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mononuclidic element
A mononuclidic element is one of the 22 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 mass. Thus, the element's natural isotopic abundance is dominated either by one stable isotope or by one very long-lived isotope. There are 19 elements in the first category (which are both monoisotopic and mononuclidic), and 3 (bismuth,〔Until 2003, 209Bi was thought to be in the first category. It was then found to have a half-life of 1019 years, about a billion times the age of the universe. See Bismuth〕 thorium and protactinium) in the second category (mononuclidic but not monoisotopic, since they have zero, not one, stable nuclides). A list of the 22 mononuclidic elements is given at the end of this article.
Of the 26 ''monoisotopic elements'' that, by definition, have only one stable isotope, there exist 7 (26 minus 19 = 7) which are nevertheless ''not'' considered mononuclidic, due to the presence of a significant fraction of a very long-lived (primordial) radioisotope occurring in their natural abundance. These elements are vanadium, rubidium, indium, lanthanum, europium, rhenium and lutetium.
== Use in metrology ==
Mononuclidic elements are of scientific importance because their atomic weights can be measured to high accuracy, since there is minimal uncertainty associated with the isotopic abundances present in a given sample. Another way of stating this, is that, for these elements, the relative atomic mass and atomic mass are the same.〔N. E. Holden, "Standard Atomic Weight Values for the Mononuclidic Elements - 2001," (BNL-NCS-68362 ), Brookhaven National Laboratory (2001)〕
In practice, only 11 of the mononuclidic elements are used in standard atomic weight metrology. These are aluminium, bismuth, caesium, cobalt, gold, manganese, phosphorus, scandium, sodium, terbium, and thorium.〔(IUPAC list of mononuclidics for metrology purposes )〕
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