Mononuclidic element

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

A mononuclidic element is one of the 22 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 either by one stable isotope or by one very long-lived isotope. There are 19 elements in the first category (which are both monoisotopic and mononuclidic), and 3 (bismuth,[1] thorium and protactinium) in the second category (mononuclidic but not monoisotopic, since they have zero, not one, stable nuclides). A list of the 22 mononuclidic elements is given at the end of this article.

Of the 26 monoisotopic elements that, by definition, have only one stable isotope, there exist 7 (26 minus 19 = 7) which are nevertheless not considered mononuclidic, due to the presence of a significant fraction of a very long-lived (primordial) radioisotope occurring in their natural abundance. These elements are vanadium, rubidium, indium, lanthanum, europium, rhenium, and lutetium.

Use in metrology[edit]

Mononuclidic elements are 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 relative atomic mass and atomic mass are the same.[2]

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.[3]

Contamination by unstable trace isotopes[edit]

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; 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.

Complete list of the 22 mononuclidic elements[edit]

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 Nuclide Z (p) N (n) Isotopic mass (u) Notes
beryllium 9Be 4 5 9.012 182(3)
fluorine 19F 9 10 18.998 403 2(5)
sodium 23Na 11 12 22.989 770(2)
aluminium 27Al 13 14 26.981 538(2)
phosphorus 31P 15 16 30.973 761(2)
scandium 45Sc 21 24 44.955 910(8)
manganese 55Mn 25 30 54.938 049(9)
cobalt 59Co 27 32 58.933 200(9)
arsenic 75As 33 42 74.921 60(2)
yttrium 89Y 39 50 88.905 85(2)
niobium 93Nb 41 52 92.906 38(2)
rhodium 103Rh 45 58 102.905 50(2)
iodine 127I 53 74 126.904 47(3)
caesium 133Cs 55 78 132.905 45(2)
praseodymium 141Pr 59 82 140.907 65(2)
terbium 159Tb 65 94 158.925 34(2)
holmium 165Ho 67 98 164.930 32(2)
thulium 169Tm 69 100 168.934 21(2)
gold 197Au 79 118 196.966 55(2)
bismuth 209Bi 83 126 208.980 38(2) (radioactive)
thorium 232Th 90 142 232.038 1(1) (radioactive)
protactinium 231Pa 91 140 231.035 88(2) (radioactive)

See also[edit]


  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
  2. ^ N. E. Holden, "Standard Atomic Weight Values for the Mononuclidic Elements - 2001," BNL-NCS-68362, Brookhaven National Laboratory (2001)
  3. ^ IUPAC list of mononuclidics for metrology purposes