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Technetium

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                43           molybdenum ← technetium → ruthenium
                Mn
                ↑
                Tc
                ↓
                Re

                                  Periodic Table - Extended Periodic Table

                                                                   General
                                   Name, Symbol, Number technetium, Tc, 43
                                         Chemical series transition metals
                                              Group, Period, Block 7, 5, d
                                             Appearance silvery gray metal
                                                     Atomic mass (0) g/mol
                                     Electron configuration [Kr] 4d^5 5s^2
                                       Electrons per shell 2, 8, 18, 13, 2
                                                       Physical properties
                                                               Phase solid
                                         Density (near r.t.) 11 g·cm^−3
                                                     Melting point 2430  K
                                                    (2157 ° C, 3915 ° F)
                                                      Boiling point 4538 K
                                                    (4265 ° C, 7709 ° F)
                                         Heat of fusion 33.29 kJ·mol^−1
                                   Heat of vaporization 585.2 kJ·mol^−1
                          Heat capacity (25 °C) 24.27 J·mol^−1·K^−1

   CAPTION: Vapor pressure (extrapolated)

                                      P/Pa   1    10  100  1 k  10 k 100 k
                                     at T/K 2727 2998 3324 3726 4234 4894

                                                         Atomic properties
                                               Crystal structure hexagonal
                                                        Oxidation states 7
                                                   (strongly acidic oxide)
                                     Electronegativity 1.9 (Pauling scale)
                                              Electron affinity -53 kJ/mol
                                       Ionization energies 1st: 702 kJ/mol
                                                          2nd: 1470 kJ/mol
                                                          3rd: 2850 kJ/mol
                                                      Atomic radius 135 pm
                                              Atomic radius (calc.) 183 pm
                                                    Covalent radius 156 pm
                                                             Miscellaneous
                                                 Magnetic ordering no data
                       Thermal conductivity (300 K) 50.6 W·m^−1·K^−1
                                             CAS registry number 7440-26-8
                                                         Selected isotopes

                CAPTION: Main article: Isotopes of technetium

                          iso    NA    half-life   DM    DE ( MeV)    DP
                        ^95 mTc syn   61 d         ε   -             ^95Mo
                                                   γ   0.204, 0.582,
                                                       0.835         -
                                                   IT  0.0389, e     ^95Tc
                        ^96Tc   syn   4.3 d        ε   -             ^96Mo
                                                   γ   0.778, 0.849,
                                                       0.812         -
                        ^97Tc   syn   2.6×10^6 y   ε   -             ^97Mo
                        ^97 mTc syn   90 d         IT  0.965, e      ^97Tc
                        ^98Tc   syn   4.2×10^6 y   β^- 0.4           ^98Ru
                                                   γ   0.745, 0.652  -
                        ^99Tc   trace 2.111×10^5 y β^- 0.294         ^99Ru
                        ^99 mTc trace 6.01 h       IT  0.142, 0.002  ^99Tc
                                                   γ   0.140         -

                                                                References

   Technetium ( IPA: /tɛkˈniʃɪəm/ or /tɛkˈniːʃɪəm/) is a chemical element
   that has the symbol Tc and the atomic number 43. The chemical
   properties of this silvery grey, radioactive, crystalline transition
   metal are intermediate between rhenium and manganese. Its short-lived
   isotope ^99 mTc is used in nuclear medicine for a wide variety of
   diagnostic tests. ^99Tc is used as a gamma ray-free source of beta
   particles, and its pertechnetate ion (TcO[4]^-) could find use as an
   anodic corrosion inhibitor for steel.

   Before the element was discovered, many of the properties of element 43
   were predicted by Dmitri Mendeleev. Mendeleev noted a gap in his
   periodic table and called the element ekamanganese. In 1937 its isotope
   ^97Tc became the first element to be artificially produced, hence its
   name (from the Greek τεχνητος, meaning "artificial"). Most technetium
   produced on Earth is a by-product of fission of uranium-235 in nuclear
   reactors and is extracted from nuclear fuel rods. No isotope of
   technetium has a half-life longer than 4.2 million years (^98Tc), so
   its detection in red giants in 1952 helped bolster the theory that
   stars can produce heavier elements. On earth, technetium occurs
   naturally only in uranium ores as a product of spontaneous fission; the
   quantities are minute but have been measured.

Notable characteristics

   Technetium is a silvery-grey radioactive metal with an appearance
   similar to platinum. However, it is commonly obtained as a grey powder.
   Its position in the periodic table is between rhenium and manganese and
   as predicted by the periodic law its properties are intermediate
   between those two elements. This element is unusual among the lighter
   elements in that it has no stable isotopes and is therefore extremely
   rare on Earth. Technetium plays no natural biological role and is not
   normally found in the human body.

   The metal form of technetium slowly tarnishes in moist air. Its oxides
   are TcO[2] and Tc[2]O[7]. Under oxidizing conditions technetium (VII)
   will exist as the pertechnetate ion, TcO[4]^-. Common oxidation states
   of technetium include 0, +2, +4, +5, +6 and +7. When in powder form
   technetium will burn in oxygen. It dissolves in aqua regia, nitric
   acid, and concentrated sulfuric acid, but it is not soluble in
   hydrochloric acid. It has characteristic spectral lines at 363 nm, 403
   nm, 410 nm, 426 nm, 430 nm, and 485 nm.

   The metal form is slightly paramagnetic, meaning its magnetic dipoles
   align with external magnetic fields even though technetium is not
   normally magnetic. The crystal structure of the metal is hexagonal
   close-packed. Pure metallic single-crystal technetium becomes a type II
   superconductor at 7.46 K; irregular crystals and trace impurities raise
   this temperature to 11.2 K for 99.9% pure technetium powder. Below this
   temperature technetium has a very high magnetic penetration depth, the
   largest among the elements apart from niobium.

   Technetium is produced in quantity by nuclear fission, and spreads more
   readily than many radionuclides. In spite of the importance of
   understanding its toxicity in animals and humans, experimental evidence
   is scant. It appears to have low chemical toxicity, and even lower
   radiological toxicity.

   When one is working in a laboratory context, all isotopes of technetium
   must be handled carefully. The most common isotope, technetium-99, is a
   weak beta emitter; such radiation is stopped by the walls of laboratory
   glassware. Soft X-rays are emitted when the beta particles are stopped,
   but as long as the body is kept more than 30 cm away these should pose
   no problem. The primary hazard when working with technetium is
   inhalation of dust; such radioactive contamination in the lungs can
   pose a significant cancer risk. For most work, careful handling in a
   fume hood is sufficient; a glove box is not needed.

Applications

Nuclear medicine

   ^99mTc ("m" indicates that this is a metastable nuclear isomer) is used
   in radioactive isotope medical tests, for example as a radioactive
   tracer that medical equipment can detect in the body. It is well suited
   to the role because it emits readily detectable 140 keV gamma rays, and
   its half-life is 6.01 hours (meaning that about fifteen sixteenths of
   it decays to ^99Tc in 24 hours). Klaus Schwochau's book Technetium
   lists 31 radiopharmaceuticals based on ^99mTc for imaging and
   functional studies of the brain, myocardium, thyroid, lungs, liver,
   gallbladder, kidneys, skeleton, blood and tumors.

   Immunoscintigraphy incorporates ^99mTc into a monoclonal antibody, an
   immune system protein capable of binding to cancer cells. A few hours
   after injection, medical equipment is used to detect the gamma rays
   emitted by the ^99mTc; higher concentrations indicate where the tumor
   is. This technique is particularly useful for detecting hard-to-find
   cancers, such as those affecting the intestine. These modified
   antibodies are sold by the German company Hoechst under the name "
   Scintium".

   When ^99mTc is combined with a tin compound it binds to red blood cells
   and can therefore be used to map circulatory system disorders. A
   pyrophosphate ion with ^99mTc adheres to calcium deposits in damaged
   heart muscle, making it useful to gauge damage after a heart attack.
   The sulfur colloid of ^99mTc is scavenged by the spleen, making it
   possible to image the structure of the spleen.

   Radiation exposure due to diagnostic treatment involving Tc-99m can be
   kept low. While ^99mTc is quite radioactive (allowing small amounts to
   be easily detected) it has a short half-life, after which it decays
   into the less radioactive ^99Tc. In the form administered in these
   medical tests (usually pertechnetate) both isotopes are quickly
   eliminated from the body, generally within a few days.

Industrial

   Technetium-99 decays almost entirely by beta decay, emitting beta
   particles with very consistent low energies and no accompanying gamma
   rays. Moreover, its very long half-life means that this emission
   decreases very slowly with time. It can also be extracted to a high
   chemical and isotopic purity from radioactive waste. For these reasons,
   it is a NIST standard beta emitter, used for equipment calibration.

   ^95mTc, with a half-life of 61 days, is used as a radioactive tracer to
   study the movement of technetium in the environment and in plant and
   animal systems.

   Like rhenium and palladium, technetium can serve as a catalyst. For
   certain reactions, for example the dehydrogenation of isopropyl
   alcohol, it is a far more effective catalyst than either rhenium or
   palladium. Of course, its radioactivity is a major problem in finding
   safe applications.

   Under certain circumstances, a small concentration (5×10^−5 mol/ L) of
   the pertechnetate ion in water can protect iron and carbon steels from
   corrosion. For this reason, pertechnetate could find use as an anodic
   corrosion inhibitor for steel, although technetium's radioactivity
   poses problems. While (for example) CrO[4]^2− can also inhibit
   corrosion, it requires a concentration ten times as high. In one
   experiment, a test specimen was kept in an aqueous solution of
   pertechnetate for 20 years and was still uncorroded. The mechanism by
   which pertechnetate prevents corrosion is not well-understood, but
   seems to involve the reversible formation of a thin surface layer. One
   theory holds that the pertechnetate reacts with the steel surface to
   form a layer of technetium dioxide which prevents further corrosion;
   the same effect explains how iron powder can be used to remove
   pertechnetate from water. ( Activated carbon can also be used for the
   same effect.) The effect disappears rapidly if the concentration of
   pertechnetate falls below the minimum concentration or if too high a
   concentration of other ions is added. The radioactive nature of
   technetium (3 M Bq per liter at the concentrations required) makes this
   corrosion protection impractical in almost all situations.
   Nevertheless, corrosion protection by pertechnetate ions was proposed
   (but never adopted) for use in boiling water reactors.

   Technetium-99 has also been proposed for use in optolectric nuclear
   batteries. ^99Tc's beta decay electrons would stimulate an excimer
   mixture, and the light would power a photocell. The battery would
   consist of an excimer mixture of argon/xenon in a pressure vessel with
   an internal mirrored surface, finely divided ^99Tc, and an intermittent
   ultrasonic stirrer, illuminating a photocell with a bandgap tuned for
   the excimer. If the pressure-vessel is carbon fibre/ epoxy, the weight
   to power ratio is said to be comparable to an air-breathing engine with
   fuel tanks.

History

Pre-discovery search

   Dmitri Mendeleev predicted technetium's properties before it was
   discovered.
   Enlarge
   Dmitri Mendeleev predicted technetium's properties before it was
   discovered.

   For a number of years there was a gap in the periodic table between
   molybdenum (element 42) and ruthenium (element 44). Many early
   researchers were eager to be the first to discover and name the missing
   element; its location in the table suggested that it should be easier
   to find than other undiscovered elements. It was first thought to have
   been found in platinum ores in 1828. It was given the name polinium but
   it turned out to be impure iridium. Then in 1846 the element ilmenium
   was claimed to have been discovered but was determined to be impure
   niobium. This mistake was repeated in 1847 with the "discovery" of
   pelopium. Dmitri Mendeleev predicted that this missing element, as part
   of other predictions, would be chemically similar to manganese and gave
   it the name ekamanganese.

   In 1877, the Russian chemist Serge Kern reported discovering the
   missing element in platinum ore. Kern named what he thought was the new
   element davyum, after the noted English chemist Sir Humphry Davy, but
   it was determined to be a mixture of iridium, rhodium and iron. Another
   candidate, lucium, followed in 1896 but it was determined to be
   yttrium. Then in 1908 the Japanese chemist Masataka Ogawa found
   evidence in the mineral thorianite for what he thought indicated the
   presence of element 43. Ogawa named the element nipponium, after Japan
   (which is Nippon in Japanese). Later analysis indicated the presence of
   rhenium (element 75), not element 43.

Disputed 1925 discovery

   German chemists Walter Noddack, Otto Berg and Ida Tacke (later Mrs.
   Noddack) reported the discovery of element 43 in 1925 and named it
   masurium (after Masuria in eastern Prussia). The group bombarded
   columbite with a beam of electrons and deduced element 43 was present
   by examining X-ray diffraction spectrograms. The wavelength of the
   X-rays produced is related to the atomic number by a formula derived by
   Henry Moseley in 1913. The team claimed to detect a faint X-ray signal
   at a wavelength produced by element 43. Contemporary experimenters
   could not replicate the discovery, and in fact it was dismissed as an
   error for many years.

   It was not until 1998 that this dismissal began to be questioned. John
   T. Armstrong of the National Institute of Standards and Technology ran
   computer simulations of the experiments and obtained results very close
   to those reported by the 1925 team; the claim was further supported by
   work published by David Curtis of the Los Alamos National Laboratory
   measuring the (tiny) natural occurrence of technetium. Debate still
   exists as to whether the 1925 team actually did discover element 43.

Official discovery and later history

   Discovery of element 43 has traditionally been assigned to a 1937
   experiment in Sicily conducted by Carlo Perrier and Emilio Segrè. The
   University of Palermo researchers found the technetium isotope ^97Tc in
   a sample of molybdenum given to Segrè by Ernest Lawrence the year
   before (Segrè visited Berkeley in the summer of 1936). The sample had
   previously been bombarded by deuterium nuclei in the University of
   California, Berkeley cyclotron for several months. University of
   Palermo officials tried unsuccessfully to force them to name their
   discovery panormium, after the Latin name for Palermo, Panormus. The
   researchers instead named element 43 after the Greek word technètos,
   meaning "artificial", since it was the first element to be artificially
   produced.

   In 1952 astronomer Paul W. Merrill in California detected the spectral
   signature of technetium (in particular, light at 403.1 nm, 423.8 nm,
   426.8 nm, and 429.7 nm) in light from S-type red giants. These massive
   stars near the end of their lives were rich in this short-lived
   element, meaning nuclear reactions within the stars must be producing
   it. This evidence was used to bolster the then unproven theory that
   stars are where heavier elements are produced. More recently, such
   observations provided evidence that elements were being formed by
   neutron capture in the s-process.

   Since its discovery, there have been many searches in terrestrial
   materials for natural sources. In 1962, technetium-99 was isolated and
   identified in pitchblende from the Belgian Congo in very small
   quantities (about 0.2 ng/kg); there it originates as a spontaneous
   fission product of uranium-238. This discovery was made by B.T. Kenna
   and P.K. Kuroda. There is also evidence that the Oklo natural nuclear
   fission reactor produced significant amounts of technetium-99, which
   has since decayed to ruthenium-99.

Occurrence and production

   Since technetium is unstable, only minute traces occur naturally in the
   Earth's crust as a spontaneous fission product of uranium. In 1999
   David Curtis (see above) estimated that a kilogram of uranium contains
   1 nanogram (1×10^−9 g) of technetium. Extraterrestrial technetium was
   found in some red giant stars (S-, M-, and N-types) that contain an
   absorption line in their spectrum indicating the presence of this
   element.

   In contrast with the rare natural occurrence, bulk quantities of
   technetium-99 are produced each year from spent nuclear fuel rods,
   which contain various fission products. The fission of a gram of the
   rare isotope uranium-235 in nuclear reactors yields 27 mg of ^99Tc,
   giving technetium a fission yield of 6.1%. Other fissionable isotopes
   also produce similar yields of technetium.

   It is estimated that up to 1994, about 49,000 T Bq (78 metric tons) of
   technetium was produced in nuclear reactors, which is by far the
   dominant source of terrestrial technetium. However, only a fraction of
   the production is used commercially. As of 2005, technetium-99 is
   available to holders of an ORNL permit for US$83/g plus packing
   charges.

   The actual production of technetium-99 from spent nuclear fuel is a
   long process. During fuel reprocessing, it appears in the waste liquid,
   which is highly radioactive. After sitting for several years, the
   radioactivity has fallen to a point where extraction of the long-lived
   isotopes, including technetium-99, becomes feasible. Several chemical
   extraction processes are used yielding technetium-99 metal of high
   purity.

   The meta stable (a state where the nucleus is in an excited state)
   isotope ^99mTc is produced as a fission product from the fission of
   uranium or plutonium in nuclear reactors. Due to the fact that used
   fuel is allowed to stand for several years before reprocessing, all
   ^99Mo and ^99mTc will have decayed by the time that the fission
   products are separated from the major actinides in conventional nuclear
   reprocessing. The PUREX raffinate will contain a high concentration of
   technetium as TcO[4]^- but almost all of this will be ^99Tc. The vast
   majority of the ^99mTc used in medical work is formed from ^99Mo which
   is formed by the neutron activation of ^98Mo. ^99Mo has a half-life of
   67 hours, so short-lived ^99mTc (half-life: 6 hours), which results
   from its decay, is being constantly produced. The hospital then
   chemically extracts the technetium from the solution by using a
   technetium-99m generator ("technetium cow").

   The normal technetium cow is an alumina column which contains
   molybdenum, as aluminium has a small neutron cross sectional it would
   be likely that an alumina column bearing inactive ^98Mo could be
   irradated with neutrons to make the radioactive column for the
   technetium cow. By working in this way, there is no need for the
   complex chemical steps which would be required to separate molybdenum
   from the fission product mixture. As an alternative method, an enriched
   uranium target can be irradated with neutrons to form ^99Mo as a
   fission product.

   Other technetium isotopes are not produced in significant quantities by
   fission; when needed, they are manufactured by neutron irradiation of
   parent isotopes (for example, ^97Tc can be made by neutron irradiation
   of ^96Ru).

Part of radioactive waste

   Since the yield of technetium-99 as a product of the nuclear fission of
   both uranium-235 and plutonium-239 is moderate, it is present in
   radioactive waste of fission reactors and is produced when a fission
   bomb is detonated. The amount of artificially produced technetium in
   the environment exceeds its natural occurrence to a large extent. This
   is due to release by atmospheric nuclear testing along with the
   disposal and processing of high-level radioactive waste. Due to its
   high fission yield and relatively high half-life, technetium-99 is one
   of the main components of nuclear waste. Its decay, measured in
   becquerels per amount of spent fuel, is dominant at about 10^4 to 10^6
   years after the creation of the nuclear waste.

   An estimated 160 T Bq (about 250 kg) of technetium-99 was released into
   the environment up to 1994 by atmospheric nuclear tests. The amount of
   technetium-99 from nuclear reactors released into the environment up to
   1986 is estimated to be on the order of 1000 TBq (about 1600 kg),
   primarily by nuclear fuel reprocessing; most of this was discharged
   into the sea. In recent years, reprocessing methods have improved to
   reduce emissions, but as of 2005 the primary release of technetium-99
   into the environment is by the Sellafield plant, which released an
   estimated 550 TBq (about 900 kg) from 1995-1999 into the Irish Sea.
   From 2000 onwards the amount has been limited by regulation to 90 TBq
   (about 140 kg) per year.

   As a result of nuclear fuel reprocessing, technetium has been
   discharged into the sea in a number of locations, and some seafood
   contains tiny but measurable quantities. For example, lobster from west
   Cumbria contains small amounts of technetium. The anaerobic,
   spore-forming bacteria in the Clostridium genus are able to reduce
   Tc(VII) to Tc(IV). Clostridia bacteria play a role in reducing iron,
   manganese and uranium, thereby affecting these elements' solubility in
   soil and sediments. Their ability to reduce technetium may determine a
   large part of Tc's mobility in industrial wastes and other subsurface
   environments.

   The long half-life of technetium-99 and its ability to form an anionic
   species makes it (along with ^129I) a major concern when considering
   long-term disposal of high-level radioactive waste. In addition, many
   of the processes designed to remove fission products from medium-active
   process streams in reprocessing plants are designed to remove cationic
   species like cesium (e.g., ^137Cs) and strontium (e.g., ^90Sr). Hence
   the pertechinate is able to escape through these treatment processes.
   Current disposal options favour burial in geologically stable rock. The
   primary danger with such a course is that the waste is likely to come
   into contact with water, which could leach radioactive contamination
   into the environment. The anionic pertechinate and iodide are less able
   to absorb onto the surfaces of minerals so they are likely to be more
   mobile. For comparison plutonium, uranium, and cesium are much more
   able to bind to soil particles. For this reason, the environmental
   chemistry of technetium is an active area of research. An alternative
   disposal method, transmutation, has been demonstrated at CERN for
   technetium-99. This transmutation process is one in which the
   technetium (^99Tc as a metal target) is bombarded with neutrons to form
   the shortlived ^100Tc (half life = 16 seconds) which decays by beta
   decay to ruthenium (^100Ru). One disadvantage of this process is the
   need for a very pure technetium target, while small traces of other
   fission products are likely to slightly increase the activity of the
   irradated target if small traces of the minor actinides (such as
   americium and curium) are present in the target then they are likely to
   undergo fission to form fission products. In this way a small activity
   and amount of minor actinides leads to a very high level of
   radioactivity in the irradated target. The formation of ^106Ru (half
   life 374 days) from the fresh fission is likely to increase the
   activity of the final ruthenium metal, which will then require a longer
   cooling time after irradation before the ruthenium can be used.

Isotopes

   Technetium is one of the two elements in the first 82 that have no
   stable isotopes. The other such element is promethium. The most stable
   radioisotopes are ^98Tc ( half-life of 4.2 Ma), ^97Tc (half-life: 2.6
   Ma) and ^99Tc (half-life: 211.1 ka).

   Twenty-two other radioisotopes have been characterized with atomic
   masses ranging from 87.933 u (^88Tc) to 112.931 u (^113Tc). Most of
   these have half-lives that are less than an hour; the exceptions are
   ^93Tc (half-life: 2.75 hours), ^94Tc (half-life: 4.883 hours), ^95Tc
   (half-life: 20 hours), and ^96Tc (half-life: 4.28 days).

   Technetium also has numerous meta states. ^97mTc is the most stable,
   with a half-life of 90.1 days (0.097 MeV). This is followed by ^95mTc
   (half life: 61 days, 0.038 MeV), and ^99mTc (half-life: 6.01 hours,
   0.143 MeV). ^99mTc only emits gamma rays, subsequently decaying to
   ^99Tc.

   For isotopes lighter than the most stable isotope, ^98Tc, the primary
   decay mode is electron capture, giving molybdenum. For the heavier
   isotopes, the primary mode is beta emission, giving ruthenium, with the
   exception that ^100Tc can decay both by beta emission and electron
   capture.

   Technetium-99 is the most common and most readily available isotope, as
   it is a major product of the fission of uranium-235. One gram of ^99Tc
   produces 6.2×10^8 disintegrations a second (that is, 0.62 G Bq/g).

Stability of technetium isotopes

   Technetium and promethium are remarkable among the light elements in
   that they have no stable isotopes. The reason for this is somewhat
   complicated.

   Using the liquid 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 which nuclides do not
   undergo beta decay. Nuclides that lie "up the walls" of the valley tend
   to decay by beta decay towards the centre (by emitting an electron,
   emitting a positron, or capturing an electron). For a fixed number of
   nucleons A, the binding energies lie on one or more parabolas, 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 four instances).
   However, if this happens, there can be no stable isotope with an even
   number of neutrons and an even number of protons.

   For technetium (Z=43), the valley of beta stability is centered at
   around 98 nucleons. However, for every number of nucleons from 95 to
   102, there is already at least one stable nuclide of either molybdenum
   (Z=42) or ruthenium (Z=44). 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.
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