Mononuclidic element

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 19 elements in the first category (which are both monoisotopic and mononuclidic), and 2 (bismuth[lower-alpha 1] 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.

Not to be confused with the 26 monoisotopic elements defined as having only one stable nuclide.
  Mononuclidic and monoisotopic (19 elements)
   Two mononuclidic, but radioactive elements (bismuth and protactinium)

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[2]
Metre Length Krypton Transition wavelength 6 Redefined in 1983[3]
Multiple Temperature Water Melting point, boiling point, and triple point 2 of hydrogen and 3 of oxygen Redefined in 2019[4] 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.[5] Neither unit is part of, or recommended for use in, the SI
Mole Amount of substance Carbon Atomic mass 3 Redefined in 2019[6]
Dalton Mass Carbon Atomic mass 3 Still in use and accepted for use in (but not part of) the SI[7]
Candela Luminous intensity Platinum Luminance at melting point 6 Redefined in 1979[8]
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.[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]

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.[11]

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 1.4 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 30 years, is generated by nuclear fission. Such isotopes are used in a variety of analytical and forensic applications.

List of the 21 mononuclidic elements
See also: List of elements by stability of isotopes

Isotopic mass data from Atomic Weights and Isotopic Compositions ed. J. S. Coursey, D. J. Schwab and R. A. Dragoset, National Institute of Standards and Technology (2005).

Element Most stable nuclide Z (p) N (n) Isotopic mass (Da) Half-life Second most stable nuclide N (n) Half-life
beryllium 9Be 4 5 9.012 182(3) Stable 10Be 6 1.387(12)×106 y
fluorine 19F 9 10 18.998 403 2(5) Stable 18F 9 109.739(9) min
sodium 23Na 11 12 22.989 770(2) Stable 22Na 11 2.6018(22) y
aluminium 27Al 13 14 26.981 538(2) Stable 26Al 13 7.17(24)×105 y
phosphorus 31P 15 16 30.973 761(2) Stable 33P 18 25.35(11) d
scandium 45Sc 21 24 44.955 910(8) Stable 46Sc 25 83.79(4) d
manganese 55Mn 25 30 54.938 049(9) Stable 53Mn 28 3.7(4)×106 y
cobalt 59Co 27 32 58.933 200(9) Stable 60Co 33 5.2713(8) y
arsenic 75As 33 42 74.921 60(2) Stable 73As 40 80.30(6) d
yttrium 89Y 39 50 88.905 85(2) Stable 88Y 49 106.616(13) d
niobium 93Nb 41 52 92.906 38(2) Stable 92Nb 51 3.47(24)×107 y
rhodium 103Rh 45 58 102.905 50(2) Stable 102mRh 57 3.742(10) y
iodine 127I 53 74 126.904 47(3) Stable 129I 76 1.57(4)×107 y
caesium 133Cs 55 78 132.905 45(2) Stable 135Cs 80 2.3×106 y
praseodymium 141Pr 59 82 140.907 65(2) Stable 143Pr 84 13.57(2) d
terbium 159Tb 65 94 158.925 34(2) Stable 158Tb 93 180(11) y
holmium 165Ho 67 98 164.930 32(2) Observationally stable 163Ho 97 4570(25) y
thulium 169Tm 69 100 168.934 21(2) Observationally stable 171Tm 102 1.92(1) y
gold 197Au 79 118 196.966 55(2) Observationally stable 195Au 116 186.098(47) d
bismuth 209Bi 83 126 208.980 38(2) 2.01(8)×1019 y 210mBi 127 3.04(6)×106 y
protactinium 231Pa 91 140 231.035 88(2) 3.276(11)×104 y 233Pa 142 26.975(13) d

See also

Notes
  1. 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
  1. Housecroft, C. E.; Sharpe, A. G. (2012). Inorganic Chemistry (4th ed.). Prentice Hall. p. 2. ISBN 978-0273742753.
  2. "Second - BIPM".
  3. "Metre - BIPM".
  4. "Kelvin - BIPM".
  5. "British thermal units (Btu) - U.S. Energy Information Administration (EIA)".
  6. "Mole - BIPM".
  7. https://www.bipm.org/documents/20126/41483022/SI-Brochure-9-EN.pdf/2d2b50bf-f2b4-9661-f402-5f9d66e4b507
  8. "Candela - BIPM".
  9. "Resolution 10 - BIPM".
  10. N. E. Holden, "Standard Atomic Weight Values for the Mononuclidic Elements - 2001," BNL-NCS-68362, Brookhaven National Laboratory (2001)
  11. IUPAC list of mononuclidics for metrology purposes
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