The answer is: more protons than neutrons
The stability of the nucleus is based on the ratio of neutrons and protons present in the nucleus. if the number of protons is equal to the number neutrons then the nucleus
is stable but in case the number of protons is more than the number neutrons or the other way round the nucleus to gain stability starts interconversion of neutron to proton
or electron or decreases proton numbers. In case of nucleus containing high energy, the nucleus releases it. Such nuclei which release energy to gain stability are called
radioactive nuclei and the phenomenon is called radioactivity.
