List of elements by stability of isotopes

This is a list of the chemical elements and their isotopes, listed in terms of stability.

It has been suggested that this article or section be merged with Primordial nuclide. (Discuss)

Atomic nuclei consist of protons and neutrons, which attract each other through the strong nuclear force, while protons repel each other via the electric force due to their positive charge. These two forces compete, leading to some combinations of neutrons and protons being more stable than others. Neutrons stabilize the nucleus, because they attract each other and protons equally by the strong nuclear force, which helps offsetting the electrical repulsion between protons. As a result, as the number of protons increases, an increasing ratio of neutrons to protons is needed to form a stable nucleus. However, if too many or too few neutrons are present with regard to the optimum ratio, the nucleus becomes unstable and subject to certain types of nuclear decay. Unstable isotopes decay through various radioactive decay pathways, most commonly alpha decay, beta decay, or spontaneous fission.

Overview

Isotope half lifes. Note that the darker more stable isotope region departs from the line of protons (Z) = neutrons (N), as the element number Z becomes larger

Of the first 82 elements in the periodic table, 80 have isotopes considered to be stable.^[1] Technetium, promethium (atomic numbers 43 and 61, respectively^[a]) and all the elements with an atomic number over 82 have only isotopes that are known to decompose through radioactive decay. They are not expected to have any stable, undiscovered ones; that's why lead is considered the heaviest stable element. However, it is possible that some isotopes that are presently considered stable will be revealed to decay with extremely long half-times (as was the case in 2003 with bismuth-209 which had been previously considered to be stable).^[2][3] This list depicts what is agreed upon by the consensus of the scientific community as of 2008.^[1]

For each of the 80 stable elements the number of the stable isotopes is given. Of these elements, only one (tin) has 10 stable isotopes, one (xenon) has nine isotopes, five have seven isotopes, eight have six isotopes, nine have five isotopes, nine have four, five have three stable isotopes, 16 have two stable isotopes, and 26 have a single stable isotope.^[1] Thus, there are presently 256 stable nuclides known (counting Ta-180m as stable, since its decay has not been observed).

Additionally, about 23 of these 80 elements also have unstable isotopes with a half-life more than or comparable with the age of the Solar System (~10⁹ years or more).^[b] One of these is Ta-180m which is predicted to have a half life in excess of 10¹⁵ years, but has never been observed to decay. The even longer half life of 7.7 x 10²⁴ years of tellurium-128 was measured by a unique method of detecting radiogenic daughter xenon-128 and is presently the longest known experimentally measured half life.^[4]. Another notable example is the only naturally-occurring isotope of bismuth, which has been predicted to be unstable with a very long half-life, but has only recently been observed to decay. Because of their long half-times, such isotopes are still found on Earth in various abundances, and together with the stable isotopes they are called primordial isotopes. All the primordial isotopes are given in order of their decreasing abundance on Earth.^[c]

The other 37 discovered elements have isotopes which are all known to be radioactive. The elements in this list are ordered according to the lifetime of their most stable isotope.^[1] Of these, four (bismuth, thorium, uranium and plutonium) are primordial because they have long enough half-times to still be found on Earth,^[d] while all the others are produced either by radioactive decay or are synthesized in laboratories and nuclear reactors. Only 13 of these 37 elements have isotopes with a half-time of at least 100 years. Every known isotope of remaining 24 elements is highly radioactive, they are used in academic research and sometimes in industry and medicine.^[e] Some of the heavier elements in the periodic table may be revealed to have yet-undiscovered isotopes with longer lifetimes than those listed here.^[f]

Elements by number of primordial isotopes

An even number of protons or of neutrons are more stable (lower binding energy) because of pairing effects, so even-even nuclides are much more stable than odd-odd. One effect is that there are few stable odd-odd nuclides, but another effect is to prevent beta decay of many even-even nuclides into another even-even nuclide of the same mass number but lower energy, because decay proceeding one step at a time would have to pass through an odd-odd nuclide of higher energy. This makes for a larger number of stable even-even nuclides, up to three for some mass numbers, and up to seven for some atomic (proton) numbers. Double beta decay directly from even-even to even-even skipping over an odd-odd nuclide is only occasionally possible, and even then with a halflife greater than a billion times the age of the universe.

Elements with no primordial isotopes

Periodic table colored according to the number of stable isotopes. Elements with odd atomic numbers have only one or two stable isotopes, while elements with even atomic numbers all have three or more stable isotopes, except for the first three: helium, beryllium, and carbon.

Periodic table with elements colored according to the half-life of their most stable isotope.      Stable elements;      Radioactive elements with very long-lived isotopes. Their half-live of over four million years confers them very small, if not negligible radioactivities;      Radioactive elements that may present low health hazards. Their most stable isotopes have half-lives between 800 and 34.000 years. Because of this, they usually have some commercial applications;      Radioactive elements that are known to pose high safety risks. Their most stable isotopes have half-lifes between one day and 103 years. Their radioactivities confers them little potential for commercial uses;      Highly radioactive elements. Their most stable isotopes have half-lifes between one day and several minutes. They pose severe health risks. Few of them receive uses outside basic research;      Extremely radioactive elements. Very little is known about these elements due to their extreme instability and radioactivitiy.

See also

• Table of nuclides
• Island of stability

Footnotes

• ^a  See stability of technetium isotopes for a detailed discussion as to why technetium and promethium have no stable isotopes.
• ^b Isotopes that have a half-life of more than about 10⁸ yr may still be found on Earth, but only those with half-lives above 7×10⁸ yr (as of ²³⁵U) are found in appreciable quantities. The present list neglects a few isotopes with half lives about 10⁸ yr because they have been measured in tiny quantities on Earth. Uranium-234 with its half-life of 246,000 yr and natural isotopic abundance 0.0055% is a special case: it is a decay product of Uranium-238 rather than a primordial nuclide.
• ^c There are unstable isotopes with extremely long half-lives that are also found on Earth, and some of them are even more abundant than all the stable isotopes of given element (for example, beta-active ¹⁸⁷Re is twice more abundant than stable ¹⁸⁵Re). Also, a bigger natural abundance of an isotope just implies that its formation was favored by the stellar nucleosynthesis precessed that produces the matter constitutes now the Solar System and the Earth (see also Formation and evolution of the Solar System).
• ^d While bismuth and thorium have only one primordial isotope, uranium has three isotopes that are found in nature (²³⁸U,²³⁵U and ²³⁴U). Plutonium-244 is a special case because its half-time (80 Myr) is barely enough to allow it to still be found in trace quantities on Earth.
• ^e See many different industrial and medical applications of radioactive elements in Radionuclide, Nuclear medicine, Common beta emitters, Commonly-used gamma emitting isotopes, Fluorine-18, Cobalt-60, Strontium-90, Technetium-99m, Iodine-123, Iodine-124, Promethium-147, Iridium-192 etc.
• ^f For elements with a higher atomic number than californium (with Z>98) there might exist undiscovered isotopes that are more stable than the known ones.
• ^g Legend: a=year, d=day, h=hour, min=minute, s=second.
• ^h These values are not purely derived from experimental data, but at least partly from systematic trends.
• ⁱ None of the elements with an atomic number above 111 have yet been confirmed by IUPAC.

References