Nuclide


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, and its nuclear energy state.
The word nuclide was proposed by Truman P. Kohman in 1947. Kohman originally 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 originally intended to focus on the nucleus.

Nuclides vs isotopes

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. Even in the case of the very lightest elements, where the ratio of neutron number to atomic number varies the most between isotopes, it usually has only a small effect, but it matters in some circumstances. Since isotope is the older term, it is better known than nuclide, and is still occasionally used in contexts in which nuclide might be more appropriate, such as nuclear technology and nuclear medicine.

Types of nuclides

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.
DesignationCharacteristicsExampleRemarks
Isotopesequal proton number ,,
Isotonesequal neutron number ,,
Isobarsequal mass number ,, see beta decay
Isodiaphersequal neutron excess ,, Examples are isodiaphers with neutron excess 1.

A nuclide and its alpha decay product are isodiaphers.
Mirror nucleineutron and proton number exchanged

,
Nuclear isomerssame proton number and mass number,

but with different energy states		, m=metastable

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 often 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, and isotones are nuclides of equal neutron number but different proton numbers. Likewise, nuclides with the same neutron excess are called isodiaphers. The name isotone was derived from the name isotope to emphasize that in the first group of nuclides it is the number of neutrons that is constant, whereas in the second the number of protons.
See Isotope#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, but different states of excitation. An example is the two states of the single isotope 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, and has never been observed to decay to the ground state. or 180W ).
There are 252 nuclides in nature that have never been observed to decay. They occur among the 80 different elements that have one or more stable isotopes. See stable nuclide and primordial nuclide. Unstable nuclides are radioactive and are called radionuclides. Their decay products are called radiogenic nuclides. 252 stable and about 87 unstable nuclides exist naturally on Earth, for a total of about 339 naturally occurring nuclides on Earth.

Origins of naturally occurring radionuclides

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 of uranium is still fairly abundant in nature, but the shorter-lived isotope is 138 times rarer. About 34 of these nuclides have been discovered.
The second group of radionuclides that exist naturally consists of radiogenic nuclides such as , an isotope of radium, which are formed by radioactive decay. They occur in the decay chains of primordial isotopes of uranium or thorium. Some of these nuclides are very short-lived, such as isotopes of francium. There exist about 51 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 spontaneous radioactive decay but instead involves a natural nuclear reaction. These occur when atoms react with natural neutrons, or are bombarded directly with cosmic rays. The latter, if non-primordial, are called cosmogenic nuclides. Other types of natural nuclear reactions produce nuclides that are said to be nucleogenic nuclides.
An example of nuclides made by nuclear reactions, are cosmogenic that is made by cosmic ray bombardment of other elements, and nucleogenic which is still being created by neutron bombardment of natural as a result of natural fission in uranium ores. Cosmogenic nuclides may be either stable or radioactive. If they are stable, their existence must be deduced against a background of stable nuclides, since every known stable nuclide is present on Earth primordially.

Artificially produced nuclides

Beyond the 339 naturally occurring nuclides, more than 3000 radionuclides of varying half-lives have been artificially produced and characterized.
The known nuclides are shown in Table of nuclides. A list of primordial nuclides is given sorted by element, at List of elements by stability of isotopes. List of nuclides is sorted by half-life, for the 905 nuclides with half-lives longer than one hour.

Summary table for numbers of each class of nuclides

This is a summary table for the 905 nuclides with half-lives longer than one hour, given in list of nuclides. Note that numbers are not exact, and may change slightly in the future, if some "stable" nuclides are observed to be radioactive with very long half-lives.
Stability classNumber of nuclidesRunning totalNotes on running total
Theoretically stable to all but proton decay9090Includes 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 from niobium-93 onward; other mechanisms possible for heavier nuclides. All considered "stable" until decay detected.162252Total of classically stable nuclides.
Radioactive primordial nuclides.34286Total primordial elements include bismuth, thorium, and uranium, plus all stable nuclides.
Radioactive non-primordial, but naturally occurring on Earth.~ 53~ 339Carbon-14 ; daughters of radioactive primordials, such as francium, etc., and nucleogenic nuclides from natural nuclear reactions that are other than those from cosmic rays.
Radioactive synthetic. Includes most useful radioactive tracers.556905
Radioactive synthetic.>2400>3300Includes all well-characterized synthetic nuclides.

Nuclear properties and stability

Atomic nuclei consist of protons and neutrons bound together by the residual strong force. Because protons are positively charged, they repel each other. Neutrons, which are electrically neutral, stabilize the nucleus in two ways. Their copresence pushes protons slightly apart, reducing the electrostatic repulsion between the protons, and they exert the attractive nuclear force on each other and on protons. For this reason, one or more neutrons are necessary for two or more protons to be bound into a nucleus. As the number of protons increases, so does the ratio of neutrons to protons necessary to ensure a stable nucleus. For example, although the neutron–proton ratio of is 1:2, the neutron–proton ratio of is greater than 3:2. A number of lighter elements have stable nuclides with the ratio 1:1. The nuclide is observationally the heaviest stable nuclide with the same number of neutrons and protons;. All stable nuclides heavier than calcium-40 contain more neutrons than protons.

Even and odd nucleon numbers

The proton–neutron ratio is not the only factor affecting nuclear stability. It depends also on evenness or oddness of its atomic number Z, neutron number N and, consequently, of their sum, the mass number A. Oddness of both Z and N tends to lower the nuclear binding energy, making odd nuclei, generally, less stable. This remarkable difference of nuclear binding energy between neighbouring nuclei, especially of odd-A isobars, has important consequences: unstable isotopes with a nonoptimal number of neutrons or protons decay by beta decay, electron capture or other exotic means, such as spontaneous fission and cluster decay.
The majority of stable nuclides are even-proton–even-neutron, where all numbers Z, N, and A are even. The odd-A stable nuclides are divided into odd-proton–even-neutron, and even-proton–odd-neutron nuclides. Odd-proton–odd-neutron nuclides are the least common.