nuclide
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A nuclide (or nucleide, from
nucleus Nucleus (plural, : nuclei) is a Latin word for the seed inside a fruit. It most often refers to: *Atomic nucleus, the very dense central region of an atom *Cell nucleus, a central organelle of a eukaryotic cell, containing most of the cell's DNA ...
, also known as nuclear species) is a class of atoms characterized by their number of
proton A proton is a stable subatomic particle, symbol , H+, or 1H+ with a positive electric charge of +1 ''e'' elementary charge. Its mass is slightly less than that of a neutron and 1,836 times the mass of an electron (the proton–electron mass ...
s, ''Z'', their number of
neutron The neutron is a subatomic particle, symbol or , which has a neutral (not positive or negative) charge, and a mass slightly greater than that of a proton. Protons and neutrons constitute the atomic nucleus, nuclei of atoms. Since protons and ...
s, ''N'', and their nuclear energy state. The word ''nuclide'' was coined by Truman P. Kohman in 1947. Kohman defined ''nuclide'' 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.


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 (referring to individual nuclear species) emphasizes nuclear properties over chemical properties, while the
isotope Isotopes are two or more types of atoms that have the same atomic number (number of protons in their nuclei) and position in the periodic table (and hence belong to the same chemical element), and that differ in nucleon numbers (mass numbe ...
concept (grouping all atoms of each element) emphasizes chemical over nuclear. The
neutron The neutron is a subatomic particle, symbol or , which has a neutral (not positive or negative) charge, and a mass slightly greater than that of a proton. Protons and neutrons constitute the atomic nucleus, nuclei of atoms. Since protons and ...
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. For hydrogen, the lightest element, the isotope effect is large enough to affect biological systems strongly. In the case of helium,
helium-4 Helium-4 () is a stable isotope of the element helium. It is by far the more abundant of the two naturally occurring isotopes of helium, making up about 99.99986% of the helium on Earth. Its nucleus is identical to an alpha particle, and consis ...
obeys Bose–Einstein statistics, while
helium-3 Helium-3 (3He see also helion (chemistry), helion) is a light, stable isotope of helium with two protons and one neutron (the most common isotope, helium-4, having two protons and two neutrons in contrast). Other than Isotopes of hydrogen#Hydrog ...
obeys Fermi-Dirac statistics. 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. A set of nuclides with equal proton number (
atomic number The atomic number or nuclear charge number (symbol ''Z'') of a chemical element is the charge number of an atomic nucleus. For ordinary nuclei, this is equal to the proton number (''n''p) or the number of protons found in the nucleus of every ...
), i.e., of the same
chemical element A chemical element is a species of atoms that have a given number of protons in their atomic nucleus, nuclei, including the pure Chemical substance, substance consisting only of that species. Unlike chemical compounds, chemical elements canno ...
but different
neutron number The neutron number, symbol ''N'', is the number of neutrons in a nuclide. Atomic number (proton number) plus neutron number equals mass number: . The difference between the neutron number and the atomic number is known as the neutron excess: . ...
s, are called
isotope Isotopes are two or more types of atoms that have the same atomic number (number of protons in their nuclei) and position in the periodic table (and hence belong to the same chemical element), and that differ in nucleon numbers (mass numbe ...
s 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). In similar manner, a set of nuclides with equal mass number ''A'', but different
atomic number The atomic number or nuclear charge number (symbol ''Z'') of a chemical element is the charge number of an atomic nucleus. For ordinary nuclei, this is equal to the proton number (''n''p) or the number of protons found in the nucleus of every ...
, are called isobars (isobar = equal in weight), and
isotone Two nuclides are isotones if they have the same neutron number ''N'', but different proton number ''Z''. For example, Isotopes of boron, boron-12 and carbon-13 nuclei both contain 7 neutrons, and so are isotones. Similarly, 36S, 37Cl, 38Ar, 39 ...
s are nuclides of equal neutron number but different proton numbers. Likewise, nuclides with the same neutron excess (''N'' − ''Z'') 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 (n) that is constant, whereas in the second the number of protons (p). 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 (thus making them by definition the same isotope), 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 (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 The ground state of a quantum-mechanical system is its stationary state of lowest energy In physics, energy (from Ancient Greek: wikt:ἐνέργεια#Ancient_Greek, ἐνέργεια, ''enérgeia'', “activity”) is the physical qu ...
nuclear isomer is the nuclide tantalum-180m (), which has a
half-life Half-life (symbol ) is the time required for a quantity (of substance) to reduce to half of its initial value. The term is commonly used in nuclear physics to describe how quickly unstable atoms undergo radioactive decay or how long stable ato ...
in excess of 1,000 trillion 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 to 180Hf (86%) or 180W (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. See
stable nuclide Stable nuclides are nuclides that are not radioactive and so (unlike radionuclides) do not spontaneously undergo radioactive decay Radioactive decay (also known as nuclear decay, radioactivity, radioactive Decay chain, disintegration, or n ...
and
primordial nuclide In geochemistry, geophysics and nuclear physics, primordial nuclides, also known as primordial isotopes, are nuclides found on Earth that have existed in their current form since before Formation of the Earth, Earth was formed. Primordial nuclid ...
. Unstable nuclides are
radioactive Radioactive decay (also known as nuclear decay, radioactivity, radioactive Decay chain, disintegration, or nuclear disintegration) is the process by which an unstable atomic nucleus loses energy by radiation. A material containing unstable nucl ...
and are called
radionuclide A radionuclide (radioactive nuclide, radioisotope or radioactive isotope) is a nuclide that has excess nuclear energy, making it unstable. This excess energy can be used in one of three ways: emitted from the nucleus as gamma radiation; transfe ...
s. Their
decay product In nuclear physics, a decay product (also known as a daughter product, daughter isotope, radio-daughter, or daughter nuclide) is the remaining nuclide left over from radioactive decay. Radioactive decay often proceeds via a sequence of steps (de ...
s ('daughter' products) are called radiogenic nuclides. In total, 251 stable and about 88 unstable (radioactive) 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 Earth is the third planet from the Sun and the only astronomical object known to harbor life. While large list of largest lakes and seas in the Solar System, volumes of water can be found throughout the Solar System, only water distributi ...
(for practical purposes, these are difficult to detect with half-lives less than 10% of the age of the Earth) (). These are remnants of nucleosynthesis that occurred in stars before the formation of the
Solar System The Solar SystemCapitalization of the name varies. The International Astronomical Union, the authoritative body regarding astronomical nomenclature, specifies capitalizing the names of all individual astronomical objects but uses mixed "Solar S ...
. For example, the isotope (t1/2 = ) of
uranium Uranium is a chemical element with the Symbol (chemistry), symbol U and atomic number 92. It is a silvery-grey metal in the actinide series of the periodic table. A uranium atom has 92 protons and 92 electrons, of which 6 are valence electrons. ...
is still fairly abundant in nature, but the shorter-lived isotope (t1/2 = ) is 138 times rarer. About 34 of these nuclides have been discovered (see
List of nuclides This list of nuclides shows observed nuclides that either are stable nuclide, stable or, if radioactive, have half-lives longer than one hour. This represents isotopes of the first 105 elements, except for elements 87 (francium) and 102 (nobelium). ...
and
Primordial nuclide In geochemistry, geophysics and nuclear physics, primordial nuclides, also known as primordial isotopes, are nuclides found on Earth that have existed in their current form since before Formation of the Earth, Earth was formed. Primordial nuclid ...
for details). The second group of radionuclides that exist naturally consists of radiogenic nuclides such as (t1/2 = ), an isotope of
radium Radium is a chemical element with the Symbol (chemistry), symbol Ra and atomic number 88. It is the sixth element in alkaline earth metal, group 2 of the periodic table, also known as the alkaline earth metals. Pure radium is silvery-white, but ...
, which are formed by
radioactive decay Radioactive decay (also known as nuclear decay, radioactivity, radioactive Decay chain, disintegration, or nuclear disintegration) is the process by which an unstable atomic nucleus loses energy by radiation. A material containing unstable nucl ...
. 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 Radioactive decay (also known as nuclear decay, radioactivity, radioactive Decay chain, disintegration, or nuclear disintegration) is the process by which an unstable atomic nucleus loses energy by radiation. A material containing unstable nucl ...
(i.e., only one atom involved with no incoming particle) but instead involves a natural
nuclear reaction In nuclear physics Nuclear physics is the field of physics that studies atomic nuclei and their constituents and interactions, in addition to the study of other forms of nuclear matter. Nuclear physics should not be confused with atomic phy ...
. These occur when atoms react with natural neutrons (from cosmic rays,
spontaneous fission Spontaneous fission (SF) is a form of radioactive decay that is found only in very heavy chemical elements. The nuclear binding energy of the elements reaches its maximum at an atomic mass number of about 56 (e.g., iron-56); spontaneous breakdow ...
, or other sources), or are bombarded directly with
cosmic ray Cosmic rays are high-energy particles or clusters of particles (primarily represented by protons or atomic nuclei) that move through space at nearly the speed of light. They originate from the Sun, from outside of the Solar System in our own ...
s. The latter, if non-primordial, are called
cosmogenic nuclide Cosmogenic nuclides (or cosmogenic isotopes) are rare nuclides (isotopes) created when a high-energy cosmic ray interacts with the atomic nucleus, nucleus of an ''in situ'' Solar System atom, causing nucleons (protons and neutrons) to be expelled ...
s. 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 (
radiocarbon Carbon-14, C-14, or radiocarbon, is a radioactive isotope of carbon with an atomic nucleus containing 6 protons and 8 neutrons. Its presence in organic materials is the basis of the radiocarbon dating method pioneered by Willard Libby and colle ...
) that is made by
cosmic ray Cosmic rays are high-energy particles or clusters of particles (primarily represented by protons or atomic nuclei) that move through space at nearly the speed of light. They originate from the Sun, from outside of the Solar System in our own ...
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 This list of nuclides shows observed nuclides that either are stable nuclide, stable or, if radioactive, have half-lives longer than one hour. This represents isotopes of the first 105 elements, except for elements 87 (francium) and 102 (nobelium). ...
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 tableTable data is derived by counting members of the list; references for the list data itself are given below in the reference section in
list of nuclides This list of nuclides shows observed nuclides that either are stable nuclide, stable or, if radioactive, have half-lives longer than one hour. This represents isotopes of the first 105 elements, except for elements 87 (francium) and 102 (nobelium). ...
.
for the 905 nuclides with half-lives longer than one hour, given in
list of nuclides This list of nuclides shows observed nuclides that either are stable nuclide, stable or, if radioactive, have half-lives longer than one hour. This represents isotopes of the first 105 elements, except for elements 87 (francium) and 102 (nobelium). ...
. 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.


Nuclear properties and stability

Atomic nuclei other than hydrogen have 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 (see graph). 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 (calcium-40) is observationally the heaviest stable nuclide with the same number of neutrons and protons (theoretically, the heaviest stable one is sulfur-32). 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 even or odd parity 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 Nuclear binding energy in experimental physics is the minimum energy that is required to disassemble the atomic nucleus, nucleus of an atom into its constituent protons and neutrons, known collectively as nucleons. The binding energy for stable n ...
, 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 In nuclear physics, beta decay (β-decay) is a type of radioactive decay in which a beta particle (fast energetic electron or positron) is emitted from an atomic nucleus, transforming the original nuclide to an isobar (nuclide), isobar of that ...
(including positron decay),
electron capture Electron capture (K-electron capture, also K-capture, or L-electron capture, L-capture) is a process in which the proton-rich nucleus of an electrically neutral atom absorbs an inner atomic electron, usually from the K or L electron shells. Thi ...
or more exotic means, such as
spontaneous fission Spontaneous fission (SF) is a form of radioactive decay that is found only in very heavy chemical elements. The nuclear binding energy of the elements reaches its maximum at an atomic mass number of about 56 (e.g., iron-56); spontaneous breakdow ...
and
cluster decay Cluster decay, also named heavy particle radioactivity or heavy ion radioactivity, is a rare type of nuclear decay in which an atomic nucleus emits a small "cluster" of neutrons and protons, more than in an alpha particle, but less than a typic ...
. 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 (roughly evenly) into odd-proton–even-neutron, and even-proton–odd-neutron nuclides. Odd-proton–odd-neutron nuclides (and nuclei) are the least common.


See also

*
Isotope Isotopes are two or more types of atoms that have the same atomic number (number of protons in their nuclei) and position in the periodic table (and hence belong to the same chemical element), and that differ in nucleon numbers (mass numbe ...
(much more information on abundance of stable nuclides) * List of elements by stability of isotopes *
List of nuclides This list of nuclides shows observed nuclides that either are stable nuclide, stable or, if radioactive, have half-lives longer than one hour. This represents isotopes of the first 105 elements, except for elements 87 (francium) and 102 (nobelium). ...
(sorted by half-life) * Table of nuclides * Alpha nuclide * Monoisotopic element * Mononuclidic element * Primordial element *
Radionuclide A radionuclide (radioactive nuclide, radioisotope or radioactive isotope) is a nuclide that has excess nuclear energy, making it unstable. This excess energy can be used in one of three ways: emitted from the nucleus as gamma radiation; transfe ...
* Hypernucleus


References


External links

*
Livechart - Table of Nuclides
at The International Atomic Energy Agency {{Authority control Nuclear physics