Radionuclide

A radionuclide (radioactive nuclide, radioisotope or radioactive isotope) is a nuclide that is unstable and known to undergo radioactive decay into a different nuclide, which may be another radionuclide (see decay chain) or be stable. Radiation emitted by radionuclides is almost always ionizing radiation because it is energetic enough to liberate an electron from another atom.

Radioactive decay is a random process at the level of single atoms: it is impossible to predict when one particular atom will decay. However, for a collection of atoms of a single nuclide, the decay rate (considered as a statistical average), and thus the half-life (t_1/2) for that nuclide, can be calculated from the measurement of the decay. The range of the half-lives of radioactive atoms has no known limits and spans a time range of over 55 orders of magnitude.

All the chemical elements have radionuclides - even the lightest element, hydrogen, has one well-known radionuclide, tritium (though helium, lithium, and boron have none with half-life over a second). Elements heavier than lead (Z > 82), and the elements technetium and promethium, have only radionuclides and do not exist in stable forms, though bismuth can be treated as stable with the half-life of its natural isotope being over a trillion times longer than the current age of the universe.