Nuclide

Last updated

A nuclide (or nucleide, from nucleus, also known as nuclear species) is a class of atoms characterized by their number of protons, Z, their number of neutrons, N, and their nuclear energy state. [1]

Contents

The word nuclide was coined by the American nuclear physicist Truman P. Kohman in 1947. [2] [3] 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 concept (grouping all atoms of each element) 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. For hydrogen, the lightest element, the isotope effect is large enough to affect biological systems strongly. In the case of helium, helium-4 obeys Bose–Einstein statistics, while helium-3 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.

DesignationCharacteristicsExampleRemarks
Isotopes equal proton number (Z 1 = Z2) 12
6
C
, 13
6
C
, 14
6
C
see neutron capture
Isotones equal neutron number (N 1 = N2) 13
6
C
, 14
7
N
, 15
8
O
see proton capture
Isobars equal mass number (Z1 + N1 = Z2 + N2) 17
7
N
, 17
8
O
, 17
9
F
see beta decay
Isodiaphersequal neutron excess (N1 − Z1 = N2 − Z2) 13
6
C
, 15
7
N
, 17
8
O
Examples are isodiaphers with neutron excess 1.

A nuclide and its alpha decay product are isodiaphers. [4]

Mirror nuclei neutron and proton number exchanged

(Z1 = N2and Z2 = N1)

3
1
H
, 3
2
He
see positron emission
Nuclear isomers same proton number and mass number,

but with different energy states

99
43
Tc
, 99m
43
Tc
m=metastable (long-lived excited state)

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). In similar manner, 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. 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). [5]

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 99
43
Tc
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 nuclear isomer is the nuclide tantalum-180m ( 180m
73
Ta
), 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. (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 and primordial nuclide. Unstable nuclides are radioactive and are called radionuclides. Their decay products ('daughter' products) are called radiogenic nuclides.

Origins of naturally occurring radionuclides

Natural radionuclides may be conveniently subdivided into three types. [6] First, those whose half-lives t1/2 are at least 2% as long as the age of the Earth (for practical purposes, these are difficult to detect with half-lives less than 10% of 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 238
U
(t1/2 = 4.5×109 years) of uranium is still fairly abundant in nature, but the shorter-lived isotope 235
U
(t1/2 = 0.7×109 years) is 138 times rarer. About 34 of these nuclides have been discovered (see List of nuclides and Primordial nuclide for details).

The second group of radionuclides that exist naturally consists of radiogenic nuclides such as 226
Ra
(t1/2 = 1602 years), 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 (i.e., only one atom involved with no incoming particle) but instead involves a natural nuclear reaction. These occur when atoms react with natural neutrons (from cosmic rays, spontaneous fission, or other sources), 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 14
C
(radiocarbon) that is made by cosmic ray bombardment of other elements, and nucleogenic 239
Pu
which is still being created by neutron bombardment of natural 238
U
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 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 [7] 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 nuclides Running total Notes on running total
Theoretically stable to all but proton decay 9090Includes 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.161251Total of classically stable nuclides.
Radioactive primordial nuclides.35286Total primordial elements include bismuth, thorium, and uranium, plus all stable nuclides.
Radioactive (half-life > 1 hour). Includes most useful radioactive tracers.619905 Carbon-14 (and other cosmogenic nuclides generated by cosmic rays); daughters of radioactive primordials, such as francium, etc., and nucleogenic nuclides from natural nuclear reactions that are other than those from cosmic rays (such as neutron absorption from spontaneous nuclear fission or neutron emission). Also many synthetic nuclides.
Radioactive synthetic (half-life < 1 hour).>2400>3300Includes all well-characterized synthetic nuclides.

Nuclear properties and stability

Stability of nuclides by (Z, N), an example of a table of nuclides:
Black - stable (all are primordial)
Red - primordial radioactive
Other - radioactive, with decreasing stability from orange to white Isotopes and half-life.svg
Stability of nuclides by (Z, N), an example of a table of nuclides:
Black – stable (all are primordial)
Red – primordial radioactive
Other – radioactive, with decreasing stability from orange to white

Atomic nuclei other than hydrogen 1
1
H
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 3
2
He
is 1:2, the neutron–proton ratio of 238
92
U
is greater than 3:2. A number of lighter elements have stable nuclides with the ratio 1:1 (Z = N). The nuclide 40
20
Ca
(calcium-40) 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

Even/odd Z, N, and A
AEvenOddTotal
Z,NEEOOEOOE
Stable14555348251
150101
Long-lived2244535
269
All primordial16795753286
176110

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, 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 (including positron decay), electron capture or more 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 (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

Related Research Articles

<span class="mw-page-title-main">Atom</span> Smallest unit of a chemical element

Atoms are the basic particles of the chemical elements. An atom consists of a nucleus of protons and generally neutrons, surrounded by an electromagnetically bound swarm of electrons. The chemical elements are distinguished from each other by the number of protons that are in their atoms. For example, any atom that contains 11 protons is sodium, and any atom that contains 29 protons is copper. Atoms with the same number of protons but a different number of neutrons are called isotopes of the same element.

A radionuclide (radioactive nuclide, radioisotope or radioactive isotope) is a nuclide that has excess numbers of either neutrons or protons, giving it excess nuclear energy, and making it unstable. This excess energy can be used in one of three ways: emitted from the nucleus as gamma radiation; transferred to one of its electrons to release it as a conversion electron; or used to create and emit a new particle (alpha particle or beta particle) from the nucleus. During those processes, the radionuclide is said to undergo radioactive decay. These emissions are considered ionizing radiation because they are energetic enough to liberate an electron from another atom. The radioactive decay can produce a stable nuclide or will sometimes produce a new unstable radionuclide which may undergo further decay. Radioactive decay is a random process at the level of single atoms: it is impossible to predict when one particular atom will decay. However, for a collection of atoms of a single nuclide the decay rate, and thus the half-life (t1/2) for that collection, can be calculated from their measured decay constants. The range of the half-lives of radioactive atoms has no known limits and spans a time range of over 55 orders of magnitude.

<span class="mw-page-title-main">Stable nuclide</span> Nuclide that does not undergo radioactive decay

Stable nuclides are nuclides that are not radioactive and so do not spontaneously undergo radioactive decay. When such nuclides are referred to in relation to specific elements, they are usually termed stable isotopes.

Nucleosynthesis is the process that creates new atomic nuclei from pre-existing nucleons and nuclei. According to current theories, the first nuclei were formed a few minutes after the Big Bang, through nuclear reactions in a process called Big Bang nucleosynthesis. After about 20 minutes, the universe had expanded and cooled to a point at which these high-energy collisions among nucleons ended, so only the fastest and simplest reactions occurred, leaving our universe containing hydrogen and helium. The rest is traces of other elements such as lithium and the hydrogen isotope deuterium. Nucleosynthesis in stars and their explosions later produced the variety of elements and isotopes that we have today, in a process called cosmic chemical evolution. The amounts of total mass in elements heavier than hydrogen and helium remains small, so that the universe still has approximately the same composition.

<span class="mw-page-title-main">Island of stability</span> Predicted set of isotopes of relatively more stable superheavy elements

In nuclear physics, the island of stability is a predicted set of isotopes of superheavy elements that may have considerably longer half-lives than known isotopes of these elements. It is predicted to appear as an "island" in the chart of nuclides, separated from known stable and long-lived primordial radionuclides. Its theoretical existence is attributed to stabilizing effects of predicted "magic numbers" of protons and neutrons in the superheavy mass region.

<span class="mw-page-title-main">Radioactive decay</span> Emissions from unstable atomic nuclei

Radioactive decay is the process by which an unstable atomic nucleus loses energy by radiation. A material containing unstable nuclei is considered radioactive. Three of the most common types of decay are alpha, beta, and gamma decay. The weak force is the mechanism that is responsible for beta decay, while the other two are governed by the electromagnetism and nuclear force.

<span class="mw-page-title-main">Decay chain</span> Series of radioactive decays

In nuclear science, the decay chain refers to a series of radioactive decays of different radioactive decay products as a sequential series of transformations. It is also known as a "radioactive cascade". The typical radioisotope does not decay directly to a stable state, but rather it decays to another radioisotope. Thus there is usually a series of decays until the atom has become a stable isotope, meaning that the nucleus of the atom has reached a stable state.

<span class="mw-page-title-main">Mass number</span> Number of heavy particles in the atomic nucleus

The mass number (symbol A, from the German word: Atomgewicht, "atomic weight"), also called atomic mass number or nucleon number, is the total number of protons and neutrons (together known as nucleons) in an atomic nucleus. It is approximately equal to the atomic (also known as isotopic) mass of the atom expressed in atomic mass units. Since protons and neutrons are both baryons, the mass number A is identical with the baryon number B of the nucleus (and also of the whole atom or ion). The mass number is different for each isotope of a given chemical element, and the difference between the mass number and the atomic number Z gives the number of neutrons (N) in the nucleus: N = AZ.

<span class="mw-page-title-main">Isotone</span> Different nuclides with the same neutron number

In nuclear physics, isotones are nucleides of the different chemical elements. They have the same neutron number, but different nucleon number due to different proton number. For example, boron-12 and carbon-13 nuclei both contain 7 neutrons, and so are isotones. Similarly, 36S, 37Cl, 38Ar, 39K, and 40Ca nuclei are all isotones of 20 because they all contain 20 neutrons.

<span class="mw-page-title-main">Isotopes of hydrogen</span> Hydrogen with different numbers of neutrons

Hydrogen (1H) has three naturally occurring isotopes, sometimes denoted 1
H
, 2
H
, and 3
H
. 1
H
and 2
H
are stable, while 3
H
has a half-life of 12.32(2) years. Heavier isotopes also exist, all of which are synthetic and have a half-life of less than one zeptosecond (10−21 s). Of these, 5
H
is the least stable, while 7
H
is the most.

Cosmic ray spallation, also known as the x-process, is a set of naturally occurring nuclear reactions causing nucleosynthesis; it refers to the formation of chemical elements from the impact of cosmic rays on an object. Cosmic rays are highly energetic charged particles from beyond Earth, ranging from protons, alpha particles, and nuclei of many heavier elements. About 1% of cosmic rays also consist of free electrons.

Beryllium (4Be) has 11 known isotopes and 3 known isomers, but only one of these isotopes is stable and a primordial nuclide. As such, beryllium is considered a monoisotopic element. It is also a mononuclidic element, because its other isotopes have such short half-lives that none are primordial and their abundance is very low. Beryllium is unique as being the only monoisotopic element with both an even number of protons and an odd number of neutrons. There are 25 other monoisotopic elements but all have odd atomic numbers, and even numbers of neutrons.

A table or chart of nuclides is a two-dimensional graph of isotopes of the elements, in which one axis represents the number of neutrons and the other represents the number of protons in the atomic nucleus. Each point plotted on the graph thus represents a nuclide of a known or hypothetical chemical element. This system of ordering nuclides can offer a greater insight into the characteristics of isotopes than the better-known periodic table, which shows only elements and not their isotopes. The chart of the nuclides is also known as the Segrè chart, after the Italian physicist Emilio Segrè.

<span class="mw-page-title-main">Valley of stability</span> Characterization of nuclide stability

In nuclear physics, the valley of stability is a characterization of the stability of nuclides to radioactivity based on their binding energy. Nuclides are composed of protons and neutrons. The shape of the valley refers to the profile of binding energy as a function of the numbers of neutrons and protons, with the lowest part of the valley corresponding to the region of most stable nuclei. The line of stable nuclides down the center of the valley of stability is known as the line of beta stability. The sides of the valley correspond to increasing instability to beta decay. The decay of a nuclide becomes more energetically favorable the further it is from the line of beta stability. The boundaries of the valley correspond to the nuclear drip lines, where nuclides become so unstable they emit single protons or single neutrons. Regions of instability within the valley at high atomic number also include radioactive decay by alpha radiation or spontaneous fission. The shape of the valley is roughly an elongated paraboloid corresponding to the nuclide binding energies as a function of neutron and atomic numbers.

<span class="mw-page-title-main">Isotope</span> Different atoms of the same element

Isotopes are distinct nuclear species of the same chemical element. They have the same atomic number and position in the periodic table, but differ in nucleon numbers due to different numbers of neutrons in their nuclei. While all isotopes of a given element have similar chemical properties, they have different atomic masses and physical properties.

<span class="mw-page-title-main">Isobar (nuclide)</span> Atoms of different elements with the same number of nucleons

Isobars are atoms (nuclides) of different chemical elements that have the same number of nucleons. Correspondingly, isobars differ in atomic number but have the same mass number. An example of a series of isobars is 40S, 40Cl, 40Ar, 40K, and 40Ca. While the nuclei of these nuclides all contain 40 nucleons, they contain varying numbers of protons and neutrons.

<span class="mw-page-title-main">Monoisotopic element</span> Element that has only a single stable isotope

A monoisotopic element is an element which has only a single stable isotope (nuclide). There are 26 such elements, as listed.

<span class="mw-page-title-main">Even and odd atomic nuclei</span> Nuclear physics classification method

In nuclear physics, properties of a nucleus depend on evenness or oddness of its atomic number Z, neutron number N and, consequently, of their sum, the mass number A. Most importantly, oddness of both Z and N tends to lower the nuclear binding energy, making odd nuclei generally less stable. This effect is not only experimentally observed, but is included in the semi-empirical mass formula and explained by some other nuclear models, such as the nuclear shell model. This difference of nuclear binding energy between neighbouring nuclei, especially of odd-A isobars, has important consequences for beta decay.

<span class="mw-page-title-main">Neutron–proton ratio</span> Ratio of neutrons to protons in an atomic nucleus

The neutron–proton ratio of an atomic nucleus is the ratio of its number of neutrons to its number of protons. Among stable nuclei and naturally occurring nuclei, this ratio generally increases with increasing atomic number. This is because electrical repulsive forces between protons scale with distance differently than strong nuclear force attractions. In particular, most pairs of protons in large nuclei are not far enough apart, such that electrical repulsion dominates over the strong nuclear force, and thus proton density in stable larger nuclei must be lower than in stable smaller nuclei where more pairs of protons have appreciable short-range nuclear force attractions.

References

  1. IUPAC (1997). "Nuclide". In A. D. McNaught; A. Wilkinson (eds.). Compendium of Chemical Terminology. Blackwell Scientific Publications. doi:10.1351/goldbook.N04257. ISBN   978-0-632-01765-2.
  2. Kohman, Truman P. (1947). "Proposed New Word: Nuclide". American Journal of Physics. 15 (4): 356–7. Bibcode:1947AmJPh..15..356K. doi:10.1119/1.1990965.
  3. Belko, Mark (1 May 2010). "Obituary: Truman P. Kohman / Chemistry professor with eyes always on stars". Pittsburgh Post-Gazette. Archived from the original on 14 December 2019. Retrieved 29 April 2018.
  4. 1 2 Sharma, B.K. (2001). Nuclear and Radiation Chemistry (7th ed.). Krishna Prakashan Media. p. 78. ISBN   978-81-85842-63-9.
  5. Cohen, E. R.; Giacomo, P. (1987). "Symbols, units, nomenclature and fundamental constants in physics". Physica A . 146 (1): 1–68. Bibcode:1987PhyA..146....1.. CiteSeerX   10.1.1.1012.880 . doi:10.1016/0378-4371(87)90216-0.
  6. "Types of Isotopes: Radioactive". SAHRA. Archived from the original on 17 October 2021. Retrieved 12 November 2016.
  7. Table 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.