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A nuclide (from nucleus, originally from Latin, meaning kernel of a nut) is an atomic species characterized by the specific constitution of its nucleus, i.e., by its number of protons Z, its number of neutrons N, and its excited state. [1] A set of nuclides with equal proton number (atomic number), i.e. of the same chemical element, but different neutron numbers, are called isotopes of the element. Particular nuclides are still often loosely called "isotopes", but the term "nuclide" is the correct one in general (i.e. when Z is not fixed). A set of nuclides with equal mass number A but different atomic number are called isobars (isobar = equal in weight), and isotones are nuclides of equal neutron number but different proton numbers. The name isotone has been derived from the name isotope to remind that in the first group of nuclides it is the number of neutrons (n) that is constant, whereas in the second the number of protons (p).[2] There are about 256 nuclides in nature which are so stable that they have never been observed to decay. They occur among the 80 different elements which have one or more stable isotopes. See stable nuclide and primordial nuclide. Nuclear isomers are members of a set of nuclides with equal proton number and equal mass number, but different states of excitation. An example is the two states of 9943Tc shown among the decay schemes. The most long-lived non-ground-state nuclear isomer is tantalum-180m, which has a halflife in excess of 1000 trillion years, and has not been observed to decay to tantalum-180. Unstable nuclides are radioactive and are called radionuclides. Their decay products ('daughter' products) are called radiogenic nuclides.
About 256 stable and about 83 unstable (radioactive) nuclides exist naturally on Earth. Natural radionuclides may be conveniently subdivided into three types. Firstly, those whose half-lives T½ are at least 10% as long as the age of the earth (4.6×109 years). These are remnants of nucleosynthesis that occurred in stars before the formation of the solar system. For example, the isotope 238U (T½ = 4.5×109 a) of uranium occurs in nature, but the shorter-lived isotope, 235U (T½ = 0.7 ×109 a), is 138 times rarer. The second group consists of isotopes such as 226Ra (T½ = 1602 a), an isotope of radium, which are formed in the radioactive decay chains of uranium or thorium. Some of these isotopes are very short lived, such as francium. The third group consists of nuclides which are continually being made in another fashion, such as 14C (radiocarbon) that are made by cosmic-ray bombardment of other elements, and promethium which is still being created by neutron bombardment in other stars, and has been detected there by its spectrum. More than 3000 nuclides have been artificially produced. The known nuclides are shown in charts of the nuclides (see Weblinks) [edit] See also[edit] External links
[edit] References
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