Fission
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.
When they contain an excess of neutrons
The nuclei of heavier atoms simply have more neutrons and protons than do lighter atoms.
I have the anser, but, I'm not telling you.
polymorphnulear
mitosis
More neutrons than protons.
Jerry Lewis Pietenpol has written: 'Atomic corrections to electric-quadrupole gamma decay by heavy nuclei' -- subject(s): Gamma decay, Heavy nuclei, Spectrum analysis
Telophase
Some protists have two nuclei.
Heavy water is water whose atomic structure consists of oxygen and heavy hydrogen. Heavy hydrogen, or deuterium, is hydrogen with neutrons in its nuclei.