| Nuclear physics |
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High-energy processes |
Nuclides (ornucleides, fromnucleus; also known asnuclear species) are a class of atoms characterized by their number ofprotons,Z, their number ofneutrons,N, and their nuclearenergy state.[1]
The wordnuclide was coined by the American nuclear physicistTruman P. Kohman in 1947.[2][3] Kohman definednuclide as a "species of atom characterized by the constitution of its nucleus" containing a certain number of neutrons and protons. The term thus originally focused on the nucleus.
A nuclide is an atom with a specific number of protons and neutrons in its nucleus, for example carbon-13 (13
6C) with 6 protons and 7 neutrons. The term was coined deliberately in distinction fromisotope in order to consider the nuclear properties independently of the chemical properties, thoughisotope is still used for that purpose especially wherenuclide might be unfamiliar as innuclear technology andnuclear medicine. For nuclear properties, the number ofneutrons can be practically as important as that ofprotons, as is never the case for chemical properties: even in the case of the very lightest elements, where the ratio of neutron number to atomic number varies the most between isotopes, it is a relatively small effect, and only substantial for hydrogen and helium (the latter of which has no chemistry proper). For hydrogen the isotope effect is large enough to affect biological systems strongly. In helium,4
2He obeysBose–Einstein statistics, while3
2He obeysFermi–Dirac statistics, which is responsible for sharp differences in physical properties at low temperature.
Although the words nuclide and isotope are often used interchangeably, being isotopes is actually only one relation between nuclides. The following table names some other relations.
| Designation | Characteristics | Example | Remarks |
|---|---|---|---|
| Isotopes | equal proton number (Z1 = Z2) | 12 6C,13 6C,14 6C | seeneutron capture |
| Isotones | equal neutron number (N1 = N2) | 13 6C,14 7N,15 8O | seeproton capture |
| Isobars | equal mass number (Z1 + N1 = Z2 + N2) | 17 7N,17 8O,17 9F | seebeta decay |
| Isodiaphers | equal neutron excess (N1 − Z1 = N2 − Z2) | 13 6C,15 7N,17 8O | Examples are isodiaphers with neutron excess 1. A nuclide and itsalpha decay product are isodiaphers.[4] |
| Mirror nuclei | neutron and proton number exchanged (Z1 = N2and Z2 = N1) | 3 1H,3 2He | seepositron emission |
| Nuclear isomers | same proton numberand mass number, but with different energy states | 99 43Tc,99m 43Tc | m = metastable (long-lived excited state) |
A set of nuclides with equal proton number (atomic number), i.e., of the samechemical element but differentneutron numbers, are calledisotopes[5] of the element. Particular nuclides are still often loosely called "isotopes", but the term "nuclide" is now considered the correct one in the general case when no specific element (Z value) encompasses them. In similar manner, a set of nuclides with equalmass numberA, but differentatomic number, are calledisobars (isobar = equal in weight), andisotones are nuclides of equal neutron number but different proton numbers. Likewise, nuclides with the same neutron excess (N − Z) are called isodiaphers.[4] The name isotone was derived from the name isotope to emphasize 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).[6]
SeeIsotope#Notation for an explanation of the notation used for different nuclide or isotope types.
Nuclear isomers are members of a set of nuclides with equal proton number and equal mass number (thus making them by definition the same isotope), but different states of excitation. An example is the two states of the single isotope99
43Tc shown among thedecay schemes. Each of these two states (technetium-99m and technetium-99) qualifies as a different nuclide, illustrating one way that nuclides may differ from isotopes (an isotope may consist of several different nuclides of different excitation states).
The longest-lived non-ground state nuclear isomer is the nuclidetantalum-180m (180m
73Ta), which has ahalf-life in excess of 1017 years. This nuclide occurs primordially, and has never been observed to decay to the ground state. (In contrast, the ground state nuclide tantalum-180 does not occur primordially, since it decays with a half-life of only 8 hours to180
72Hf (86%) or180
74W (14%).)
There are 251 nuclides in nature that have never been observed to decay. They occur among the 80 different elements that have one or more stable isotopes. Seestable nuclide andprimordial nuclide. Unstable nuclides areradioactive and are calledradionuclides. Theirdecay products ('daughter' products) are calledradiogenic nuclides.
Natural radionuclides may be conveniently subdivided into three types.[7] First, those whosehalf-lives exceed a few percent of theage of the Earth (about4.6×109 years) survive from its formation and are remnants ofnucleosynthesis that occurred in stars before the formation of theSolar System. For example, the isotope238
92U (t1/2 =4.463×109 years) ofuranium is still fairly abundant in nature, but the shorter-lived isotope235
92U (t1/2 =0.704×109 years) is now 138 times rarer. 35 of theseprimordial radionuclides have been identified (seeList of nuclides andPrimordial nuclide for details).
The second group of radionuclides that exist naturally consists ofradiogenic nuclides (such as226
88Ra (t1/2 =1600 years), an isotope ofradium) that are formed byradioactive decay. They occur in the decay chains of primordial isotopes of uranium or thorium. Some of these nuclides are very short-lived, such asisotopes of francium. There exist about 50 of these daughter nuclides that have half-lives too short to be primordial, and which exist in nature solely due to decay from longer lived radioactive primordial nuclides.
The third group consists of nuclides that are continuously being made in another fashion that is not simple spontaneousradioactive decay (i.e., only one atom involved with no incoming particle) but instead involves a naturalnuclear reaction. These occur when atoms react with natural neutrons (from cosmic rays,spontaneous fission, or other sources), or are bombarded directly withcosmic rays. The latter, if non-primordial, are calledcosmogenic nuclides. Other types of natural nuclear reactions produce nuclides that are said to benucleogenic nuclides. Nuclides produced as directproducts of spontaneous fission span a wide range, but every one will quickly (in geologic time) decay either to a primordial nuclide or one of the sevenlong-lived fission products, which are thus present in nature, but might be considered either radiogenic or nucleogenic.
Examples of nuclides made by nuclear reactions are cosmogenic14
6C (radiocarbon) that is made bycosmic ray bombardment of other elements and nucleogenic239
94Pu still being created by neutron bombardment of natural238
92U as a result of natural fission in uranium ores.
This is a summary table[8] for the 987 nuclides with half-lives longer than one hour, given inList of nuclides. Note that that number, while exact to present knowledge, will likely change slightly in the future, as some "stable" nuclides are observed to be radioactive with very long half-lives, and some half-lives or known radioactive ones are revised.
| Stability class | Number of nuclides | Running total | Notes on running total |
|---|---|---|---|
| Theoretically stable to all butproton decay | 90 | 90 | Includes first 40 elements. Proton decay yet to be observed. |
| Energetically unstable to one or more known decay modes, but no decay yet seen.Spontaneous fission possible for "stable" nuclides fromniobium-93 onward; other mechanisms possible for heavier nuclides. All considered "stable" until decay detected. | 161 | 251 | Total of classicallystable nuclides. |
| Radioactiveprimordial nuclides. | 35 | 286 | Total primordial elements includebismuth,thorium, anduranium, as well as all having stable nuclides. |
| Radioactive (half-life > 1 hour). Includes most usefulradioactive tracers. | 701 | 987 | Carbon-14 (and othercosmogenic nuclides generated bycosmic rays), daughters of radioactive primordials,nucleogenic nuclides from natural nuclear reactions that are other than those from cosmic rays (such as neutron absorption from spontaneousnuclear fission orneutron emission), and many synthetic nuclides. |
| Radioactive synthetic (half-life < 1 hour). | > 4000 | > 5000[9] | Includes all other characterized synthetic nuclides. |
Atomic nuclei other than1
1H, a lone proton, consist of protons and neutrons bound together by theresidual strong force, overcoming electrical repulsion between protons, and for that reason neutrons are required by bind protons together; as the number of protons increases, so does the ratio of neutrons to protons necessary for stability, as the graph illustrates. For example, although light elements up through calcium have stable nuclides with the same number of neutrons as protons, lead requires about 3 neutrons for 2 protons.
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