Why heavy nuclei are unstable?

Answer 1

Heavy, i.e. large, nuclei are unstable because their size is such that the attractive strong nuclear force starts to lose out over the repulsive effect of the electromagnetic interaction. The happens because the distance coefficient for the strong nuclear forces drops off more rapidly than does the electromagnetic interaction.

Answer 2

the heavy nuclei have vey small value of their binding energy so very less energy will be requried to split heavy nuclei that 's why heavy nuclei are unstable