The stable and non-radioactive elements can also be changed into radioactive elements by bombarding them with protons, neutrons or alpha particles.
When a tree dies, the radio carbon-14 present inside the plant starts decaying.
Nuclei which do not emit radiations naturally are called stable nuclei.
In general, most of the nuclei with atomic number 1 to 82 are stable nuclei.
More than 1,000 radioactive isotopes of the various elements are known.
Approximately 50 of these are found in nature; the rest are produced artificially as the direct products of nuclear reactions or indirectly as the radioactive descendants of these products.