Technetium (₄₃Tc) is one of the two elements withZ < 83 that have no stableisotopes; the other such element ispromethium.^[2] It is primarily artificial, with only trace quantities existing in nature produced byspontaneous fission (there are an estimated2.5×10⁻¹³ grams of⁹⁹Tc per gram ofpitchblende)^[3] orneutron capture bymolybdenum. The element was first obtained in 1936 from bombardedmolybdenum, the first artificial element to be produced. The most stableradioisotopes are⁹⁷Tc (half-life of 4.21 million years),⁹⁸Tc (half-life: 4.2 million years), and⁹⁹Tc (half-life: 211,100 years). Given that their stated uncertainties are 16 and 30 times their difference, the half-lives of⁹⁷Tc and⁹⁸Tc are statistically indistinguishable.

Thirty-three other radioisotopes have been characterized withatomic masses ranging from⁸⁵Tc to¹²⁰Tc. Those with half-lives more than an hour have masses 93 to 96.

Technetium also has numerousmeta states.^97mTc is the most stable, with a half-life of 91.1 days (0.097 MeV), followed by^95mTc (half-life: 62.0 days, 0.039 MeV) and^99mTc (half-life: 6.01 hours, 0.143 MeV).^99mTc emits onlygamma rays while decaying to⁹⁹Tc.

For isotopes lighter than⁹⁸Tc, the primarydecay mode iselectron capture toisotopes of molybdenum. For the heavier isotopes, the primary mode isbeta emission toisotopes of ruthenium, with the exception that⁹⁸Tc and¹⁰⁰Tc can decay both by beta emission and electron capture.

Technetium-99m is the technetium isotope employed in thenuclear medicine industry. Its low-energyisomeric transition, which yields a gamma-ray at ~140.5 keV, is ideal for imaging usingSingle Photon Emission Computed Tomography (SPECT). Several technetium isotopes, such as^94mTc,⁹⁵Tc, and⁹⁶Tc, which are produced via (p,n) reactions using acyclotron onmolybdenum targets, have also been identified as potentialPositron Emission Tomography (PET) or gamma-emitting agents for medical imaging.^[4][5][6] Technetium-101 has been produced using aD-D fusion-basedneutron generator from the¹⁰⁰Mo(n,γ)¹⁰¹Mo reaction on natural molybdenum and subsequentbeta-minus decay of¹⁰¹Mo to¹⁰¹Tc. Despite its shorter half-life (14.22 minutes),¹⁰¹Tc exhibits unique decay characteristics suitable for radioisotope diagnostic ortherapeutic procedures, where it has been proposed that its implementation, as a supplement for dual-isotopic imaging or replacement for^99mTc, could be performed by on-site production and dispensing at the point of patient care.^[7]

Technetium-99 is the most common and most readily available isotope, as it is a majorfission product from fission ofactinides likeuranium andplutonium with afission product yield of 6% or more, and in fact the most significantlong-lived fission product. Lighter isotopes of technetium are almost never produced in fission because the initial fission products normally have a higher neutron/proton ratio than is stable for their mass range, and therefore undergobeta decay until reaching the ultimate product. Beta decay of fission products of mass 98 and lower, or 100, stops at stable (or very long-lived) isotopes of lower atomic number and does not reach technetium. For greater masses, the technetium isotopes are very short-lived and quickly undergo further beta decay. Therefore, the technetium inspent nuclear fuel is practically all⁹⁹Tc. In the presence offast neutrons a small amount of⁹⁸
Tc will be produced by (n,2n) "knockout" reactions. Ifnuclear transmutation of fission-derived technetium, or technetium wastes from medical applications, is desired, fast neutrons are therefore not desirable as the long lived⁹⁸
Tc increases rather than reducing the longevity of the radioactivity in the material.^{[citation needed]}

One gram of⁹⁹Tc produces6.2×10⁸ disintegrations a second (that is, 0.62 GBq/g).^[8]

Technetium has noprimordial isotopes and does not occur in nature in significant quantities, and thus astandard atomic weight cannot be given.

Technetium andpromethium are unusual light elements in that they have no stable isotopes. Using theliquid drop model for atomic nuclei, one can derive a semiempirical formula for the binding energy of a nucleus. This formula predicts a "valley of beta stability" along whichnuclides do not undergo beta decay. Nuclides that lie "up the walls" of the valley tend to beta decay towards the center (by emitting an electron, emitting apositron, or capturing an electron). For a fixed number of nucleonsA, the binding energies lie on one or moreparabolas, with the most stable nuclide at the bottom. One can have more than one parabola because isotopes with an even number of protons and an even number of neutrons are more stable than isotopes with an odd number of neutrons and an odd number of protons. A single beta decay then transforms one into the other. When there is only one parabola, there can be only one stable isotope lying on that parabola. When there are two parabolas, that is, when the number of nucleons is even, it can happen (rarely) that there is a stable nucleus with an odd number of neutrons and an odd number of protons (although this happens only in five instances:²H,⁶Li,¹⁰B,¹⁴N and^180mTa). However, if this happens, there can be no stable isotope with an even number of neutrons and an even number of protons (180 is an exception, and^180mTa is suspected not to be truly stable).

For technetium (Z = 43), the valley of beta stability is centered at around 98 nucleons. However, for every number of nucleons from 94 to 102, there is already at least one stable nuclide of eithermolybdenum (Z = 42) orruthenium (Z = 44), and theMattauch isobar rule states that two adjacentisobars cannot both be stable.^[11] For the isotopes with odd numbers of nucleons, this immediately rules out a stable isotope of technetium, since there can be only one stable nuclide with a fixed odd number of nucleons. For the isotopes with an even number of nucleons, since technetium has an odd number of protons, any isotope must also have an odd number of neutrons. In such a case, the presence of a stable nuclide having the same number of nucleons and an even number of protons rules out the possibility of a stable nucleus.^[11][12]

Daughter products other than technetium

• Isotopes of ruthenium
• Isotopes of molybdenum
• Isotopes of niobium

• Isotopes of technetium
• Technetium
• Lists of isotopes by element

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