Question

Why do only some atoms like uranium undergo nuclear fission????

Answer 1

Only heavier atoms undergo nuclear fission.

The nuclear force saturates due to its short range, meaning that heavier nuclei have protons that don't attract each other via the nuclear force but still repel each other electrostatically. This is why heavier nuclei tend to have more neutrons in relation to the number of protons - the neutrons only attract. The binding energy per nucleon, a measure of how tightly a nucleus is bound, peaks at about 60 nucleons. (There's also a sharp peak at 4; the alpha particle, which is a He-4 nucleus, is very tightly bound) So light nuclei require energy to split apart and would release energy only if you can fuse them together. You might expect that anything heavier than 120 nucleons would fission, but these nuclei are still bound together, so the two parts you would get in fission aren't likely to fly apart. It's not until you get into the elements heavier than lead (all of which are radioactive) that you find nuclei whose binding energy per nucleon is low enough that the fission fragments could tunnel apart.