The substances which disintegrate (or decay) by the spontaneous emission of radiation are called radioactive substances. Examples: Uranium, radium, polonium, thorium, actinium, etc.

The isotopes of nearly all the elements of an atomic number higher than 82 (i.e. lead) are radioactive. These are called natural radioactive substances.

The phenomenon of radioactivity cannot be due to the orbital electrons which could easily be affected by such changes. It should therefore be the property of the nucleus.

Thus, radioactivity is a nuclear phenomenon. It is the process of spontaneous emission of α, β, and γ radiations from the nuclei of atoms during their decay.

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