Stable isotope
Encyclopedia
Stable isotopes are chemical isotope
s that may or may not be radioactive, but if radioactive, have half-lives too long to be measured.
Only 90 nuclides from the first 40 elements are energetically stable to any kind of decay save proton decay
, in theory (see list of nuclides). An additional 165 are theoretically unstable to known types of decay, but no evidence of decay has ever been observed, for a total of 255 nuclides for which there is no evidence of radioactivity. By this definition, there are 255 known stable nuclides of the 80 elements which have one or more stable isotopes. A list of these is given at the end of this article.
Of the 80 elements with one or more stable isotopes, twenty-six have only a single stable isotope, and are thus termed monoisotopic
, and the rest have more than one stable isotope. One element (tin
) has ten stable isotopes, the largest number known for an element.
(whether stable or unstable) have nearly the same chemical characteristics and therefore behave almost identically in biology (a notable exception is the isotopes of hydrogen—see heavy water
). The mass differences, due to a difference in the number of neutrons, will result in partial separation of the light isotopes from the heavy isotopes during chemical reactions and during physical processes such as diffusion and vaporization. This process is called isotope fractionation
. For example, the difference in mass between the two stable isotopes of hydrogen, 1H (1 proton, no neutron, also known as protium) and 2H (1 proton, 1 neutron, also known as deuterium
) is almost 100%. Therefore, a significant fractionation will occur.
, carbon
, nitrogen
, hydrogen
and sulfur
. These isotope systems have been under investigation for many years in order to study processes of isotope fractionation in natural systems because they are relatively simple to measure. Recent advances in mass spectrometry
(i.e. multiple-collector inductively coupled plasma mass spectrometry) now enable the measurement of heavier stable isotopes, such as iron
, copper
, zinc
, molybdenum
, etc.
Stable isotopes have been used in botanical and plant biological investigations for many years, and more and more ecological and biological studies are finding stable isotopes (mostly carbon, nitrogen and oxygen) to be extremely useful. Other workers have used oxygen isotopes to reconstruct historical atmospheric temperatures, making them important tools for climate research. Measurements of ratios of one naturally occurring stable isotope to another play an important role in radiometric dating
and isotope geochemistry
, and also helpful for determining patterns of rainfall and movements of elements through living organisms, helping sort out food web dynamics in ecosystems.
s are stable (about 255; see list at the end of this article); and about 33 more (total of 288) are known radioactives with sufficiently long half-lives (also known) to occur "primordially." If the half-life of a nuclide
is comparable to, or greater than, the Earth's age (4.5 billion years), a significant amount will have survived since the formation of the Solar System
, and then is said to be primordial
. It will then contribute in that way to the natural isotopic composition of a chemical element. Primordially present radioisotopes are easily detected with half-lives as short as 700 million years (e.g., 235U
), although some primordial isotopes have been detected with half-lives as short as 80 million years (e.g., 244Pu
). However, this is the present limit of detection, as the nuclide with the next-shortest half-life (niobium-92 with half-life 34.7 million years) has not been yet been detected in nature.
Many naturally-occurring radioisotopes (another 51 or so, for a total of about 339) exhibit still shorter half-lives than 80 million years, but they are made freshly, as daughter products of decay processes of primordial nuclides (for example, radium from uranium) or from ongoing energetic reactions, such as cosmogenic nuclide
s produced by present bombardment of Earth by cosmic rays (for example, carbon-14 made from nitrogen).
Many isotopes that are classed as stable (i.e. no radioactivity has been observed for them) are predicted to have extremely long half-lives (sometimes as high as 1018 years or more). If the predicted half-life falls into an experimentally accessible range, such isotopes have a chance to move from the list of stable nuclides to the radioactive category, once their activity is observed. Good examples are bismuth-209 and tungsten-180 which were formerly classed as stable, but have been recently (2003) found to be alpha
-active. However, such nuclides do not change their status as primordial when they are found to be radioactive.
Most stable isotopes in the earth are believed to have been formed in processes of nucleosynthesis
, either in the 'Big Bang
', or in generations of stars that preceded the formation of the solar system. However, some stable isotopes also show abundance variations in the earth as a result of decay from long-lived radioactive nuclides. These decay-products are termed radiogenic isotopes, in order to distinguish them from the much larger group of 'non-radiogenic' isotopes.
may reveal a number of long-lived or even stable atoms that are heavier (and with more protons) than lead.
to lead
, with the exceptions of technetium
(#43) and promethium
(#61), which do not have any stable nuclides. As of December, 2010, there were a total of 255 known "stable" nuclides. In this definition, "stable" means a nuclide which has either never been observed to decay against the natural background. Thus, these elements have half-lives too long to be measured by any means, direct or indirect.
Only one element (tin
) has 10 stable isotopes, and one (xenon
) has nine stable isotopes. No elements have exactly eight stable isotopes, but four elements have seven stable isotopes, nine have six stable isotopes, nine have five stable isotopes, nine have four stable isotopes, five have three stable isotopes, 16 have two stable isotopes, and 26 have only a single stable isotope and are thus considered monoisotopic elements. The mean number of stable isotopes for elements which have at least one such isotope, is 255/80 = 3.2.
Just as in the case of electrons, which have the lowest energy state when they occur in pairs in a given orbital, nucleons (both protons and neutrons) exhibit a lower energy state when their number is even, rather than odd. This stability tends to prevent beta decay (in two steps) of many even-even nuclides into another even-even nuclide of the same mass number but lower energy (and of course with two more protons and two fewer neutrons), because decay proceeding one step at a time would have to pass through an odd-odd nuclide of higher energy. This makes for a larger number of stable even-even nuclides, up to three for some mass numbers, and up to seven for some atomic (proton) numbers. Conversely, of the 255 known stable nuclides, only four have both an odd number of protons and odd number of neutrons: hydrogen-2 (deuterium
), lithium-6, boron-10 and nitrogen-14. Also, only four naturally occurring, radioactive odd-odd nuclides have a half-life over a billion years: potassium-40
, vanadium-50, lanthanum-138 and tantalum-180m. Odd-odd primordial nuclide
s are rare because most odd-odd nuclei are highly unstable with respect to beta decay
, because the decay products are even-even, and are therefore more strongly bound, due to nuclear pairing effects.
Yet another effect of the instability of an odd number of either type of nucleons, is that odd-numbered elements tend to have fewer stable isotopes. Of the 26 monoisotopic element
s that have only a single stable isotope, all but one have an odd atomic number — the single exception to both rules being beryllium. All of these elements also have an even number of neutrons, with the single exception again being beryllium.
or excited level (the ground state of this nucleus is radioactive with a very short half-life of 8 hours); but the decay of the excited nuclear isomer is extremely strongly forbidden by spin-parity selection rules. It has been reported experimentally by direct observation that the half-life of 180mTa to gamma decay must be more than 1015 years. Other possible modes of 180mTa decay (beta decay, electron capture and alpha decay) have also never been observed.
are more than about 2% of the time since the supernova nucleosynthesis
of the elements from which our solar system was made. An extreme case of this is plutonium-244
, which is still detectable from primordial reservoirs, even though it has a half-life of only 80 million years (1.8% of the solar system age). There exist about 33 naturally occurring radioactive primordial nuclide
s.
In about 50 known cases, elements with shorter half-lives than plutonium-244 are naturally observed on Earth, since as they are produced by cosmic ray
s (e.g., carbon-14
), or else because (like radium
and polonium
) they occur in a decay chain
of radioactive isotopes (primarily uranium and thorium), which have long-enough half-lives to be abundant primordially.
It is expected that continuous improvement of experimental sensitivity will allow discovery of very mild radioactivity (instability) of some isotopes that are considered stable today. For example, it wasn't until 2003 that bismuth-209
(the only naturally-occurring isotope of bismuth) was shown to be very mildly radioactive. Many "stable" nuclides are possibly "meta-stable" in as much as they may be calculated to have an energy release upon several possible kinds of radioactive decays.
Only 90 nuclides from the first 40 elements are theoretically stable to any sort of decay save proton decay (which has not been observed). The rest, starting with niobium-93, are theoretically unstable to spontaneous fission.
For processes other than spontaneous fission, other theoretical decay routes for heavier elements include:
These include all nuclides of mass 201 and greater. Argon-36 is presently the lightest known "stable" nuclide which is theoretically unstable.
The positivity of energy release in these processes means that they are allowed kinematically (they do not violate the conservation of energy) and, thus, in principle, can occur. They are not observed due to strong but not absolute suppression, by spin-parity selection rules (for beta decays and isomeric transitions) or by the thickness of the potential barrier (for alpha and cluster decays and spontaneous fission).
Other predicted (but not yet observed) modes of radioactive decay are noted as: A for alpha decay, B for beta decay, BB for double beta decay, E for electron capture, EE for double electron capture, and IT for isomeric transition. Because of the curve of binding energy, all nuclides from Z = 41 (niobium) and beyond, are theoretically unstable with regard to spontaneous fission SF (see list of nuclides for details), and many of the heavier nuclides are theoretically unstable to other processes as well.
Abbreviations:
A for alpha decay, B for beta decay, BB for double beta decay, E for electron capture, EE for double electron capture, IT for isomeric transition.
AlphaDelta: Stable Isotope fractionation calculator - http://www2.ggl.ulaval.ca/cgi-bin/isotope/generisotope.cgi
Isotope
Isotopes are variants of atoms of a particular chemical element, which have differing numbers of neutrons. Atoms of a particular element by definition must contain the same number of protons but may have a distinct number of neutrons which differs from atom to atom, without changing the designation...
s that may or may not be radioactive, but if radioactive, have half-lives too long to be measured.
Only 90 nuclides from the first 40 elements are energetically stable to any kind of decay save proton decay
Proton decay
In particle physics, proton decay is a hypothetical form of radioactive decay in which the proton decays into lighter subatomic particles, such as a neutral pion and a positron...
, in theory (see list of nuclides). An additional 165 are theoretically unstable to known types of decay, but no evidence of decay has ever been observed, for a total of 255 nuclides for which there is no evidence of radioactivity. By this definition, there are 255 known stable nuclides of the 80 elements which have one or more stable isotopes. A list of these is given at the end of this article.
Of the 80 elements with one or more stable isotopes, twenty-six have only a single stable isotope, and are thus termed monoisotopic
Monoisotopic element
A monoisotopic element is one of 26 chemical elements which have only a single stable isotope . A list is given in a following section....
, and the rest have more than one stable isotope. One element (tin
Tin
Tin is a chemical element with the symbol Sn and atomic number 50. It is a main group metal in group 14 of the periodic table. Tin shows chemical similarity to both neighboring group 14 elements, germanium and lead and has two possible oxidation states, +2 and the slightly more stable +4...
) has ten stable isotopes, the largest number known for an element.
Properties of stable isotopes
Different isotopes of the same elementChemical element
A chemical element is a pure chemical substance consisting of one type of atom distinguished by its atomic number, which is the number of protons in its nucleus. Familiar examples of elements include carbon, oxygen, aluminum, iron, copper, gold, mercury, and lead.As of November 2011, 118 elements...
(whether stable or unstable) have nearly the same chemical characteristics and therefore behave almost identically in biology (a notable exception is the isotopes of hydrogen—see heavy water
Heavy water
Heavy water is water highly enriched in the hydrogen isotope deuterium; e.g., heavy water used in CANDU reactors is 99.75% enriched by hydrogen atom-fraction...
). The mass differences, due to a difference in the number of neutrons, will result in partial separation of the light isotopes from the heavy isotopes during chemical reactions and during physical processes such as diffusion and vaporization. This process is called isotope fractionation
Isotope fractionation
Isotope fractionation is the separation of a mixture of isotopes into its components.There are four types of isotope fractionation:* equilibrium fractionation* kinetic fractionation* transient kinetic isotope fractionation* mass-independent fractionation...
. For example, the difference in mass between the two stable isotopes of hydrogen, 1H (1 proton, no neutron, also known as protium) and 2H (1 proton, 1 neutron, also known as deuterium
Deuterium
Deuterium, also called heavy hydrogen, is one of two stable isotopes of hydrogen. It has a natural abundance in Earth's oceans of about one atom in of hydrogen . Deuterium accounts for approximately 0.0156% of all naturally occurring hydrogen in Earth's oceans, while the most common isotope ...
) is almost 100%. Therefore, a significant fractionation will occur.
Study of stable isotopes
Commonly analysed stable isotopes include oxygenOxygen
Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...
, carbon
Carbon
Carbon is the chemical element with symbol C and atomic number 6. As a member of group 14 on the periodic table, it is nonmetallic and tetravalent—making four electrons available to form covalent chemical bonds...
, nitrogen
Nitrogen
Nitrogen is a chemical element that has the symbol N, atomic number of 7 and atomic mass 14.00674 u. Elemental nitrogen is a colorless, odorless, tasteless, and mostly inert diatomic gas at standard conditions, constituting 78.08% by volume of Earth's atmosphere...
, hydrogen
Hydrogen
Hydrogen is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of , hydrogen is the lightest and most abundant chemical element, constituting roughly 75% of the Universe's chemical elemental mass. Stars in the main sequence are mainly...
and sulfur
Sulfur
Sulfur or sulphur is the chemical element with atomic number 16. In the periodic table it is represented by the symbol S. It is an abundant, multivalent non-metal. Under normal conditions, sulfur atoms form cyclic octatomic molecules with chemical formula S8. Elemental sulfur is a bright yellow...
. These isotope systems have been under investigation for many years in order to study processes of isotope fractionation in natural systems because they are relatively simple to measure. Recent advances in mass spectrometry
Mass spectrometry
Mass spectrometry is an analytical technique that measures the mass-to-charge ratio of charged particles.It is used for determining masses of particles, for determining the elemental composition of a sample or molecule, and for elucidating the chemical structures of molecules, such as peptides and...
(i.e. multiple-collector inductively coupled plasma mass spectrometry) now enable the measurement of heavier stable isotopes, such as iron
Iron
Iron is a chemical element with the symbol Fe and atomic number 26. It is a metal in the first transition series. It is the most common element forming the planet Earth as a whole, forming much of Earth's outer and inner core. It is the fourth most common element in the Earth's crust...
, copper
Copper
Copper is a chemical element with the symbol Cu and atomic number 29. It is a ductile metal with very high thermal and electrical conductivity. Pure copper is soft and malleable; an exposed surface has a reddish-orange tarnish...
, zinc
Zinc
Zinc , or spelter , is a metallic chemical element; it has the symbol Zn and atomic number 30. It is the first element in group 12 of the periodic table. Zinc is, in some respects, chemically similar to magnesium, because its ion is of similar size and its only common oxidation state is +2...
, molybdenum
Molybdenum
Molybdenum , is a Group 6 chemical element with the symbol Mo and atomic number 42. The name is from Neo-Latin Molybdaenum, from Ancient Greek , meaning lead, itself proposed as a loanword from Anatolian Luvian and Lydian languages, since its ores were confused with lead ores...
, etc.
Stable isotopes have been used in botanical and plant biological investigations for many years, and more and more ecological and biological studies are finding stable isotopes (mostly carbon, nitrogen and oxygen) to be extremely useful. Other workers have used oxygen isotopes to reconstruct historical atmospheric temperatures, making them important tools for climate research. Measurements of ratios of one naturally occurring stable isotope to another play an important role in radiometric dating
Radiometric dating
Radiometric dating is a technique used to date materials such as rocks, usually based on a comparison between the observed abundance of a naturally occurring radioactive isotope and its decay products, using known decay rates...
and isotope geochemistry
Isotope geochemistry
Isotope geochemistry is an aspect of geology based upon study of the relative and absolute concentrations of the elements and their isotopes in the Earth. Variations in the abundance of these isotopes, typically measured with an isotope ratio mass spectrometer or an accelerator mass spectrometer,...
, and also helpful for determining patterns of rainfall and movements of elements through living organisms, helping sort out food web dynamics in ecosystems.
Definition of stability, and natural isotopic presence
Most naturally occurring nuclideNuclide
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....
s are stable (about 255; see list at the end of this article); and about 33 more (total of 288) are known radioactives with sufficiently long half-lives (also known) to occur "primordially." If the half-life of a nuclide
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....
is comparable to, or greater than, the Earth's age (4.5 billion years), a significant amount will have survived since the formation of the Solar System
Solar System
The Solar System consists of the Sun and the astronomical objects gravitationally bound in orbit around it, all of which formed from the collapse of a giant molecular cloud approximately 4.6 billion years ago. The vast majority of the system's mass is in the Sun...
, and then is said to be primordial
Primordial nuclide
In geochemistry and geonuclear physics, primordial nuclides or primordial isotopes are nuclides found on the earth that have existed in their current form since before Earth was formed. Only 288 such nuclides are known...
. It will then contribute in that way to the natural isotopic composition of a chemical element. Primordially present radioisotopes are easily detected with half-lives as short as 700 million years (e.g., 235U
Uranium-235
- References :* .* DOE Fundamentals handbook: Nuclear Physics and Reactor theory , .* A piece of U-235 the size of a grain of rice can produce energy equal to that contained in three tons of coal or fourteen barrels of oil. -External links:* * * one of the earliest articles on U-235 for the...
), although some primordial isotopes have been detected with half-lives as short as 80 million years (e.g., 244Pu
Plutonium-244
Plutonium-244 is an isotope of plutonium that has a halflife of 80 million years. This is longer than any of the other isotopes of plutonium and longer than any actinide except for the three naturally abundant ones uranium-235 , uranium-238, and thorium-232...
). However, this is the present limit of detection, as the nuclide with the next-shortest half-life (niobium-92 with half-life 34.7 million years) has not been yet been detected in nature.
Many naturally-occurring radioisotopes (another 51 or so, for a total of about 339) exhibit still shorter half-lives than 80 million years, but they are made freshly, as daughter products of decay processes of primordial nuclides (for example, radium from uranium) or from ongoing energetic reactions, such as cosmogenic nuclide
Cosmogenic nuclide
See also Environmental radioactivity#NaturalCosmogenic nuclides are rare isotopes created when a high-energy cosmic ray interacts with the nucleus of an in situ solar system atom, causing cosmic ray spallation...
s produced by present bombardment of Earth by cosmic rays (for example, carbon-14 made from nitrogen).
Many isotopes that are classed as stable (i.e. no radioactivity has been observed for them) are predicted to have extremely long half-lives (sometimes as high as 1018 years or more). If the predicted half-life falls into an experimentally accessible range, such isotopes have a chance to move from the list of stable nuclides to the radioactive category, once their activity is observed. Good examples are bismuth-209 and tungsten-180 which were formerly classed as stable, but have been recently (2003) found to be alpha
Alpha particle
Alpha particles consist of two protons and two neutrons bound together into a particle identical to a helium nucleus, which is classically produced in the process of alpha decay, but may be produced also in other ways and given the same name...
-active. However, such nuclides do not change their status as primordial when they are found to be radioactive.
Most stable isotopes in the earth are believed to have been formed in processes of nucleosynthesis
Nucleosynthesis
Nucleosynthesis is the process of creating new atomic nuclei from pre-existing nucleons . It is thought that the primordial nucleons themselves were formed from the quark–gluon plasma from the Big Bang as it cooled below two trillion degrees...
, either in the 'Big Bang
Big Bang
The Big Bang theory is the prevailing cosmological model that explains the early development of the Universe. According to the Big Bang theory, the Universe was once in an extremely hot and dense state which expanded rapidly. This rapid expansion caused the young Universe to cool and resulted in...
', or in generations of stars that preceded the formation of the solar system. However, some stable isotopes also show abundance variations in the earth as a result of decay from long-lived radioactive nuclides. These decay-products are termed radiogenic isotopes, in order to distinguish them from the much larger group of 'non-radiogenic' isotopes.
Research areas
The so-called Island of StabilityIsland of stability
The island of stability in nuclear physics describes a set of as-yet undiscovered isotopes of transuranium elements which are theorized to be much more stable than others...
may reveal a number of long-lived or even stable atoms that are heavier (and with more protons) than lead.
Stable isotope fractionation
There are three types of isotope fractionation:- equilibrium fractionationEquilibrium fractionationEquilibrium isotope fractionation is the partial separation of isotopes between two or more substances in chemical equilibrium. Equilibrium fractionation is strongest at low temperatures, and forms the basis of the most widely used isotopic paleothermometers : D/H and 18O/16O records from ice...
- kinetic fractionationKinetic fractionationKinetic fractionation is a process that separates stable isotopes from each other by their mass during unidirectional processes.One naturally occurring example of kinetic fractionation is the evaporation of seawater to form clouds...
- mass-independent fractionationMass-independent fractionationMass-independent fractionation refers to any chemical or physical process that acts to separate isotopes, where the amount of separation does not scale in proportion with the difference in the masses of the isotopes...
Isotopes per element
Of the known chemical elements, 80 elements have at least one stable nuclide. These comprise the first 82 elements from hydrogenHydrogen
Hydrogen is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of , hydrogen is the lightest and most abundant chemical element, constituting roughly 75% of the Universe's chemical elemental mass. Stars in the main sequence are mainly...
to lead
Lead
Lead is a main-group element in the carbon group with the symbol Pb and atomic number 82. Lead is a soft, malleable poor metal. It is also counted as one of the heavy metals. Metallic lead has a bluish-white color after being freshly cut, but it soon tarnishes to a dull grayish color when exposed...
, with the exceptions of technetium
Technetium
Technetium is the chemical element with atomic number 43 and symbol Tc. It is the lowest atomic number element without any stable isotopes; every form of it is radioactive. Nearly all technetium is produced synthetically and only minute amounts are found in nature...
(#43) and promethium
Promethium
Promethium is a chemical element with the symbol Pm and atomic number 61. It is notable for being the only exclusively radioactive element besides technetium that is followed by chemical elements with stable isotopes.- Prediction :...
(#61), which do not have any stable nuclides. As of December, 2010, there were a total of 255 known "stable" nuclides. In this definition, "stable" means a nuclide which has either never been observed to decay against the natural background. Thus, these elements have half-lives too long to be measured by any means, direct or indirect.
Only one element (tin
Tin
Tin is a chemical element with the symbol Sn and atomic number 50. It is a main group metal in group 14 of the periodic table. Tin shows chemical similarity to both neighboring group 14 elements, germanium and lead and has two possible oxidation states, +2 and the slightly more stable +4...
) has 10 stable isotopes, and one (xenon
Xenon
Xenon is a chemical element with the symbol Xe and atomic number 54. The element name is pronounced or . A colorless, heavy, odorless noble gas, xenon occurs in the Earth's atmosphere in trace amounts...
) has nine stable isotopes. No elements have exactly eight stable isotopes, but four elements have seven stable isotopes, nine have six stable isotopes, nine have five stable isotopes, nine have four stable isotopes, five have three stable isotopes, 16 have two stable isotopes, and 26 have only a single stable isotope and are thus considered monoisotopic elements. The mean number of stable isotopes for elements which have at least one such isotope, is 255/80 = 3.2.
"Magic numbers" and odd and even proton and neutron count
Stability of isotopes is affected by the ratio of protons to neutrons, and also by presence of certain "magic numbers" of neutrons or protons which represent closed and filled quantum shells. These quantum shells correspond to a set of energy levels within the shell model of the nucleus; filled shells, such as the filled shell of 50 protons for tin, confers unusual stability on the nuclide. As in the case of tin, a magic number for Z, the atomic number, tends to increase the number of stable isotopes for the element.Just as in the case of electrons, which have the lowest energy state when they occur in pairs in a given orbital, nucleons (both protons and neutrons) exhibit a lower energy state when their number is even, rather than odd. This stability tends to prevent beta decay (in two steps) of many even-even nuclides into another even-even nuclide of the same mass number but lower energy (and of course with two more protons and two fewer neutrons), because decay proceeding one step at a time would have to pass through an odd-odd nuclide of higher energy. This makes for a larger number of stable even-even nuclides, up to three for some mass numbers, and up to seven for some atomic (proton) numbers. Conversely, of the 255 known stable nuclides, only four have both an odd number of protons and odd number of neutrons: hydrogen-2 (deuterium
Deuterium
Deuterium, also called heavy hydrogen, is one of two stable isotopes of hydrogen. It has a natural abundance in Earth's oceans of about one atom in of hydrogen . Deuterium accounts for approximately 0.0156% of all naturally occurring hydrogen in Earth's oceans, while the most common isotope ...
), lithium-6, boron-10 and nitrogen-14. Also, only four naturally occurring, radioactive odd-odd nuclides have a half-life over a billion years: potassium-40
Potassium-40
Potassium-40 is a radioactive isotope of potassium which has a very long half-life of 1.248 years, or about 39.38 seconds.Potassium-40 is a rare example of an isotope which undergoes all three types of beta decay. About 89.28% of the time, it decays to calcium-40 with emission of a beta particle...
, vanadium-50, lanthanum-138 and tantalum-180m. Odd-odd primordial nuclide
Primordial nuclide
In geochemistry and geonuclear physics, primordial nuclides or primordial isotopes are nuclides found on the earth that have existed in their current form since before Earth was formed. Only 288 such nuclides are known...
s are rare because most odd-odd nuclei are highly unstable with respect to beta decay
Beta decay
In nuclear physics, beta decay is a type of radioactive decay in which a beta particle is emitted from an atom. There are two types of beta decay: beta minus and beta plus. In the case of beta decay that produces an electron emission, it is referred to as beta minus , while in the case of a...
, because the decay products are even-even, and are therefore more strongly bound, due to nuclear pairing effects.
Yet another effect of the instability of an odd number of either type of nucleons, is that odd-numbered elements tend to have fewer stable isotopes. Of the 26 monoisotopic element
Monoisotopic element
A monoisotopic element is one of 26 chemical elements which have only a single stable isotope . A list is given in a following section....
s that have only a single stable isotope, all but one have an odd atomic number — the single exception to both rules being beryllium. All of these elements also have an even number of neutrons, with the single exception again being beryllium.
Nuclear isomers, including a "stable" one
The count of 255 known stable nuclides includes Ta-180m, since even though its decay and instability is automatically implied by its notation of "metastable", still this has not yet been observed. All "stable" isotopes (stable by observation, not theory) are the ground states of nuclei, with the exception of tantalum-180m, which is the nuclear isomerNuclear isomer
A nuclear isomer is a metastable state of an atomic nucleus caused by the excitation of one or more of its nucleons . "Metastable" refers to the fact that these excited states have half-lives more than 100 to 1000 times the half-lives of the other possible excited nuclear states...
or excited level (the ground state of this nucleus is radioactive with a very short half-life of 8 hours); but the decay of the excited nuclear isomer is extremely strongly forbidden by spin-parity selection rules. It has been reported experimentally by direct observation that the half-life of 180mTa to gamma decay must be more than 1015 years. Other possible modes of 180mTa decay (beta decay, electron capture and alpha decay) have also never been observed.
Primordial radioactive and naturally occurring non-primordial isotopes
Elements with more than 82 protons only have radioactive isotopes, although they can still occur naturally because their half-livesHalf-life
Half-life, abbreviated t½, is the period of time it takes for the amount of a substance undergoing decay to decrease by half. The name was originally used to describe a characteristic of unstable atoms , but it may apply to any quantity which follows a set-rate decay.The original term, dating to...
are more than about 2% of the time since the supernova nucleosynthesis
Supernova nucleosynthesis
Supernova nucleosynthesis is the production of new chemical elements inside supernovae. It occurs primarily due to explosive nucleosynthesis during explosive oxygen burning and silicon burning...
of the elements from which our solar system was made. An extreme case of this is plutonium-244
Plutonium-244
Plutonium-244 is an isotope of plutonium that has a halflife of 80 million years. This is longer than any of the other isotopes of plutonium and longer than any actinide except for the three naturally abundant ones uranium-235 , uranium-238, and thorium-232...
, which is still detectable from primordial reservoirs, even though it has a half-life of only 80 million years (1.8% of the solar system age). There exist about 33 naturally occurring radioactive primordial nuclide
Primordial nuclide
In geochemistry and geonuclear physics, primordial nuclides or primordial isotopes are nuclides found on the earth that have existed in their current form since before Earth was formed. Only 288 such nuclides are known...
s.
In about 50 known cases, elements with shorter half-lives than plutonium-244 are naturally observed on Earth, since as they are produced by cosmic ray
Cosmic ray
Cosmic rays are energetic charged subatomic particles, originating from outer space. They may produce secondary particles that penetrate the Earth's atmosphere and surface. The term ray is historical as cosmic rays were thought to be electromagnetic radiation...
s (e.g., carbon-14
Carbon-14
Carbon-14, 14C, or radiocarbon, is a radioactive isotope of carbon with a 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 colleagues , to date archaeological, geological, and hydrogeological...
), or else because (like radium
Radium
Radium is a chemical element with atomic number 88, represented by the symbol Ra. Radium is an almost pure-white alkaline earth metal, but it readily oxidizes on exposure to air, becoming black in color. All isotopes of radium are highly radioactive, with the most stable isotope being radium-226,...
and polonium
Polonium
Polonium is a chemical element with the symbol Po and atomic number 84, discovered in 1898 by Marie Skłodowska-Curie and Pierre Curie. A rare and highly radioactive element, polonium is chemically similar to bismuth and tellurium, and it occurs in uranium ores. Polonium has been studied for...
) they occur in a decay chain
Decay chain
In nuclear science, the decay chain refers to the radioactive decay of different discrete radioactive decay products as a chained series of transformations...
of radioactive isotopes (primarily uranium and thorium), which have long-enough half-lives to be abundant primordially.
Still-unobserved decay
Bismuth-209
Bismuth-209 is the isotope of bismuth with the longest half-life. It has 83 protons and 126 neutrons, and an atomic mass of 208.9803987 u. All primordial bismuth is of this isotope...
(the only naturally-occurring isotope of bismuth) was shown to be very mildly radioactive. Many "stable" nuclides are possibly "meta-stable" in as much as they may be calculated to have an energy release upon several possible kinds of radioactive decays.
Only 90 nuclides from the first 40 elements are theoretically stable to any sort of decay save proton decay (which has not been observed). The rest, starting with niobium-93, are theoretically unstable to spontaneous fission.
For processes other than spontaneous fission, other theoretical decay routes for heavier elements include:
- alpha decayAlpha decayAlpha decay is a type of radioactive decay in which an atomic nucleus emits an alpha particle and thereby transforms into an atom with a mass number 4 less and atomic number 2 less...
- 70 heavy nuclides - double beta decayDouble beta decayDouble beta decay is a radioactive decay process where a nucleus releases two beta rays as a single process.In double-beta decay, two neutrons in the nucleus are converted to protons, and two electrons and two electron antineutrinos are emitted...
(including double electron captureDouble electron captureDouble electron capture is a decay mode of atomic nucleus. For a nuclide with number of nucleons A and atomic number Z, double electron capture is only possible if the mass of the nuclide of is lower....
, electron-positron conversion and double positron decay) - 55 nuclides - beta decayBeta decayIn nuclear physics, beta decay is a type of radioactive decay in which a beta particle is emitted from an atom. There are two types of beta decay: beta minus and beta plus. In the case of beta decay that produces an electron emission, it is referred to as beta minus , while in the case of a...
- Ta-180m - electron captureElectron captureElectron capture is a process in which a proton-rich nuclide absorbs an inner atomic electron and simultaneously emits a neutrino...
- Te-123, Ta-180m - isomeric transitionIsomeric transitionAn isomeric transition is a radioactive decay process that involves emission of a gamma ray from an atom where the nucleus is in an excited metastable state, referred to in its excited state, as a nuclear isomer....
- Ta-180m - cluster decayCluster decayCluster decay is a type of nuclear decay in which a parent atomic nucleus with A nucleons and Z protons emits a cluster of Ne neutrons and Ze protons heavier than an alpha particle but lighter than a typical binary fission fragment Cluster decay (also named heavy particle radioactivity or heavy...
and spontaneous fissionSpontaneous fissionSpontaneous fission is a form of radioactive decay characteristic of very heavy isotopes. Because the nuclear binding energy reaches a maximum at a nuclear mass greater than about 60 atomic mass units , spontaneous breakdown into smaller nuclei and single particles becomes possible at heavier masses...
- many heaviest nuclides
These include all nuclides of mass 201 and greater. Argon-36 is presently the lightest known "stable" nuclide which is theoretically unstable.
The positivity of energy release in these processes means that they are allowed kinematically (they do not violate the conservation of energy) and, thus, in principle, can occur. They are not observed due to strong but not absolute suppression, by spin-parity selection rules (for beta decays and isomeric transitions) or by the thickness of the potential barrier (for alpha and cluster decays and spontaneous fission).
Summary table for numbers of each class of nuclides
This is a summary table from List of nuclides. Note that numbers are not exact, and may change slightly in the future, as nuclides are observed to be radioactive, or new half-lives are determined to some precision. Note that only the 255 have any claim to stability, but that only 90 nuclides from the first 40 elements are theoretically stable to any process but proton decay.| Type of nuclide by stability class. | Number of nuclides in class (exact number may change). | Running total of nuclides in all classes to this point. | 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. Considered stable until radioactivity confirmed. | 165 | 255 | Spontaneous fission possible for "stable" nuclides > niobium-93. Other mechanisms possible for heavier nuclides. Total is the classically stable nuclides |
| Radioactive primordial nuclide Primordial nuclide In geochemistry and geonuclear physics, primordial nuclides or primordial isotopes are nuclides found on the earth that have existed in their current form since before Earth was formed. Only 288 such nuclides are known... s. |
33 | 288 | Total primordials include Bi, U,Th, Pu, plus all stable nuclides. |
| Radioactive nonprimordial, but naturally occurring on Earth. | ~ 51 | ~ 339 | Cosmogenic nuclide Cosmogenic nuclide See also Environmental radioactivity#NaturalCosmogenic nuclides are rare isotopes created when a high-energy cosmic ray interacts with the nucleus of an in situ solar system atom, causing cosmic ray spallation... s from cosmic rays; daughters of radioactive primordials such as francium Francium Francium is a chemical element with symbol Fr and atomic number 87. It was formerly known as eka-caesium and actinium K.Actually the least unstable isotope, francium-223 It has the lowest electronegativity of all known elements, and is the second rarest naturally occurring element... , etc. |
List of observationally-stable isotopes
In the list below, 90 nuclides have no predicted energetically-possible mode of decay, save proton decay. These are unmarked.Other predicted (but not yet observed) modes of radioactive decay are noted as: A for alpha decay, B for beta decay, BB for double beta decay, E for electron capture, EE for double electron capture, and IT for isomeric transition. Because of the curve of binding energy, all nuclides from Z = 41 (niobium) and beyond, are theoretically unstable with regard to spontaneous fission SF (see list of nuclides for details), and many of the heavier nuclides are theoretically unstable to other processes as well.
Abbreviations:
A for alpha decay, B for beta decay, BB for double beta decay, E for electron capture, EE for double electron capture, IT for isomeric transition.
See also
- Table of nuclidesTable of nuclidesThe tables listed below provide information on the basic properties of all nuclides.* Neutron + Element 1 - Element 24 * Element 25 - Element 48 * Element 49 - Element 72...
- List of nuclides (905 nuclides in order of stability, all with half-lives > one hour)
- Isotope geochemistryIsotope geochemistryIsotope geochemistry is an aspect of geology based upon study of the relative and absolute concentrations of the elements and their isotopes in the Earth. Variations in the abundance of these isotopes, typically measured with an isotope ratio mass spectrometer or an accelerator mass spectrometer,...
- RadionuclideRadionuclideA radionuclide is an atom with an unstable nucleus, which is a nucleus characterized by excess energy available to be imparted either to a newly created radiation particle within the nucleus or to an atomic electron. The radionuclide, in this process, undergoes radioactive decay, and emits gamma...
- Mononuclidic element
- Primordial nuclidePrimordial nuclideIn geochemistry and geonuclear physics, primordial nuclides or primordial isotopes are nuclides found on the earth that have existed in their current form since before Earth was formed. Only 288 such nuclides are known...
- List of elements by stability of isotopes
External links
AlphaDelta: Stable Isotope fractionation calculator - http://www2.ggl.ulaval.ca/cgi-bin/isotope/generisotope.cgi
- National Isotope Development Center Reference information on isotopes, and coordination and management of isotope production, availability, and distribution
- Isotope Development & Production for Research and Applications (IDPRA) U.S. Department of Energy program for isotope production and production research and development