Mononuclidic element

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A mononuclidic element is an element that Earth naturally contains essentially as a single nuclide. This means there is one main isotope with a characteristic atomic mass, and the natural abundance is dominated by that isotope, which is stable or very long‑lived.

There are 21 such elements. Of them, 19 are both monoisotopic and mononuclidic (only one stable nuclide and only one nuclide in nature). The remaining two, bismuth and protactinium, are mononuclidic but not monoisotopic because they have no stable nuclides.

Among the 26 monoisotopic elements (those with a single stable isotope), seven are not mononuclidic in practice because natural samples still contain a significant amount of a very long‑lived radioactive isotope: vanadium, rubidium, indium, lanthanum, europium, lutetium and rhenium.

Why does this matter for units and measurements? In some definitions the ambiguity from varying isotopic compositions in natural samples can cause differences. Using a substance defined by a single isotope (or a fixed isotope ratio) can be technically difficult. The use of mononuclidic elements as reference materials avoids these issues. In fact, the most recent SI definition references cesium, a mononuclidic element.

Mononuclidic elements are scientifically important because their atomic weights can be measured very precisely—the standard atomic weight equals the isotopic mass for these elements. However, only 11 of them are used in standard atomic weight metrology: aluminium, bismuth, caesium, cobalt, gold, manganese, phosphorus, scandium, sodium, terbium and thorium.

In nuclear magnetic resonance (NMR) spectroscopy, the most sensitive stable nuclei are hydrogen‑1 (1H), fluorine‑19 (19F) and phosphorus‑31 (31P). Fluorine and phosphorus are monoisotopic, and hydrogen is nearly so.

Trace amounts of unstable isotopes can occur in natural samples. For example, beryllium‑10 (half‑life about 1.387 million years) is made by cosmic rays; iodine‑129 (half-life 16.1 million years) is produced by cosmogenic and nuclear processes; caesium‑137 (half-life 30.04 years) is a product of fission. These isotopes are useful in various analytical and forensic applications.