Mononuclidic elements

Mononuclidic elements are chemical elements which either have only one stable isotope, or for which there is only one very long-lived isotope that dominates the element's  natural isotopic abundance. There are nineteen elements in the first category, and three ( Bi,  Th and  Pa) in the second.[1] Elements such as Rb, In, and La, that have a single stable isotope and a significant fraction of a very long-lived isotope in their natural abundance, are not considered mononuclidic.

## Contents

* 1 Use in metrology
* 2 Contamination by unstable trace isotopes
* 3 Complete list
* 4 See also
* 5 References

## Use in metrology

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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.[2]

## Contamination by unstable trace isotopes

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Trace concentrations of unstable isotopes of some mononuclidic elements are found in natural samples. For example,  10Be, with a half-life of 1.5 million years, is produced by cosmic rays in the Earth's upper atmosphere;  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

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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).

Nuclide  | Z (p)  | N (n)  | isotopic mass (u)

|
|
|

9Be  | 4  | 5  | 9.012 182(3)
19F  | 9  | 10  | 18.998 403 2(5)
23Na  | 11  | 12  | 22.989 770(2)
27Al  | 13  | 14  | 26.981 538(2)
31P  | 15  | 16  | 30.973 761(2)
45Sc  | 21  | 24  | 44.955 910(8)
55Mn  | 25  | 30  | 54.938 049(9)
59Co  | 27  | 32  | 58.933 200(9)
75As  | 33  | 42  | 74.921 60(2)
89Y  | 39  | 50  | 88.905 85(2)
93Nb  | 41  | 52  | 92.906 38(2)
103Rh  | 45  | 58  | 102.905 50(2)
127I  | 53  | 74  | 126.904 47(3)
133Cs  | 55  | 78  | 132.905 45(2)
141Pr  | 59  | 82  | 140.907 65(2)
159Tb  | 65  | 94  | 158.925 34(2)
165Ho  | 67  | 98  | 164.930 32(2)
169Tm  | 69  | 100  | 168.934 21(2)
197Au  | 79  | 118  | 196.966 55(2)
209Bi  | 83  | 126  | 208.980 38(2)
232Th  | 90  | 142  | 232.0381(1)
231Pa  | 91  | 140  | 231.035 88(2)

## See also

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* Stable isotopes

## References

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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)

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