Mononuclidic element


A mononuclidic element or monotopic element is one of the 21 chemical elements that is found naturally on Earth essentially as a single 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, and 2 in the second category. 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, there exist 7 which are nevertheless not considered mononuclidic, due to the presence of a significant fraction of a very long-lived radioisotope occurring in their natural abundance. These elements are vanadium, rubidium, indium, lanthanum, europium, lutetium, and rhenium.

Use in metrology

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 standard atomic weight and atomic mass are the same.
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.
In nuclear magnetic resonance spectroscopy, the three most sensitive stable nuclei are hydrogen-1, fluorine-19 and phosphorus-31. 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, with a half-life of 1.4 million years, is produced by cosmic rays in the Earth's upper atmosphere; iodine-129, with a half-life of 15.7 million years, is produced by various cosmogenic and nuclear mechanisms; caesium-137, 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 21 mononuclidic elements

Data from ed. J. S. Coursey, D. J. Schwab and R. A. Dragoset, National Institute of Standards and Technology.
ElementNuclideZ N Isotopic mass Notes
beryllium9Be459.012 182
fluorine19F91018.998 403 2
sodium23Na111222.989 770
aluminium27Al131426.981 538
phosphorus31P151630.973 761
scandium45Sc212444.955 910
manganese55Mn253054.938 049
cobalt59Co273258.933 200
arsenic75As334274.921 60
yttrium89Y395088.905 85
niobium93Nb415292.906 38
rhodium103Rh4558102.905 50
iodine127I5374126.904 47
caesium133Cs5578132.905 45
praseodymium141Pr5982140.907 65
terbium159Tb6594158.925 34
holmium165Ho6798164.930 32
thulium169Tm69100168.934 21
gold197Au79118196.966 55
bismuth209Bi83126208.980 38
protactinium231Pa91140231.035 88