What is Radioactivity ?

Radioactivity :  Some isotopes, called radioisotopes, have unstable nuclei that release high energy particles or radiant energy, or both. An example is tritium, or ³H. Tritium has two neutrons and a proton in its nucleus. When an unstable nucleus disintegrates into particle fragments and radiation, the element gradually forms an element with lower atomic weight and number.

Radioisotopes have a unique half-life, or time period in which half of the atoms disintegrate into a nonradioactive state. The half-life of the radioisotope ¹⁴C is used in dating fossils and archeological specimens.

Biologists also use radioactive atoms such as tritium as "labels" in molecules so that their pathways in biological reactions can be traced. The radiation is detected with radiation counters or by its reaction with photographic films, which provides a track.