Mononuclidic element explained

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:

UnitDimensionReference substanceRelevant propertyNumber of common isotopesCurrent (2022) status
SecondTimeCaesiumHyperfine transition frequency1Still in use and one of the 7 SI base units[1]
MetreLengthKryptonTransition wavelength6Redefined in 1983[2]
MultipleTemperatureWaterMelting point, boiling point, and triple point2 of hydrogen and 3 of oxygenRedefined in 2019[3] or defunct
Calorie and British thermal unitEnergyWaterSpecific heat capacity2 of hydrogen and 3 of oxygenCalorie redefined in terms of the joule, BTU still in use.[4] Neither unit is part of, or recommended for use in, the SI
MoleAmount of substanceCarbonAtomic mass3Redefined in 2019[5]
DaltonMassCarbonAtomic mass3Still in use and accepted for use in (but not part of) the SI[6]
CandelaLuminous intensityPlatinumLuminance at melting point6Redefined in 1979[7]
Millimetre of mercuryPressureMercuryDensity7Redefined 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.[8] 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.[9]

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.[10]

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).

ElementMost stable nuclideZ (p)N (n)Isotopic mass (Da)Half-lifeSecond most stable nuclideN (n)Half-life
beryllium9Be459.012 182(3)Stable10Be6
fluorine19F91018.998 403 2(5)Stable18F9
sodium23Na111222.989 770(2)Stable22Na11
aluminium27Al131426.981 538(2)Stable26Al13
phosphorus31P151630.973 761(2)Stable33P18
scandium45Sc212444.955 910(8)Stable46Sc25
manganese55Mn253054.938 049(9)Stable53Mn28
cobalt59Co273258.933 200(9)Stable60Co33
arsenic75As334274.921 60(2)Stable73As40
yttrium89Y395088.905 85(2)Stable88Y49
niobium93Nb415292.906 38(2)Stable92Nb51
rhodium103Rh4558102.905 50(2)Stable102mRh57
iodine127I5374126.904 47(3)Stable129I76
caesium133Cs5578132.905 45(2)Stable135Cs80
praseodymium141Pr5982140.907 65(2)Stable143Pr84
terbium159Tb6594158.925 34(2)Stable158Tb93
holmium165Ho6798164.930 32(2)Observationally stable163Ho97
thulium169Tm69100168.934 21(2)Observationally stable171Tm102
gold197Au79118196.966 55(2)Observationally stable195Au116
bismuth209Bi83126208.980 38(2)210mBi127
protactinium231Pa91140231.035 88(2)233Pa142

See also

Notes and References

  1. Web site: Second - BIPM .
  2. Web site: Metre - BIPM .
  3. Web site: Kelvin - BIPM .
  4. Web site: British thermal units (Btu) - U.S. Energy Information Administration (EIA) .
  5. Web site: Mole - BIPM .
  6. https://www.bipm.org/documents/20126/41483022/SI-Brochure-9-EN.pdf/2d2b50bf-f2b4-9661-f402-5f9d66e4b507
  7. Web site: Candela - BIPM .
  8. Web site: Resolution 10 - BIPM .
  9. N. E. Holden, "Standard Atomic Weight Values for the Mononuclidic Elements - 2001," BNL-NCS-68362, Brookhaven National Laboratory (2001)
  10. http://www.iupac.org/publications/pac/2006/pdf/7811x2051.pdf IUPAC list of mononuclidics for metrology purposes