Atomic nuclei contain protons and neutrons. The stability of a nucleus depends upon the number of protons and neutrons are nearly equal, then the nucleus is stable. The nuclei in which the number of neutrons is more than the number of protons are unstable.
The spontaneous decay of an unstable heavier nucleus with a simultaneous emission of certain radiations is called radioactivity. For example, uranium-238 isotope ( 23892 U) disintegrates to emit α-rays (consisting of doubly charged helium ion, He2+).
Uranium-238 → Thorium-234 + α-particle
Or, 23892 U → 23490Th + 42He2+ ( α-particle)
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The substances which show radioactivity are called radioactive substances.
It may be pointed out here, that radioactivity is the property of the nucleus, i.e., during radioactive disintegrations, only the nuclei of the radioactive material undergo a change.
Atomic nuclei contain protons and neutrons. The stability of a nucleus depends upon the number of protons and neutrons are nearly equal, then the nucleus is stable. The nuclei in which the number of neutrons is more than the number of protons are unstable.
The nuclei of the lighter elements (elements having lower atomic numbers), contain nearly equal number of protons and neutrons. As a result, the nuclei of lighter elements are stable. The lighter elements do not show any radioactivity because the nuclei of lighter elements contain nearly equal number of protons and neutrons.
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On the other hand, the nuclei of heavier elements (elements having higher atomic numbers), contain more neutrons than protons. As a result, the nuclei of heavier elements are unstable, and show radioactivity. The heavier elements show radioactivity because the nuclei of these elements contain more neutrons than protons.