View source for Monoisotopic element

{{Short description|Element that has only a single stable isotope}}
{{distinguish|text=the 21 [[mononuclidic element]]s, defined as those with only one significant naturally-abundant nuclide}}

[[File:Monoisotopic, mononuclidic, radioactive elements.svg|thumb|366px|{{legend2|#00FF00}}Monoisotopic [[#Differentiation from mononuclidic elements|''and'' mononuclidic]] elements<br/>{{legend2|#00FFFF}}{{legend2|#99FFFF}} Monoisotopic, but [[primordial radionuclide]]s exist]]
A '''monoisotopic element''' is an element which has only a single [[stable isotope]] (nuclide). There are 26 such elements, as listed.

Stability is experimentally defined for chemical elements, as there are a number of stable nuclides with atomic numbers over ~{{nbs}}40 which are theoretically unstable, but apparently have half-lives so long that they have not been observed either directly or indirectly (from measurement of products) to decay.

Monoisotopic elements are characterized, except in one case, by odd numbers of protons (odd ''Z''), and even numbers of neutrons. Because of the energy gain from nuclear pairing, 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 monoisotopic exception to the odd ''Z'' rule is beryllium; its single stable, primordial isotope, beryllium-9, has 4&nbsp;protons and 5&nbsp;neutrons. This element is prevented from having a stable isotope with equal numbers of neutrons and protons ([[beryllium-8]], with 4 of each) by its instability toward [[alpha decay]], which is favored due to the extremely tight binding of [[helium#Related stability of the helium-4 nucleus and electron shell|helium-4]] 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]] has a half-life of 1.36 million years, which is too short to be [[primordial element|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]]s, which are characterized as having essentially only one isotope (nuclide) found in nature.<ref>N. E. Holden, "Standard Atomic Weight Values for the Mononuclidic Elements - 2001," [http://www.osti.gov/bridge/servlets/purl/782053-S3nk8u/native/782053.pdf BNL-NCS-68362], [[Brookhaven National Laboratory]] (2001)</ref> The reason for this is the occurrence of certain long-lived radioactive [[primordial nuclide]]s in nature, which may form admixtures with the monoisotopics, and thus prevent them from being naturally mononuclidic. This happens in the cases of seven 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]], [[rubidium]], [[indium]], [[lanthanum]], [[europium]], [[lutetium]] and [[rhenium]]. For indium and rhenium, the long-lived radionuclide is actually the most abundant isotope in nature, and the stable isotope is less abundant. 

In two additional cases ([[bismuth]]<ref>Until 2003, <sup>209</sup>Bi was thought to be in the first category. It was then found to have a [[half-life]] of 10<sup>19</sup> years, about a billion times the age of the universe. See [[Bismuth#Characteristics|Bismuth]]</ref> and [[protactinium]]), mononuclidic elements occur 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-[[mononuclidic element]]s are marked with an asterisk, and the long-lived [[primordial nuclide|primordial radioisotope]] given. In two cases (indium and rhenium), the most abundant naturally occurring isotope is the mildly radioactive one, and in the case of europium, nearly half of it is.

{{div col|colwidth=27em}}
# [[Beryllium-9]]
# [[Fluorine-19]]
# [[Sodium-23]]
# [[Aluminium-27]]
# [[Phosphorus-31]]
# [[Scandium-45]]
# [[Vanadium-51]]* naturally occurs with 0.25% of radioactive [[vanadium-50]]
# [[Manganese-55]]
# [[Cobalt-59]]
# [[Arsenic-75]]
# [[Rubidium-85]]* naturally occurs with 27.835% of radioactive [[rubidium-87]]
# [[Yttrium-89]]
# [[Niobium-93]]
# [[Rhodium-103]]
# [[Indium-113]]* naturally occurs with '''majority''' (95.7%) radioactive isotope [[indium-115]]
# [[Iodine-127]]
# [[Caesium-133]]
# [[Lanthanum-139]]* naturally occurs with 0.09% radioactive [[lanthanum-138]]
# [[Praseodymium-141]]
# [[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]]
# [[Rhenium-185]]* naturally occurs with '''majority''' (62.6%) radioactive isotope [[rhenium-187]]
# [[Gold-197]]
{{div col end}}

== See also ==
{{div col|colwidth=27em}}
* [[Primordial nuclide]]
* [[List of nuclides]] (sorted by half-life)
* [[Table of nuclides]]
* [[Isotope geochemistry]]
* [[Radionuclide]]
* [[Mononuclidic element]]
* [[Stable isotope]]
* [[List of elements by stability of isotopes]]
{{div col end}}

== References ==
{{Reflist}}

== External links ==
* [http://www.iupac.org/publications/pac/2003/pdf/7506x0683.pdf Atomic Weights of the Elements as of year 2000, with a section on monoisotopics starting page&nbsp;708]

{{DEFAULTSORT:Monoisotopic Element}}
[[Category:Isotopes]]
[[Category:Sets of chemical elements]]
[[Category:Metrology]]