Menu
Home
People
Places
Arts
History
Plants & Animals
Science
Life & Culture
Technology
Reference.org
Mononuclidic element
open-in-new
<h2 id="use-in-metrology">Use in metrology</h2> <p>Many <a href="/facts/Units_of_measurement/pvTpWbmC">units of measurement</a> 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: </p> <table><tbody><tr><th>Unit</th><th><a href="/facts/Dimensional_analysis/t6UskoCD">Dimension</a></th><th>Reference substance</th><th>Relevant property</th><th>Number of common isotopes</th><th>Current (2022) status</th></tr><tr><td><a href="/facts/Second/jtFxnJXx">Second</a></td><td><a href="/facts/Time/HceJKUGB">Time</a></td><td><a href="/facts/Caesium/dXKN1S5k">Caesium</a></td><td><a href="/facts/Caesium_standard/e8eu9bdv">Hyperfine transition frequency</a></td><td><a href="/facts/Isotopes_of_caesium/m5AYV9tp">1</a></td><td>Still in use and one of the 7 <a href="/facts/SI_base_units/M7PsTWg4">SI base units</a><a class="footnote-ref" id="fnref:3" href="#fn:3"><sup>3</sup></a></td></tr><tr><td><a href="/facts/Metre/hPhzc1CV">Metre</a></td><td><a href="/facts/Length/EwFyAqGH">Length</a></td><td><a href="/facts/Krypton/llkNlCke">Krypton</a></td><td>Transition <a href="/facts/Wavelength/kcJU0ymz">wavelength</a></td><td><a href="/facts/Isotopes_of_krypton/8hRKYepJ">6</a></td><td>Redefined in 1983<a class="footnote-ref" id="fnref:4" href="#fn:4"><sup>4</sup></a></td></tr><tr><td><a href="/facts/Conversion_of_scales_of_temperature/JBPyokVb">Multiple</a></td><td><a href="/facts/Temperature/QyGe4gmj">Temperature</a></td><td><a href="/facts/Water/0lkXnJPK">Water</a></td><td><a href="/facts/Melting_point/MY0w0h0k">Melting point</a>, <a href="/facts/Boiling_point/E2BK9QoS">boiling point</a>, and <a href="/facts/Triple_point/Npary634">triple point</a></td><td><a href="/facts/Isotopes_of_hydrogen/c3CAg2Pc">2 of hydrogen</a> and <a href="/facts/Isotopes_of_oxygen/ektDXb8B">3 of oxygen</a></td><td>Redefined in 2019<a class="footnote-ref" id="fnref:5" href="#fn:5"><sup>5</sup></a> or defunct</td></tr><tr><td><a href="/facts/Calorie/ntNMPx65">Calorie</a> and <a href="/facts/British_thermal_unit/GwIjRAPz">British thermal unit</a></td><td><a href="/facts/Energy/wd8PrQM7">Energy</a></td><td>Water</td><td><a href="/facts/Specific_heat_capacity/9P0U6YgI">Specific heat capacity</a></td><td>2 of <a href="/facts/Hydrogen/BoYHjl0J">hydrogen</a> and 3 of <a href="/facts/Oxygen/acyn6o8r">oxygen</a></td><td>Calorie redefined in terms of the <a href="/facts/Joule/9ndq9jD2">joule</a>, BTU still in use.<a class="footnote-ref" id="fnref:6" href="#fn:6"><sup>6</sup></a> Neither unit is part of, or recommended for use in, the <a href="/facts/International_System_of_Units/S7YLglNz">SI</a></td></tr><tr><td><a href="/facts/Mole_(unit)/kCMFwCc0">Mole</a></td><td><a href="/facts/Amount_of_substance/tQVyNLCp">Amount of substance</a></td><td><a href="/facts/Carbon/ukrgQexo">Carbon</a></td><td><a href="/facts/Atomic_mass/J1FJY9MV">Atomic mass</a></td><td><a href="/facts/Isotopes_of_carbon/K1LfqDNj">3</a></td><td>Redefined in 2019<a class="footnote-ref" id="fnref:7" href="#fn:7"><sup>7</sup></a></td></tr><tr><td><a href="/facts/Dalton_(unit)/IMATU3vf">Dalton</a></td><td><a href="/facts/Mass/HHdc8XmF">Mass</a></td><td>Carbon</td><td>Atomic mass</td><td>3</td><td>Still in use and accepted for use in (but not part of) the SI<a class="footnote-ref" id="fnref:8" href="#fn:8"><sup>8</sup></a></td></tr><tr><td><a href="/facts/Candela/grEDstnw">Candela</a></td><td><a href="/facts/Luminous_intensity/uUSDSAKR">Luminous intensity</a></td><td><a href="/facts/Platinum/qJC6lQf7">Platinum</a></td><td><a href="/facts/Luminance/yAqK94ny">Luminance</a> at melting point</td><td><a href="/facts/Isotopes_of_platinum/G5CS1TkP">6</a></td><td>Redefined in 1979<a class="footnote-ref" id="fnref:9" href="#fn:9"><sup>9</sup></a></td></tr><tr><td><a href="/facts/Millimetre_of_mercury/qVG9oG1d">Millimetre of mercury</a></td><td><a href="/facts/Pressure/eyc37GRG">Pressure</a></td><td><a href="/facts/Mercury_(element)/WlxfoHva">Mercury</a></td><td><a href="/facts/Density/92p6hh9I">Density</a></td><td><a href="/facts/Isotopes_of_mercury/gVVXzNeh">7</a></td><td>Redefined in terms of the <a href="/facts/Pascal_(unit)/Am1To2Qm">pascal</a>, not part of, or recommended for use in, the SI</td></tr></tbody></table> <p>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 <a href="/facts/Kelvin/E3trzC4C">kelvin</a> before 2007.<a class="footnote-ref" id="fnref:10" href="#fn:10"><sup>10</sup></a> If the definition refers only to one isotope (as that of the dalton does) or to a specific isotope ratio, e.g. <a href="/facts/Vienna_Standard_Mean_Ocean_Water/coVVQaza">Vienna Standard Mean Ocean Water</a>, 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 <a href="/facts/2019_revision_of_the_SI/g0h07jCX">the most recent iteration</a> of the SI is caesium, a mononuclidic element. </p><p>Mononuclidic elements are also of scientific importance because their <a href="/facts/Atomic_weight/A3vbuz2n">atomic weights</a> can be <a href="/facts/Metrology/HC0MmLdb">measured</a> 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 <a href="/facts/Standard_atomic_weight/8arqbQsq">standard atomic weight</a> and <a href="/facts/Atomic_mass/J1FJY9MV">atomic mass</a> are the same.<a class="footnote-ref" id="fnref:11" href="#fn:11"><sup>11</sup></a> </p><p>In practice, only 11 of the mononuclidic elements are used in standard atomic weight metrology. These are <a href="/facts/Aluminium/7wmR8jYE">aluminium</a>, <a href="/facts/Bismuth/75FLjkoX">bismuth</a>, <a href="/facts/Caesium/dXKN1S5k">caesium</a>, <a href="/facts/Cobalt/gqhScEw4">cobalt</a>, <a href="/facts/Gold/1znknojp">gold</a>, <a href="/facts/Manganese/j9ELd1TL">manganese</a>, phosphorus, <a href="/facts/Scandium/TJRt1chg">scandium</a>, sodium, <a href="/facts/Terbium/ICF3PMED">terbium</a>, and <a href="/facts/Thorium/ysdWH470">thorium</a>.<a class="footnote-ref" id="fnref:12" href="#fn:12"><sup>12</sup></a> </p><p>In <a href="/facts/Nuclear_magnetic_resonance_spectroscopy/SFKgdthI">nuclear magnetic resonance spectroscopy</a> (NMR), the three most sensitive stable nuclei are <a href="/facts/Hydrogen-1/c3CAg2Pc">hydrogen-1</a> (1H), <a href="/facts/Fluorine-19/nlSCwKKe">fluorine-19</a> (19F) and <a href="/facts/Phosphorus-31/vE2sGdt2">phosphorus-31</a> (31P). Fluorine and phosphorus are monoisotopic, with hydrogen nearly so. <a href="/facts/Proton_nuclear_magnetic_resonance/g9AGxvSP">1H NMR</a>, <a href="/facts/Fluorine-19_nuclear_magnetic_resonance_spectroscopy/uMuD7QbT">19F NMR</a> and <a href="/facts/Phosphorus-31_nuclear_magnetic_resonance/0t8CaSjY">31P NMR</a> allow for identification and study of compounds containing these elements. </p> <h2 id="contamination-by-unstable-trace-isotopes">Contamination by unstable trace isotopes</h2> <p>Trace concentrations of unstable isotopes of some mononuclidic elements are found in natural samples. For example, <a href="/facts/Beryllium-10/oz3rQBLB">beryllium-10</a> (10Be), with a <a href="/facts/Half-life/kGVH8BAB">half-life</a> of 1.4 million years, is produced by <a href="/facts/Cosmic_ray/uxCVdTKd">cosmic rays</a> in the <a href="/facts/Earth/HLLmDfj0">Earth</a>'s <a href="/facts/Upper_atmosphere/5yH29VP8">upper atmosphere</a>; <a href="/facts/Iodine-129/oJDlGf5H">iodine-129</a> (129I), with a <a href="/facts/Half-life/kGVH8BAB">half-life</a> of 15.7 million years, is produced by various cosmogenic and nuclear mechanisms; <a href="/facts/Caesium-137/NrY5Tb6j">caesium-137</a> (137Cs), with a <a href="/facts/Half-life/kGVH8BAB">half-life</a> of 30 years, is generated by <a href="/facts/Nuclear_fission/LfkIylvm">nuclear fission</a>. Such isotopes are used in a variety of analytical and forensic applications. </p> <h2 id="list-of-the-21-mononuclidic-elements">List of the 21 mononuclidic elements</h2> <p class="note">See also: <a href="/facts/List_of_elements_by_stability_of_isotopes/Bm0ogJmR">List of elements by stability of isotopes</a></p> <p>Isotopic mass data from <a href="http://physics.nist.gov/PhysRefData/Compositions/">Atomic Weights and Isotopic Compositions</a> ed. J. S. Coursey, D. J. Schwab and R. A. Dragoset, <a href="/facts/National_Institute_of_Standards_and_Technology/gr9Fnh85">National Institute of Standards and Technology</a> (2005). </p> <table><tbody><tr><th>Element</th><th>Most stable nuclide</th><th>Z (<a href="/facts/Proton/clCjepXP">p</a>)</th><th>N (<a href="/facts/Neutron/oRCM1geb">n</a>)</th><th>Isotopic mass (<a href="/facts/Dalton_(unit)/IMATU3vf">Da</a>)</th><th><a href="/facts/Half-life/kGVH8BAB">Half-life</a></th><th>Second most stable nuclide</th><th>N (<a href="/facts/Neutron/oRCM1geb">n</a>)</th><th>Half-life</th></tr><tr><td><a href="/facts/Beryllium/dv295voP">beryllium</a></td><td><a href="/facts/Isotopes_of_beryllium/bNvHB5xg">9Be</a></td><td>4</td><td>5</td><td>9.012 182(3)</td><td>Stable</td><td>10Be</td><td>6</td><td>1.387(12)×106 y</td></tr><tr><td><a href="/facts/Fluorine/YUV0Fb8m">fluorine</a></td><td><a href="/facts/Isotopes_of_fluorine/nlSCwKKe">19F</a></td><td>9</td><td>10</td><td>18.998 403 2(5)</td><td>Stable</td><td>18F</td><td>9</td><td>109.739(9) min</td></tr><tr><td><a href="/facts/Sodium/eYuSPu2V">sodium</a></td><td><a href="/facts/Isotopes_of_sodium/dGhdEPWX">23Na</a></td><td>11</td><td>12</td><td>22.989 770(2)</td><td>Stable</td><td>22Na</td><td>11</td><td>2.6018(22) y</td></tr><tr><td><a href="/facts/Aluminium/7wmR8jYE">aluminium</a></td><td><a href="/facts/Isotopes_of_aluminium/8nsVdoC5">27Al</a></td><td>13</td><td>14</td><td>26.981 538(2)</td><td>Stable</td><td><a href="/facts/Aluminium-26/INwQYapg">26Al</a></td><td>13</td><td>7.17(24)×105 y</td></tr><tr><td><a href="/facts/Phosphorus/27xCeIqs">phosphorus</a></td><td><a href="/facts/Isotopes_of_phosphorus/vE2sGdt2">31P</a></td><td>15</td><td>16</td><td>30.973 761(2)</td><td>Stable</td><td>33P</td><td>18</td><td>25.35(11) d</td></tr><tr><td><a href="/facts/Scandium/TJRt1chg">scandium</a></td><td><a href="/facts/Isotopes_of_scandium/avIUruhT">45Sc</a></td><td>21</td><td>24</td><td>44.955 910(8)</td><td>Stable</td><td>46Sc</td><td>25</td><td>83.79(4) d</td></tr><tr><td><a href="/facts/Manganese/j9ELd1TL">manganese</a></td><td><a href="/facts/Isotopes_of_manganese/NUERgknG">55Mn</a></td><td>25</td><td>30</td><td>54.938 049(9)</td><td>Stable</td><td>53Mn</td><td>28</td><td>3.7(4)×106 y</td></tr><tr><td><a href="/facts/Cobalt/gqhScEw4">cobalt</a></td><td><a href="/facts/Isotopes_of_cobalt/jIIqGRE3">59Co</a></td><td>27</td><td>32</td><td>58.933 200(9)</td><td>Stable</td><td><a href="/facts/Cobalt-60/hjDPVA8M">60Co</a></td><td>33</td><td>5.2713(8) y</td></tr><tr><td><a href="/facts/Arsenic/k1c4f44D">arsenic</a></td><td><a href="/facts/Isotopes_of_arsenic/VvoUD0pG">75As</a></td><td>33</td><td>42</td><td>74.921 60(2)</td><td>Stable</td><td>73As</td><td>40</td><td>80.30(6) d</td></tr><tr><td><a href="/facts/Yttrium/BlX6rj99">yttrium</a></td><td><a href="/facts/Isotopes_of_yttrium/GS71S6HJ">89Y</a></td><td>39</td><td>50</td><td>88.905 85(2)</td><td>Stable</td><td>88Y</td><td>49</td><td>106.616(13) d</td></tr><tr><td><a href="/facts/Niobium/3zNI8AKp">niobium</a></td><td><a href="/facts/Isotopes_of_niobium/nbT6c0vs">93Nb</a></td><td>41</td><td>52</td><td>92.906 38(2)</td><td>Stable</td><td>92Nb</td><td>51</td><td>3.47(24)×107 y</td></tr><tr><td><a href="/facts/Rhodium/xmBd9aFG">rhodium</a></td><td><a href="/facts/Isotopes_of_rhodium/fYAb2dz8">103Rh</a></td><td>45</td><td>58</td><td>102.905 50(2)</td><td>Stable</td><td><a href="/facts/Nuclear_isomer/L8DHcqnk">102mRh</a></td><td>57</td><td>3.742(10) y</td></tr><tr><td><a href="/facts/Iodine/JlfMecMf">iodine</a></td><td><a href="/facts/Isotopes_of_iodine/HxgH1yJD">127I</a></td><td>53</td><td>74</td><td>126.904 47(3)</td><td>Stable</td><td><a href="/facts/Iodine-129/oJDlGf5H">129I</a></td><td>76</td><td>1.57(4)×107 y</td></tr><tr><td><a href="/facts/Caesium/dXKN1S5k">caesium</a></td><td><a href="/facts/Isotopes_of_caesium/m5AYV9tp">133Cs</a></td><td>55</td><td>78</td><td>132.905 45(2)</td><td>Stable</td><td>135Cs</td><td>80</td><td>2.3×106 y</td></tr><tr><td><a href="/facts/Praseodymium/2lnRJ9dH">praseodymium</a></td><td><a href="/facts/Isotopes_of_praseodymium/8J7u8hCJ">141Pr</a></td><td>59</td><td>82</td><td>140.907 65(2)</td><td>Stable</td><td>143Pr</td><td>84</td><td>13.57(2) d</td></tr><tr><td><a href="/facts/Terbium/ICF3PMED">terbium</a></td><td><a href="/facts/Isotopes_of_terbium/W0Wxkne3">159Tb</a></td><td>65</td><td>94</td><td>158.925 34(2)</td><td>Stable</td><td>158Tb</td><td>93</td><td>180(11) y</td></tr><tr><td><a href="/facts/Holmium/C73kJVtp">holmium</a></td><td><a href="/facts/Isotopes_of_holmium/zNJ7HJqT">165Ho</a></td><td>67</td><td>98</td><td>164.930 32(2)</td><td><a href="/facts/Observationally_stable/vC0VVvqN">Observationally stable</a></td><td>163Ho</td><td>96</td><td>4570(25) y</td></tr><tr><td><a href="/facts/Thulium/G2905xhd">thulium</a></td><td><a href="/facts/Isotopes_of_thulium/eqq1cw1P">169Tm</a></td><td>69</td><td>100</td><td>168.934 21(2)</td><td>Observationally stable</td><td>171Tm</td><td>102</td><td>1.92(1) y</td></tr><tr><td><a href="/facts/Gold/1znknojp">gold</a></td><td><a href="/facts/Isotopes_of_gold/9a1b4sIe">197Au</a></td><td>79</td><td>118</td><td>196.966 55(2)</td><td>Observationally stable</td><td>195Au</td><td>116</td><td>186.098(47) d</td></tr><tr><td><a href="/facts/Bismuth/75FLjkoX">bismuth</a></td><td><a href="/facts/Bismuth-209/Bxu7JrUs">209Bi</a></td><td>83</td><td>126</td><td>208.980 38(2)</td><td>2.01(8)×1019 y</td><td><a href="/facts/Isotopes_of_bismuth/DrbTkEzW">210mBi</a></td><td>127</td><td>3.04(6)×106 y</td></tr><tr><td><a href="/facts/Protactinium/v87VqTPf">protactinium</a></td><td><a href="/facts/Isotopes_of_protactinium/AVGw4A8U">231Pa</a></td><td>91</td><td>140</td><td>231.035 88(2)</td><td>3.276(11)×104 y</td><td>233Pa</td><td>142</td><td>26.975(13) d</td></tr></tbody></table> <h2 id="see-also">See also</h2> <ul><li><a href="/facts/Primordial_element/lFAUqHDv">Primordial element</a></li> <li><a href="/facts/List_of_nuclides/RSRMmg77">Table of nuclides sorted by half-life</a></li> <li><a href="/facts/Table_of_nuclides/wb9TQwN7">Table of nuclides</a></li> <li><a href="/facts/Isotope_geochemistry/Pq2IXawb">Isotope geochemistry</a></li> <li><a href="/facts/Radionuclide/5WpCfbdq">Radionuclide</a></li> <li><a href="/facts/List_of_elements_by_stability_of_isotopes/Bm0ogJmR">List of elements by stability of isotopes</a></li></ul> <h2 id="notes">Notes</h2> <h2 id="references">References</h2> <ol> <li id="fn:1"><p>Housecroft, C. E.; Sharpe, A. G. (2012). Inorganic Chemistry (4th ed.). Prentice Hall. p. 2. ISBN 978-0273742753. <a href="978-0273742753" target="_blank">978-0273742753</a> <a href="#fnref:1" class="footnote-back-ref">↩</a></p></li> <li id="fn:2"><p>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 <a href="/wiki/Half-life" target="_blank">/wiki/Half-life</a> <a href="#fnref:2" class="footnote-back-ref">↩</a></p></li> <li id="fn:3"><p>"Second - BIPM". <a href="https://www.bipm.org/en/history-si/second" target="_blank">https://www.bipm.org/en/history-si/second</a> <a href="#fnref:3" class="footnote-back-ref">↩</a></p></li> <li id="fn:4"><p>"Metre - BIPM". <a href="https://www.bipm.org/en/history-si/metre" target="_blank">https://www.bipm.org/en/history-si/metre</a> <a href="#fnref:4" class="footnote-back-ref">↩</a></p></li> <li id="fn:5"><p>"Kelvin - BIPM". <a href="https://www.bipm.org/en/history-si/kelvin" target="_blank">https://www.bipm.org/en/history-si/kelvin</a> <a href="#fnref:5" class="footnote-back-ref">↩</a></p></li> <li id="fn:6"><p>"British thermal units (Btu) - U.S. Energy Information Administration (EIA)". <a href="https://www.eia.gov/energyexplained/units-and-calculators/british-thermal-units.php" target="_blank">https://www.eia.gov/energyexplained/units-and-calculators/british-thermal-units.php</a> <a href="#fnref:6" class="footnote-back-ref">↩</a></p></li> <li id="fn:7"><p>"Mole - BIPM". <a href="https://www.bipm.org/en/history-si/mole" target="_blank">https://www.bipm.org/en/history-si/mole</a> <a href="#fnref:7" class="footnote-back-ref">↩</a></p></li> <li id="fn:8"><p>"SI Brochure - 9th ed". Bureau International des Poids et Mesures. <a href="https://www.bipm.org/documents/20126/41483022/SI-Brochure-9-EN.pdf/2d2b50bf-f2b4-9661-f402-5f9d66e4b507" target="_blank">https://www.bipm.org/documents/20126/41483022/SI-Brochure-9-EN.pdf/2d2b50bf-f2b4-9661-f402-5f9d66e4b507</a> <a href="#fnref:8" class="footnote-back-ref">↩</a></p></li> <li id="fn:9"><p>"Candela - BIPM". <a href="https://www.bipm.org/en/history-si/candela" target="_blank">https://www.bipm.org/en/history-si/candela</a> <a href="#fnref:9" class="footnote-back-ref">↩</a></p></li> <li id="fn:10"><p>"Resolution 10 - BIPM". <a href="https://www.bipm.org/en/committees/cg/cgpm/23-2007/resolution-10" target="_blank">https://www.bipm.org/en/committees/cg/cgpm/23-2007/resolution-10</a> <a href="#fnref:10" class="footnote-back-ref">↩</a></p></li> <li id="fn:11"><p>N. E. Holden, "Standard Atomic Weight Values for the Mononuclidic Elements - 2001," BNL-NCS-68362, Brookhaven National Laboratory (2001) <a href="https://www.osti.gov/bridge/servlets/purl/782053-S3nk8u/native/782053.pdf" target="_blank">https://www.osti.gov/bridge/servlets/purl/782053-S3nk8u/native/782053.pdf</a> <a href="#fnref:11" class="footnote-back-ref">↩</a></p></li> <li id="fn:12"><p>IUPAC list of mononuclidics for metrology purposes <a href="http://www.iupac.org/publications/pac/2006/pdf/7811x2051.pdf" target="_blank">http://www.iupac.org/publications/pac/2006/pdf/7811x2051.pdf</a> <a href="#fnref:12" class="footnote-back-ref">↩</a></p></li> </ol>