Monoisotopic element

Not to be confused with the 22 mononuclidic elements, defined as those with only one significant naturally-abundant nuclide.

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 ~ 30 which are theorically 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 element, which typically requires at least a completely paired set of neutrons to offset into stability.

The single mononuclidic exception is beryllium, which has 4 protons and 5 neutrons. This isotope 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).

Differentiation from similar term

Set A is the 26 monoisotopic elements and B the 22 mononuclidic elements. The intersection consists of 19 elements that are both, but each set contains elements that are of one sort but not the other. There are 7 monoisotopics that are not mononuclidic, and there are 3 mononuclidics that are not monoisotopic.

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.^[1] 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 minus 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, the long-lived radionuclide is actually the most abundant isotope in nature, and the stable isotope is less abundant.

In 3 additional cases (bismuth,^[2] thorium, and protactinium), mononuclidic elements occur primordially which are not monoisotopic because the naturally-occurring primordial nuclide consists entirely of a single radioisotope (radionuclide), and thus the element has no stable isotopes at all. For an element to be monoisotopic, it must have one stable nuclide.

List of (observationally-stable) monoisotopic elements, ordered by atomic number and weight

Non-mononuclidic elements are marked with an asterisk, and the long-lived primordial radioisotope given. In two notable cases (indium and rhenium), the highest abundance naturally occurring isotope is the mildly radioactive one, and in the case of europium, nearly half of it is.

1. Beryllium-9
2. Fluorine-19
3. Sodium-23
4. Aluminium-27
5. Phosphorus-31
6. Scandium-45
7. Vanadium-51* naturally occurs with 0.25% of radioactive vanadium-50
8. Manganese-55
9. Cobalt-59
10. Arsenic-75
11. Rubidium-85* naturally occurs with 27.835% of radioactive rubidium-87
12. Yttrium-89
13. Niobium-93
14. Rhodium-103
15. Indium-113* naturally occurs with majority (95.7%) radioactive isotope indium-115
16. Iodine-127
17. Caesium-133
18. Lanthanum-139* naturally occurs with 0.09% radioactive lanthanum-138
19. Praseodymium-141
20. Europium-153* naturally occurs with 47.8% radioactive europium-151
21. Terbium-159
22. Holmium-165
23. Thulium-169
24. Lutetium-175* naturally occurs with 2.59% radioactive lutetium-176
25. Rhenium-185* naturally occurs with majority (62.6%) radioactive isotope rhenium-187
26. Gold-197

See also

• Primordial element
• Primordial nuclide
• Table of nuclides sorted by half-life
• Table of nuclides
• Isotope geochemistry
• Radionuclide
• Mononuclidic element
• Stable isotope
• List of elements by stability of isotopes
• List of elements by nuclear stability

External links

• Atomic Weights of the Elements as of year 2000, with a section on monoisotopics starting page 708

References

1. ^ N. E. Holden, "Standard Atomic Weight Values for the Mononuclidic Elements - 2001," BNL-NCS-68362, Brookhaven National Laboratory (2001)
2. ^ 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