A nuclide 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, its nuclear energy state. The word nuclide was proposed by Truman P. Kohman in 1947. Kohman suggested nuclide as referring to a "species of atom characterized by the constitution of its nucleus" defined by containing a certain number of neutrons and protons; the word thus was intended to focus on the nucleus. A nuclide is a species of an atom with a specific number of protons and neutrons in the nucleus, for example carbon-13 with 6 protons and 7 neutrons; the nuclide concept emphasizes nuclear properties over chemical properties, while the isotope concept emphasizes chemical over nuclear. The neutron number has large effects on nuclear properties, but its effect on chemical reactions is negligible for most elements. In the case of the lightest elements, where the ratio of neutron number to atomic number varies the most between isotopes, it has only a small effect, although it does matter in some circumstances.
Since isotope is the older term, it is better known than nuclide, is still sometimes used in contexts where nuclide might be more appropriate, such as nuclear technology and nuclear medicine. Although the words nuclide and isotope are used interchangeably, being isotopes is only one relation between nuclides; the following table names some other relations. A set of nuclides with equal proton number, i.e. of the same chemical element but different neutron numbers, are called isotopes of the element. Particular nuclides are still loosely called "isotopes", but the term "nuclide" is the correct one in general. In similar manner, a set of nuclides with equal mass number A, but different atomic number, are called isobars, isotones are nuclides of equal neutron number but different proton numbers. Nuclides with the same neutron excess are called isodiaphers; the name isotone has been derived from the name isotope to emphasize that in the first group of nuclides it is the number of neutrons, constant, whereas in the second the number of protons.
See Isotope #Notation for an explanation of the notation used for different isotope types. 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 the single isotope 9943Tc shown among the decay schemes; each of these two states qualifies as a different nuclide, illustrating one way that nuclides may differ from isotopes. The longest-lived non-ground state nuclear isomer is the nuclide tantalum-180m, which has a half-life in excess of 1,000 trillion years; this nuclide occurs primordially, has never been observed to decay to the ground state.. There are 252 nuclides in nature, they occur among the 80 different elements. See stable nuclide and primordial nuclide. Unstable nuclides are called radionuclides, their decay products are called radiogenic nuclides. 252 stable and about 87 unstable nuclides exist on Earth, for a total of about 339 occurring nuclides on Earth. Natural radionuclides may be conveniently subdivided into three types.
First, those whose half-lives t1/2 are at least 2% as long as the age of the Earth. These are remnants of nucleosynthesis that occurred in stars before the formation of the solar system. For example, the isotope 238U of uranium is still abundant in nature, but the shorter-lived isotope 235U is 138 times rarer. About 34 of these nuclides have been discovered; the second group of radionuclides that exist consists of radiogenic nuclides such as 226Ra, an isotope of radium, which are formed by radioactive decay. They occur in the decay chains of primordial isotopes of thorium; some of these nuclides are short-lived, such as isotopes of francium. There exist about 51 of these daughter nuclides that have half-lives too short to be primordial, which exist in nature due to decay from longer lived radioactive primordial nuclides; the third group consists of nuclides that are continuously being made in another fashion, not simple spontaneous radioactive decay but instead involves a natural nuclear reaction.
These occur when atoms are bombarded directly with cosmic rays. The latter, if non-primordial, are called cosmogenic nuclides. Other types of natural nuclear reactions produce nuclides. An example of nuclides made by nuclear reactions, are cosmogenic 14C, made by cosmic ray bombardment of other elements, nucleogenic 239Pu, still being cr
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