A mononuclidic element or monotopic element[1] 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 mass. Thus, the element's natural isotopic abundance is dominated by one isotope that is either stable or very long-lived. There are 19elements in the first category (which are both monoisotopic and mononuclidic), and 2 (bismuth[a] and protactinium) in the second category (mononuclidic but not monoisotopic, since they have zero, not one, stable nuclides). A list of the 21 mononuclidic elements is given at the end of this article.
Many units of measurement were historically, or are still, defined with reference to the properties of specific substances that, in many cases, occurred in nature as mixes of multiple isotopes, for example:
Redefined in terms of the pascal, not part of, or recommended for use in, the SI
Since samples taken from different natural sources can have subtly different isotopic ratios, the relevant properties can differ between samples. If the definition simply refers to a substance without addressing the isotopic composition, this can lead to some level of ambiguity in the definition and variation in practical realizations of the unit by different laboratories, as was observed with the kelvin before 2007.[9] If the definition refers only to one isotope (as that of the dalton does) or to a specific isotope ratio, e.g. Vienna Standard Mean Ocean Water, this removes a source of ambiguity and variation, but adds layers of technical difficulty (preparing samples of a desired isotopic ratio) and uncertainty (regarding how much an actual reference sample differs from the nominal ratio). The use of mononuclidic elements as reference material sidesteps these issues and notably the only substance referenced in the most recent iteration of the SI is caesium, a mononuclidic element.
Mononuclidic elements are also 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 standard atomic weight and atomic mass are the same.[10]
Trace concentrations of unstable isotopes of some mononuclidic elements are found in natural samples. For example, beryllium-10 (10Be), with a half-life of 1.4 million years, is produced by cosmic rays in the Earth's upper atmosphere; iodine-129 (129I), with a half-life of 15.7 million years, is produced by various cosmogenic and nuclear mechanisms; caesium-137 (137Cs), with a half-life of 30years, is generated by nuclear fission. Such isotopes are used in a variety of analytical and forensic applications.
↑ 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
References
↑ Housecroft, C. E.; Sharpe, A. G. (2012). Inorganic Chemistry (4thed.). Prentice Hall. p.2. ISBN978-0273742753.
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