Monoisotopic element

A monoisotopic element is one of 26 chemical elements which have only a single stable isotope (nuclide). A list is given in a following section.

Stability is experimentally defined for chemical elements, as there are a number of stable nuclides with atomic numbers over ~ 40 which are theoretically unstable, but apparently have half-lives so long that they have not been observed directly or indirectly (from measurement of products) to decay.

Monoisotopic elements are characterized, except in a single case, by odd numbers of protons (odd Z), and even numbers of neutrons. Because of the energy gain from nuclear pairing effects, the odd number of protons imparts instability to isotopes of an odd Z, which in heavier elements requires a completely paired set of neutrons to offset this tendency into stability. (The four stable nuclides with odd Z and odd neutron numbers are hydrogen-2, lithium-6, boron-10, and nitrogen-14).

The single mononuclidic exception to the odd Z rule is beryllium, which has 4 protons and 5 neutrons. This element is prevented from having equal numbers of neutrons and protons (4 of each) by the instability toward double-alpha decay, which is favored due to the extremely tight binding of helium-4 nuclei. It is prevented from having a stable isotope with 4 protons and 6 neutrons by the very large mismatch in proton/neutron ratio for such a light element. (Nevertheless, beryllium-10 has a half-life of 1.36 million years, which is too short to be primordial, but still indicates unusual stability for a light isotope with such an imbalance.)

The set of monoisotopic elements overlap but are not the same as the set of 22 mononuclidic elements, which are characterized as having essentially only one isotope (nuclide) found in nature. The reason for this is the occurrence of certain long-lived radioactive primordial nuclides in nature, which may form admixtures with the monoisotopics, and thus prevent them from being naturally mononuclidic. This happens in the cases of 7 (26 − 19 = 7) of the monoisotopic elements. These isotopes are monoisotopic, but due to the presence of the long lived radioactive primordial nuclide, are not mononuclidic. These elements are vanadium, rubidium, indium, lanthanum, europium, rhenium and lutetium. See the list below; in two noted cases (indium and rhenium), the long-lived radionuclide is actually the most abundant isotope in nature, and the stable isotope is less abundant.

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