively charged, naturally repel each other due to the electromagnetic force. In small nuclei, this repulsion is counterbalanced by the strong nuclear force, which is a short-range force that attracts protons and neutrons to each other. However, as the nucleus gets larger, the strong nuclear force can't effectively hold the protons together against their electromagnetic repulsion because its range is limited.

In heavy nuclei, such as uranium or plutonium, the number of protons is so large that the repulsive electromagnetic forces become significant. The strong nuclear force is not sufficient to hold the nucleus together stably. This makes the nucleus unstable and prone to breaking apart, or undergoing fission.

Fission is a nuclear reaction in which the nucleus of an atom splits into two or more smaller nuclei, along with the release of a large amount of energy. This process is facilitated by the absorption of a neutron, which increases the instability of the nucleus. The resulting fragments are also usually radioactive, leading to a chain reaction of further fissions if not controlled.

In summary, the tendency for heavy nuclei to undergo fission is a result of the interplay between the strong nuclear force and the electromagnetic force. The strong nuclear force, while powerful, has a limited range and is unable to effectively counterbalance the repulsion between the large number of protons in heavy nuclei. This results in an unstable nucleus that is prone to fission.