Nucleus of an atom become radio-active when increase in nuclear force.
PACE WITH INCREASE IN THE REPULSIVE FORCE :
i.e.when there are too many neutrons compared to protons or too many protons compared to neutrons in the nucleus of atom. Or The nucleus of the atom becomes radioactive if it is a radio-isotope, i.e. number of neutrons in the nucleus exceeds the number of protons inside it.
For example :
