Mononuclidic element

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Mononuclidic and monoisotopic (19 elements)
Two mononuclidic, but radioactive elements (bismuth and protactinium) Monoisotopic, mononuclidic, radioactive elements.svg
  Mononuclidic and monoisotopic (19 elements)
   Two mononuclidic, but radioactive elements (bismuth and protactinium)

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 [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.

Contents

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 substanceRelevant propertyNumber of common isotopesCurrent (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 CarbonAtomic mass3Still 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

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).

ElementMost stable nuclideZ (p)N (n)Isotopic mass (Da) Half-life Second most stable nuclideN (n)Half-life
beryllium 9Be 459.012 182(3)Stable10Be61.387(12)×106 y
fluorine 19F 91018.998 403 2(5)Stable18F9109.739(9) min
sodium 23Na 111222.989 770(2)Stable22Na112.6018(22) y
aluminium 27Al 131426.981 538(2)Stable 26Al 137.17(24)×105 y
phosphorus 31P 151630.973 761(2)Stable33P1825.35(11) d
scandium 45Sc 212444.955 910(8)Stable46Sc2583.79(4) d
manganese 55Mn 253054.938 049(9)Stable53Mn283.7(4)×106 y
cobalt 59Co 273258.933 200(9)Stable 60Co 335.2713(8) y
arsenic 75As 334274.921 60(2)Stable73As4080.30(6) d
yttrium 89Y 395088.905 85(2)Stable88Y49106.616(13) d
niobium 93Nb 415292.906 38(2)Stable92Nb513.47(24)×107 y
rhodium 103Rh 4558102.905 50(2)Stable 102mRh 573.742(10) y
iodine 127I 5374126.904 47(3)Stable 129I 761.57(4)×107 y
caesium 133Cs 5578132.905 45(2)Stable135Cs802.3×106 y
praseodymium 141Pr 5982140.907 65(2)Stable143Pr8413.57(2) d
terbium 159Tb 6594158.925 34(2)Stable158Tb93180(11) y
holmium 165Ho 6798164.930 32(2) Observationally stable 163Ho964570(25) y
thulium 169Tm 69100168.934 21(2)Observationally stable171Tm1021.92(1) y
gold 197Au 79118196.966 55(2)Observationally stable195Au116186.098(47) d
bismuth 209Bi 83126208.980 38(2)2.01(8)×1019 y 210mBi 1273.04(6)×106 y
protactinium 231Pa 91140231.035 88(2)3.276(11)×104 y233Pa14226.975(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. "SI Brochure - 9th ed". Bureau International des Poids et Mesures.
  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