Monoisotopic, mononuclidic, radioactive elements

A monoisotopic element is an element which has only a single

stable isotope The term stable isotope has a meaning similar to stable nuclide, but is preferably used when speaking of nuclides of a specific element. Hence, the plural form stable isotopes usually refers to isotopes of the same element. The relative abundanc ...

(nuclide). There are only 26 elements that have this property. A list is given in a following section. Stability is experimentally defined for chemical elements, as there are a number of stable nuclides with atomic numbers over ~40 which are theoretically unstable, but apparently have half-lives so long that they have not been observed directly or indirectly (from measurement of products) to decay. Monoisotopic elements are characterized, except in a single case, by odd numbers of protons (odd ''Z''), and even numbers of neutrons. Because of the energy gain from nuclear pairing effects, the odd number of protons imparts instability to isotopes of an odd ''Z'', which in heavier elements requires a completely paired set of neutrons to offset this tendency into stability. (The five stable nuclides with odd ''Z'' and odd neutron numbers are hydrogen-2, lithium-6, boron-10, nitrogen-14, and tantalum-180m1.) The single mononuclidic exception to the odd ''Z'' rule is beryllium; its single stable, primordial isotope, beryllium-9, has 4 protons and 5 neutrons. This element is prevented from having a stable isotope with equal numbers of neutrons and protons (

beryllium-8 Beryllium-8 (8Be, Be-8) is a radionuclide with 4 neutrons and 4 protons. It is an unbound resonance and nominally an isotope of beryllium. It decays into two alpha particles with a half-life on the order of 8.19 seconds. This has important r ...

, with 4 of each) by its instability toward

alpha decay Alpha decay or α-decay is a type of radioactive decay in which an atomic nucleus emits an alpha particle (helium nucleus) and thereby transforms or 'decays' into a different atomic nucleus, with a mass number that is reduced by four and an atom ...

, which is favored due to the extremely tight binding of

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 ...

nuclei. It is prevented from having a stable isotope with 4 protons and 6 neutrons by the very large mismatch in proton/neutron ratio for such a light element. (Nevertheless,

beryllium-10 Beryllium-10 (10Be) is a radioactive isotope of beryllium. It is formed in the Earth's atmosphere mainly by cosmic ray spallation of nitrogen and oxygen. Beryllium-10 has a half-life of 1.39 × 106 years, and decays by beta decay to stable boron- ...

has a half-life of 1.36 million years, which is too short to be primordial, but still indicates unusual stability for a light isotope with such an imbalance.)

Differentiation from mononuclidic elements

The set of monoisotopic elements overlap but are not the same as the set of 21

mononuclidic element A mononuclidic element or monotopic element is one of the 21 chemical elements that is found naturally on Earth essentially as a single nuclide (which may, or may not, be a stable nuclide). This single nuclide will have a characteristic atomic m ...

s, which are characterized as having essentially only one isotope (nuclide) found in nature. The reason for this is the occurrence of certain long-lived radioactive

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 Earth was formed. Primordial nuclides were present in the ...

s in nature, which may form admixtures with the monoisotopics, and thus prevent them from being naturally mononuclidic. This happens in the cases of 7 () of the monoisotopic elements. These isotopes are monoisotopic, but due to the presence of the long-lived radioactive primordial nuclide, are not mononuclidic. These elements are

vanadium Vanadium is a chemical element with the symbol V and atomic number 23. It is a hard, silvery-grey, malleable transition metal. The elemental metal is rarely found in nature, but once isolated artificially, the formation of an oxide layer ( pas ...

rubidium Rubidium is the chemical element with the symbol Rb and atomic number 37. It is a very soft, whitish-grey solid in the alkali metal group, similar to potassium and caesium. Rubidium is the first alkali metal in the group to have a density higher ...

indium Indium is a chemical element with the symbol In and atomic number 49. Indium is the softest metal that is not an alkali metal. It is a silvery-white metal that resembles tin in appearance. It is a post-transition metal that makes up 0.21 parts p ...

lanthanum Lanthanum is a chemical element with the symbol La and atomic number 57. It is a soft, ductile, silvery-white metal that tarnishes slowly when exposed to air. It is the eponym of the lanthanide series, a group of 15 similar elements between lantha ...

europium Europium is a chemical element with the symbol Eu and atomic number 63. Europium is the most reactive lanthanide by far, having to be stored under an inert fluid to protect it from atmospheric oxygen or moisture. Europium is also the softest lanth ...

lutetium Lutetium is a chemical element with the symbol Lu and atomic number 71. It is a silvery white metal, which resists corrosion in dry air, but not in moist air. Lutetium is the last element in the lanthanide series, and it is traditionally counted am ...

and

rhenium Rhenium is a chemical element with the symbol Re and atomic number 75. It is a silvery-gray, heavy, third-row transition metal in group 7 of the periodic table. With an estimated average concentration of 1 part per billion (ppb), rhenium is one ...

. See the list below; in two noted cases (indium and rhenium), the long-lived radionuclide is actually the most abundant isotope in nature, and the stable isotope is less abundant. In 2 additional cases (

bismuth Bismuth is a chemical element with the Symbol (chemistry), symbol Bi and atomic number 83. It is a post-transition metal and one of the pnictogens, with chemical properties resembling its lighter group 15 siblings arsenic and antimony. Elemental ...

Until 2003, ²⁰⁹Bi was thought to be in the first category. It was then found to have 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 ...

of 10¹⁹ years, about a billion times the age of the universe. See

Bismuth Bismuth is a chemical element with the Symbol (chemistry), symbol Bi and atomic number 83. It is a post-transition metal and one of the pnictogens, with chemical properties resembling its lighter group 15 siblings arsenic and antimony. Elemental ...

and

protactinium Protactinium (formerly protoactinium) is a chemical element with the symbol Pa and atomic number 91. It is a dense, silvery-gray actinide metal which readily reacts with oxygen, water vapor and inorganic acids. It forms various chemical compounds ...

), mononuclidic elements occur primordially which are not monoisotopic because the naturally occurring nuclide is radioactive, and thus the element has no stable isotopes at all. For an element to be monoisotopic, it must have one stable nuclide.

List of (observationally-stable) monoisotopic elements, ordered by atomic number and weight

Non-

s are marked with an asterisk, and the long-lived primordial radioisotope given. In two notable cases (indium and rhenium), the highest abundance naturally occurring isotope is the mildly radioactive one, and in the case of europium, nearly half of it is. #

Beryllium-9 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 isot ...

Fluorine-19 Fluorine (9F) has 18 known isotopes ranging from to (with the exception of ) and two isomers ( and ). Only fluorine-19 is stable and naturally occurring in more than trace quantities; therefore, fluorine is a monoisotopic and mononuclidic elem ...

Sodium-23 There are 22 isotopes of sodium (11Na), ranging from to , and two isomers ( and ). is the only stable (and the only primordial) isotope. It is considered a monoisotopic element and it has a standard atomic weight of . Sodium has two radioact ...

Aluminium-27 Aluminium or ''aluminum'' (13Al) has 22 known isotopes from 22Al to 43Al and 4 known isomers. Only 27Al (stable isotope) and 26Al (radioactive isotope, t1/2 = ) occur naturally, however 27Al comprises nearly all natural aluminium. Other than 26 ...

Phosphorus-31 Although phosphorus (15P) has 23 isotopes from 25P to 47P, only 31P is stable; as such, phosphorus is considered a monoisotopic element. The longest-lived radioactive isotopes are 33P with a half-life of 25.34 days and 32P with a half-life of 14.2 ...

Scandium-45 Naturally occurring scandium (21Sc) is composed of one stable isotope, 45Sc. Twenty-five radioisotopes have been characterized, with the most stable being 46Sc with a half-life of 83.8 days, 47Sc with a half-life of 3.35 days, and 48Sc with a hal ...

Vanadium-51 Naturally occurring vanadium (23V) is composed of one stable isotope 51V and one radioactive isotope 50V with a half-life of 1.5×1017 years. 24 artificial radioisotopes have been characterized (in the range of mass number between 40 and 65) with ...

* naturally occurs with 0.25% of radioactive

vanadium-50 Naturally occurring vanadium (23V) is composed of one stable isotope 51V and one radioactive isotope 50V with a half-life of 1.5×1017 years. 24 artificial radioisotopes have been characterized (in the range of mass number between 40 and 65) with t ...

Manganese-55 Naturally occurring manganese (25Mn) is composed of one stable isotope, 55Mn. 25 radioisotopes have been characterized, with the most stable being 53Mn with a half-life of 3.7 million years, 54Mn with a half-life of 312.3 days, and 52Mn with a hal ...

Cobalt-59 Naturally occurring cobalt (Co) consists of a single stable isotope, Co. Twenty-eight radioisotopes have been characterized; the most stable are Co with a half-life of 5.2714 years, Co (271.8 days), Co (77.27 days), and Co (70.86 days). All other ...

# Arsenic-75 #

Rubidium-85 Rubidium (37Rb) has 36 isotopes, with naturally occurring rubidium being composed of just two isotopes; 85Rb (72.2%) and the radioactive 87Rb (27.8%). Normal mixes of rubidium are radioactive enough to fog photographic film in approximately 30 ...

* naturally occurs with 27.835% of radioactive

rubidium-87 Rubidium (37Rb) has 36 isotopes, with naturally occurring rubidium being composed of just two isotopes; 85Rb (72.2%) and the radioactive 87Rb (27.8%). Normal mixes of rubidium are radioactive enough to fog photographic film in approximately 30 ...

# Yttrium-89 #

Niobium-93 Naturally occurring niobium (41Nb) is composed of one stable isotope (93Nb). The most stable radioisotope is 92Nb with a half-life of 34.7 million years. The next longest-lived niobium isotopes are 94Nb (half-life: 20,300 years) and 91Nb with a ...

Rhodium-103 Naturally occurring rhodium (45Rh) is composed of only one stable isotope, 103Rh. The most stable radioisotopes are 101Rh with a half-life of 3.3 years, 102Rh with a half-life of 207 days, and 99Rh with a half-life of 16.1 days. Thirty other radio ...

# Indium-113* naturally occurs with majority (95.7%) radioactive isotope indium-115 #

Iodine-127 There are 37 known isotopes of iodine (53I) from 108I to 144I; all undergo radioactive decay except 127I, which is stable. Iodine is thus a monoisotopic element. Its longest-lived radioactive isotope, 129I, has a half-life of 15.7 million yea ...

Caesium-133 Caesium (55Cs) has 40 known isotopes, making it, along with barium and mercury, one of the elements with the most isotopes. The atomic masses of these isotopes range from 112 to 151. Only one isotope, 133Cs, is stable. The longest-lived radioisoto ...

Lanthanum-139 Naturally occurring lanthanum (57La) is composed of one stable (139La) and one radioactive (138La) isotope, with the stable isotope, 139La, being the most abundant (99.91% natural abundance). There are 38 radioisotopes that have been characteriz ...

* naturally occurs with 0.09% radioactive

lanthanum-138 Naturally occurring lanthanum (57La) is composed of one stable (139La) and one radioactive (138La) isotope, with the stable isotope, 139La, being the most abundant (99.91% natural abundance). There are 38 radioisotopes that have been characterize ...

Praseodymium-141 Naturally occurring praseodymium (59Pr) is composed of one stable isotope, 141Pr. Thirty-eight radioisotopes have been characterized with the most stable being 143Pr, with a half-life of 13.57 days and 142Pr, with a half-life of 19.12 hours. All o ...

# Europium-153* naturally occurs with 47.8% radioactive europium-151 # Terbium-159 # Holmium-165 # Thulium-169 # Lutetium-175* naturally occurs with 2.59% radioactive

lutetium-176 Naturally occurring lutetium (71Lu) is composed of one stable isotope 175Lu (97.41% natural abundance) and one long-lived radioisotope, 176Lu with a half-life of 3.78 × 1010 years (2.59% natural abundance). Thirty-five radioisotopes have been cha ...

# Rhenium-185* naturally occurs with majority (62.6%) radioactive isotope

rhenium-187 Naturally occurring rhenium (75Re) is 37.4% 185Re, which is stable (although it is predicted to decay), and 62.6% 187Re, which is unstable but has a very long half-life (4.12×1010 years). Among elements with a known stable isotope, only indium ...

Gold-197 Gold (79Au) has one stable isotope, 197Au, and 36 radioisotopes, with 195Au being the most stable with a half-life of 186 days. Gold is currently considered the heaviest monoisotopic element. Bismuth formerly held that distinction until alpha-dec ...

References

External links

Atomic Weights of the Elements as of year 2000, with a section on monoisotopics starting page 708
{{DEFAULTSORT:Monoisotopic Element Isotopes Sets of chemical elements Metrology

Differentiation from mononuclidic elements

List of (observationally-stable) monoisotopic elements, ordered by atomic number and weight

See also

References

External links