Atomic species
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]
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
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]
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
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]
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 (
Z
1
= Z
2
)
|
12
6
C
,
13
6
C
,
14
6
C
|
see
neutron capture
|
Isotones
|
equal neutron number (
N
1
= N
2
)
|
13
6
C
,
14
7
N
,
15
8
O
|
see
proton capture
|
Isobars
|
equal mass number (Z
1
+ N
1
= Z
2
+ N
2
)
|
17
7
N
,
17
8
O
,
17
9
F
|
see
beta decay
|
Isodiaphers
|
equal neutron excess (N
1
? Z
1
= N
2
? Z
2
)
|
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
(Z
1
= N
2
and
Z
2
= N
1
)
|
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 isoto
n
e was derived from the name isoto
p
e 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
180
Hf (86%) or
180
W (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
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]
Natural radionuclides may be conveniently subdivided into three types.
[6]
First, those whose
half-lives
t
1/2
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
×
10
9
years
). These are remnants of
nucleosynthesis
that occurred in stars before the formation of the
Solar System
. For example, the isotope
238
U
(t
1/2
=
4.5
×
10
9
years
) of
uranium
is still fairly abundant in nature, but the shorter-lived isotope
235
U
(t
1/2
=
0.7
×
10
9
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
(t
1/2
=
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
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]
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
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]
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 class
|
Number of nuclides
|
Running total
|
Notes on running total
|
Theoretically stable to all but
proton 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 from
niobium-93
onward; other mechanisms possible for heavier nuclides. All considered "stable" until decay detected.
|
161
|
251
|
Total of classically
stable nuclides
.
|
Radioactive
primordial nuclides
.
|
35
|
286
|
Total primordial elements include
bismuth
,
thorium
, and
uranium
, plus all stable nuclides.
|
Radioactive (half-life > 1 hour). Includes most useful
radioactive tracers
.
|
619
|
905
|
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
|
>3300
|
Includes all well-characterized synthetic nuclides.
|
Nuclear properties and stability
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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
[
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]
Even/odd
Z
,
N
, and
A
A
|
Even
|
Odd
|
Total
|
Z
,
N
|
EE
|
OO
|
EO
|
OE
|
Stable
|
145
|
5
|
53
|
48
|
251
|
150
|
101
|
Long-lived
|
22
|
4
|
4
|
5
|
35
|
26
|
9
|
All primordial
|
167
|
9
|
57
|
53
|
286
|
176
|
110
|
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
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]
References
[
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]
External links
[
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]
|
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Representations
| |
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Images
| |
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Articles on isotopes of an element
| |
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Various tables and lists of the nuclides
|