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 mass. Thus, the element's natural isotopic abundance is dominated by one isotope that is either stable or very long-lived. There are 19 elements in the first category (which are both monoisotopic and mononuclidic), and 2 (bismuth 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.
Of the 26 monoisotopic elements that, by definition, have only one stable isotope, seven are not considered mononuclidic, due to the presence of a significant fraction of a very long-lived (primordial) radioisotope. These elements are vanadium, rubidium, indium, lanthanum, europium, lutetium, and rhenium.
Use in metrology 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: Unit Dimension Reference substance Relevant property Number of common isotopes Current (2022) status Second Time Caesium Hyperfine transition frequency 1 Still in use and one of the 7 SI base units Metre Length Krypton Transition wavelength 6 Redefined in 1983 Multiple Temperature Water Melting point, boiling point, and triple point 2 of hydrogen and 3 of oxygen Redefined in 2019 or defunct Calorie and British thermal unit Energy Water Specific heat capacity 2 of hydrogen and 3 of oxygen Calorie redefined in terms of the joule, BTU still in use. Neither unit is part of, or recommended for use in, the SI Mole Amount of substance Carbon Atomic mass 3 Redefined in 2019 Dalton Mass Carbon Atomic mass 3 Still in use and accepted for use in (but not part of) the SI Candela Luminous intensity Platinum Luminance at melting point 6 Redefined in 1979 Millimetre of mercury Pressure Mercury Density 7 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. 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 (isotopic) atomic mass are the same."Monoisotopic elements". CIAAW. 2021. CIAAW use monoisotopic to mean what we call mononuclidic, and also omit europium and rhenium from the note.
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 - This appears to be obsolete, and thorium is no longer considered mononuclidic either.
In nuclear magnetic resonance spectroscopy (NMR), the three most sensitive stable nuclei are hydrogen-1 (1H), fluorine-19 (19F) and phosphorus-31 (31P). Fluorine and phosphorus are monoisotopic, with hydrogen nearly so. 1H NMR, 19F NMR and 31P NMR allow for identification and study of compounds containing these elements.
Contamination by unstable trace isotopes Trace concentrations of unstable isotopes of some mononuclidic elements are found in natural samples. For example, beryllium-10 (10Be), with a half-life of 1.387 million years, is produced by cosmic rays in the Earth's upper atmosphere; iodine-129 (129I), with a half-life of 16.1 million years, is produced by various cosmogenic and nuclear mechanisms; caesium-137 (137Cs), with a half-life of 30.04 years, is generated by nuclear fission. Such isotopes are used in a variety of analytical and forensic applications.
List of the 21 mononuclidic elements
Element Most stable Z (p) N (n) Isotopic mass (Da) Half-life Next most stable N (n) Half-life beryllium 9Be45 9.012 183 06(8)Stable10Be6 fluorine 19F910 18.998 403 1621(9)Stable18F9 sodium 23Na1112 22.989 769 2820(19)Stable22Na11 aluminium 27Al1314 26.981 538 41(5)Stable26Al13 phosphorus 31P1516 30.973 761 9977(8)Stable33P18 scandium 45Sc2124 44.955 907 1(7)Stable46Sc25 manganese 55Mn2530 54.938 043 04(28)Stable53Mn28 cobalt 59Co2732 58.933 193 5(4)Stable60Co33 arsenic 75As3342 74.921 594 6(9)Stable73As40 yttrium 89Y3950 88.905 838 2(4)Stable88Y49 niobium 93Nb4152 92.906 3732(16)Stable92Nb51 rhodium 103Rh4558 102.905 4941(25)Stable101Rh57 iodine 127I5374 126.904 473(4)Stable129I76 caesium 133Cs5578 132.905 451 959(9)Stable135Cs80 praseodymium 141Pr5982 140.907 6596(16)Stable143Pr84 terbium 159Tb6594 158.925 3537(12)Stable158Tb93 holmium 165Ho6798 164.930 329 1(8) Observationally stable163Ho96 thulium 169Tm69100 168.934 219 0(8) Observationally stable171Tm102 gold 197Au79118 196.966 570 1(6) Observationally stable195Au116 bismuth 209Bi83126 208.980 3986(15)210mBi127 protactinium 231Pa91140 231.035 8825(19)233Pa142
See also Primordial element List of nuclides - Table of nuclides sorted by half-life Table of nuclides Isotope geochemistry Radionuclide List of elements by stability of isotopes
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