Technetium - American Chemical Society

Article pubs.acs.org/jchemeduc

Technetium: The First Radioelement on the Periodic Table Erik V. Johnstone,*,† Mary Anne Yates,‡ Frederic Poineau,† Alfred P. Sattelberger,†,‡ and Kenneth R. Czerwinski† †

Department of Chemistry and Biochemistry, University of NevadaLas Vegas, Las Vegas, Nevada 89154, United States Argonne National Laboratory, Argonne, Illinois 60439, United States

‡

ABSTRACT: The radioactive nature of technetium is discussed using a combination of introductory nuclear physics concepts and empirical trends observed in the chart of the nuclides and the periodic table of the elements. Trends such as the enhanced stability of nucleon pairs, magic numbers, and Mattauch’s rule are described. The concepts of nuclear binding energies and the nuclear shell model are introduced and used to explain the relative stability of radionuclides and, in particular, the isotopes of technetium. KEYWORDS: Upper-Division Undergraduate, Graduate Education/Research, Inorganic Chemistry, Nuclear/Radiochemistry, Transition Elements

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INTRODUCTION Radioactivity, or the spontaneous emission of ionizing energy from the nucleus of a disintegrating atom, is a natural phenomenon that is integral to the construct of the physical world. It is ubiquitous, arising from both natural and artificial sources. Humans are routinely exposed to it through cosmic and primordial radiation, medical procedures, and its use in consumer goods.1,2 Since its discovery by Henri Becquerel in 1896,3 the study of radioactivity has culminated in the identification of many different radioelements and isotopes, the synthesis of new elements and isotopes, and the production of nuclear energy and weapons.4,5 One of the radioactive elements of particular interest is the first sequential radioelement on the periodic table, technetium. The element technetium is centrally located in the periodic table. Most chemistry courses do not include discussions on this element. In general, the chemistry of technetium is less explored than that of its neighbors. This can primarily be attributed to its radioactive nature. Previous education efforts have been based on discussing radioactivity as a natural phenomenon.1,2 These papers used the naturally occurring radionuclides to provide the readers with information on radioactive decay, exposure, and dose. In a similar manner, the natural radioactive properties of technetium, and its location in the middle of the transition series, provides an excellent educational opportunity to introduce chemistry students to radioactivity and nuclear physics. The history of technetium begins in 1869 with Dmitri I. Mendeleev, who is credited with assembling and arranging the approximately 60 known elements at the time into the first periodic table of elements. Using the table, he was able to predict the existence and possible properties of elements that had yet to be discovered.6 Of these hypothesized elements, Mendeleev predicted the discoveries of gallium, scandium, and germanium originally denoted as eka-aluminum, eka-boron, and eka-silicon, where the prefix eka means “first” in Sanskrit and refers to their chemical similarities and chronological arrangement. © XXXX American Chemical Society and Division of Chemical Education, Inc.

Along with these initially predicted elements was ekamanganese with an atomic weight of 100, which filled the gap underneath manganese and was also expected to have similar chemical properties.7−10 With the discovery of the electron in 1897 by J. J. Thompson and the proton by Ernest Rutherford in 1919, the ordering of elements in the periodic table was no longer arranged by increasing atomic mass, but instead by atomic charge. The coincidental discovery of isotopes, or different atomic masses for the same element, by Frederick Soddy and H. G. J. Moseley’s observation that characteristic Kα X-rays for elements were based on proportionality to charge, helped predict missing elements for Z = 43, 61, 72, and 75. The latter two were discovered as hafnium in 1923 and rhenium in 1925, respectively. Element Z = 61, promethium, was not discovered until 1945.11 The discovery of element 43 was made possible by the invention of the cyclotron in Berkeley, California, by Ernest O. Lawrence. The cyclotron allowed scientists to synthesize new elements and isotopes by bombarding targets with high-energy charged particles.12 Collaborations between Lawrence and the Italian radiochemists Emilio Segrè and Carlo Perrier led to the exchange of cyclotron-irradiated materials, including an irradiated molybdenum foil. In 1937, Segrè and Perrier were able to isolate element Z = 43 from the molybdenum foil.13 This new element’s chemical behavior was similar to that of manganese, and even more so to that of rhenium. The element was later named technetium with the symbol Tc derived from the Greek word for artificial, τεχνητoζ.́ 14 Technetium is the lightest inherently radioactive element on the periodic table. Its isotopes include masses from 85Tc to 120 Tc, where the longest-lived isotopes are 97Tc (t1/2 = 4.2 × 6 10 years), 98Tc (t1/2 = 4.2 × 106 years), and 99Tc (t1/2 = 2.1 × 105 years). Although it is referred to as an artificial element, Received: May 11, 2016 Revised: December 29, 2016

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DOI: 10.1021/acs.jchemed.6b00343 J. Chem. Educ. XXXX, XXX, XXX−XXX

Journal of Chemical Education

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protons, neutrons, and α particles, as well as the discovery of fission. H. L. Johnston made an attempt in 1931 to prepare a periodic representation of nuclides, but admitted it was far out of date by the time it was released. The current representation (Figure 1) was first published in 1935 by Giorgio Fea.22 It was later expanded by Emilio Segrè, Glenn Seaborg, and others from 1944 to 1958.23 As with the periodic table of the elements, having an organized visualization of the nuclides allows researchers to identify patterns and trends that help reveal the underlying physics of nuclear structure. In brief, the nuclides are displayed on a graph with the number of protons (atomic number = Z) on the ordinate and the number of neutrons, N, on the abscissa. The atomic weight, A, is approximated as A = Z + N. Nuclides with the same A are isobars; nuclides with the same Z are isotopes, and nuclides with the same N are isotones. Of the known nuclides, 266 are stable. The values of (N, Z) for the stable nuclides in comparison to Z = N are presented in Figure 1. At low Z numbers (Z ≤ 7), the pattern of stable nuclides, otherwise known as the valley of stability (see discussion below in the section on Atomic Masses, Binding Energies, and Mass Parabolas), follows N = Z. As the number of protons and corresponding neutrons that inhabit the nucleus is increased, this trend quickly diminishes, and the valley of stability moves toward neutron-rich nuclides. This can be explained by the necessary mass compensation of the neutrons in the nucleus to overcome the Coulombic repulsions of the protons in proximity to each other. Typically, unstable nuclides found above the valley of stability (low N/Z or proton-rich) will decay through β+ emission or by electron capture, while those below it (high N/Z or neutron-rich) will decay through β− emission. The decays result in the conversion of a nucleon, forming isotopes that approach stable nuclides at the bottom of the valley of stability. In radioactive isotopes with A > 210, α decay is also a competitive reaction. Empirical evidence suggests that there are useful distinctions to draw by classifying stable nuclides based on whether the proton or neutron number is even or odd (Table 1).25 Of the known

technetium is naturally occurring; however, it is only formed in ultratrace quantities from the spontaneous fission of natural 238 U. Additionally, it has also been observed in stars that undergo the s-process.15,16 Nevertheless, almost all of the technetium present on Earth is a result of nuclear energy production in uranium-fueled reactors. Because isotopes with atomic weight (A) = 99 have a relatively high production yield from the fission of 235U, i.e., cumulative thermal neutron fission yield ∼6.1%, most of the technetium is isotopically present as 99Tc. The typical production of 99Tc in uranium-fueled light water reactors is approximately 2 g per day for every 100 MW of thermal energy.17 The appreciable amounts of 99Tc formed in the generation of nuclear power and the role of 99mTc in radiopharmaceutical applications have sparked interest in exploring and developing technetium chemistry.18 Technetium exhibits extensive redox chemistry with 9 oxidation states ranging from −1 to +7, where technetium complexes with the metal atom in valence +7 to +3 are often used as precursors in synthetic chemistry.19 However, due to its radioactivity and late discovery, there is relatively little known about its chemistry in comparison with the neighboring stable elements.17−21 Because technetium is the lightest radioelement located centrally among the transition metals, one question often arises: Why is it radioactive or, more specifically, why are there no stable isotopes of technetium? One can provide a simple explanation based on empirical evidence derived from nuclear trends in the chart of the nuclides. In this work, we use a combination of these trends along with nuclear binding energies and the nuclear shell model to give a simple and reasonably straightforward explanation of the innate radioactivity of technetium.

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CHART OF THE NUCLIDES AND THE NUCLEAR SHELL MODEL In the 1920s and 1930s, the number of known isotopes was exploding due to the wide range of experiments taking place around the world involving the bombardment of atoms with

Figure 1. Values of (N, Z) for the stable nuclides in comparison to N = Z. Data for the graphic has been collected from the chart of the nuclides.24 B



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Table 1. Distribution of Stable Nuclides by Even or Odd A, Z, and N A

Z

N

Number of Stable Nuclides

Even Odd Odd Even

Even Even Odd Odd

Even Odd Even Odd

159 53 50 4

Table 2. Stable Odd A Isotopes with Odd Z, Even N Configurations near Technetium Stable Isotope 89

Y 93 Nb 103 Rh 107 Ag 109 Ag

stable nuclides, the majority (159) is composed of even proton and even neutron numbers; this is significant because even Z, even N configurations result in paired nucleon stability. In almost equivalent amounts to each other are the odd A stable nuclei with either even Z and odd N or odd Z and even N. Each of these combinations only account for roughly 20% of the existing stable elements. There are only four stable nuclei with odd Z and odd N configurations. These four even A nuclides (i.e., 2H, 6Li, 10B, 14N) only exist at low Z numbers. Beyond 14 N, there are no stable nuclides with both odd proton and odd neutron numbers; likewise, nucleon pairing for these odd Z, odd N configurations is not favorable.25 The enhanced stability of even numbers of particles suggested the existence of pairing of nucleons just as pairing of electrons is observed in atoms. Another significant trend in the chart of the nuclides is the number of stable nuclei with Z or N equal to 2, 4, 8, 20, 28, 50, 82, and 126. These observations led to the development of the nuclear shell model, which is now the basis for understanding nuclear structure. It was first proposed by W. M. Elsasser in 1934 and clarified by Maria Goeppert Mayer in 1948.26 The nuclear shell model describes the pairing and distribution of nucleons (protons and neutrons) within discrete energy levels in the nucleus, similar to the atomic shell model for electrons.27 The labels for the energy levels progress as with electrons: s, p, d, f, etc. Proton and neutron shells are filled individually according to the Pauli principle where closed shells result in configurations much more stable in comparison to those with non-filled arrangements. These filled-shell configurations are referred to as magic numbers. A clear example of the enhanced stability due to magic numbers is tin (Z = 50), which has 10 different stable isotopes, the highest number for any element. When both the numbers of neutrons and protons in a nucleus result in filled shells, it is considered to be doubly magic. Examples of these nuclei include the stable, neutron-rich 48Ca (Z = 20, N = 28) and the radionuclide 132Sn, which is important in fission product distribution. Magic and doubly magic isotopes illustrate the close relationship between paired nucleons and relative nuclear stability.25 These observable trends within the chart of the nuclides lead to a better understanding of why there are no stable isotopes of technetium. Hypothetically, if there were a stable isotope of technetium, it would have to fulfill these necessary characteristics: (1) It would have to fall at the bottom of the valley of stability. The odd Z elements near technetium that have stable isotopes are 89Y, 93Nb, 103Rh, 107Ag, and 109Ag (Table 2). These isotopes have N/Z ratios from 1.26 to 1.32. Using this ratio range, the expected stable isotopes of technetium would be in the range 97 ≤ A ≤ 99 (Table 3). (2) Because technetium has an odd atomic number (Z = 43), no stable even A configurations are attainable, since this would result from an odd Z and odd N configuration.

Z

N

N/Z

39 41 45 47 47

50 52 58 60 62

1.28 1.26 1.29 1.28 1.32

Table 3. N/Z Ratios of Odd A Technetium Isotopes with Odd Z, Even N Configurations for A = 97 and 99

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Atomic Weight

Z

N

N/Z

97 99

43 43

54 56

1.26 1.30

A hypothetical stable technetium isotope would have an odd A with an odd Z and even N configuration. This reduces the selection of isotopes to those for A = 97 and 99. Yet, even under these circumstances, neither of these isotopes are stable. It is noted that 97Tc and 99Tc are by far the longest-lived isotopes in this group and so could be thought of as “closest” to stability. So, what else accounts for the inherent radioactive nature of technetium? Mattauch’s isobar rule is another empirical trend within the chart of the nuclides that can be used for evaluating nuclear stability.

MATTAUCH’S RULE Mattauch’s empirical isobar rule states that no two adjacent nuclides in an isobar can both be stable.28 Josef Mattauch proposed this in 1934 in the early days of nuclear physics research as a tool to help understand trends in nuclear structure. To apply Mattauch’s rule to technetium, one must evaluate stable isotopes of the two neighboring elements on the chart of the nuclides, molybdenum (Z = 42) and ruthenium (Z = 44). As previously determined from observable trends within the chart of the nuclides, the only stable technetium isotopes that could potentially exist in accordance with the shell model would be for those with A = 97 and 99. For these technetium isotopes to be stable, molybdenum and ruthenium must not have stable isotopes with identical atomic mass numbers. However, molybdenum and ruthenium exhibit six and seven stable isotopes, respectively, between A = 92 and A = 104. It is noted that the isotope 100Mo, while radioactive, has a half-life of 7.3 × 1018 years and decays by double β decay, making it a “nearly” stable isotope. For both elements, five of the stable isotopes are consecutively numbered, i.e., 94Mo to 98Mo and 98 Ru to 102Ru. The condition of five stable, consecutive isotopes is shown by other elements (Table 4). Some of these trends are based on shell properties. Elements with five stable, consecutive isotopes with neutron or proton number within two of the magic numbers comprise all the elements up to barium, including molybdenum and ruthenium, in Table 4. For molybdenum and ruthenium, the neutron numbers are close to the magic number 50. While this may impart some stability, it does not effectively describe the five consecutive, stable isotopes for these elements. However, the shell model describes a subshell at Z = 40, as well as Z = 70. This subshell near Z = 40 and A ≈ 100 is the subject of numerous studies on nuclear stability.29−32 The results show interplay in nuclear properties between the shell and subshell C



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where a, b, and c are functions of A reflecting the size of the nucleus and Coulomb forces, and δ is related to the pairing energy of the nucleons. For a given A isobar (suite of nuclei with the same value of A), a, b, and c are constants, so the plot of the constituent masses forms a parabola.33 Examples of this are presented in Figures 3 and 4. Stable isotopes will occupy the lowest possible energy at a minimum

Table 4. Elements with at Least Five Stable Consecutive Isotopesa Element

Z

Stable Isotopes in Row

Total Stable Isotopes

Ti Mo Ru Sn Xe Ba Gd Dy Yb Hf Hg

22 42 44 50 54 56 64 66 70 72 80

46−50 94−98 98−102 114−120 128−132 134−138 154−158 160−164 170−174 176−180 198−202

5 6 7 10 9 6 6b 7 7 5c 7

a

The list excludes long-lived naturally occurring isotopes with a measured natural abundance. b152Gd is 0.2% abundance with t1/2 = 1 × 1014 years. c174Hf is 0.2% abundance with t1/2 = 2 × 1015 years.

structures in the nucleus. Generally, there appears to be enhanced stability for the even Z isotopes. For the subject of this discussion, elements molybdenum and ruthenium having five consecutive stable isotopes above and below technetium (Figure 2) indicate that the isotopes 97Tc and 99Tc cannot be stable in accordance with the observations that form the basis of Mattauch’s rule.28

Figure 3. Isobar parabola for odd mass number nuclides demonstrating the tendency of neutron-rich nuclides to successively β− decay and proton-rich nuclides to successively β+ or electron capture decay toward a stable nuclide at the minimum or “valley”.

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ATOMIC MASSES, BINDING ENERGIES, AND MASS PARABOLAS Another way to explain the radioactivity of technetium involves the concepts of binding energies or mass defects. As mentioned above, the atomic weight, A, equals the sum of the number of protons, Z, and neutrons, N, and is usually presented in atomic mass units (AMU) where the mass of the carbon atom is defined as identically 12 AMU. This can also be expressed as energy in million electron volts (MeV) through the relationship E = mc2. In reality, however, the actual mass of a nucleus, M, differs from A by the amount of energy required to dissociate the nucleus into its constituent nucleons, EB, referred to as the mass defect, or binding energy. The relationship between mass defect and binding energy is the expression of the value in either mass or energy, again using E = mc2. This is represented in eq 1, where MH is the mass of the hydrogen atom (938.77 MeV) and MN is the mass of the neutron (939.55 MeV), both in energy units. M = ZMH + (A − Z)MN − E B

This may be rewritten

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(1)

as

M = aZ + bZ + c − δA−1 2

of the parabola. Unstable nuclei to the left of the minimum are neutron-rich and decay by β− emission. Nuclides to the right of the minimum are proton-rich, and hence decay by β+ or electron capture. If A is large enough, generally greater than 210, then α decay can also occur. Each mass parabola can be treated as an individual cross-section from the valley of stability referred to above in the discussion of the chart of the nuclides. It is in fact these energy parabolas that form the “valley”. For odd A masses from odd Z and even N or even Z and odd N, there can only be one minimum value for the parabola (Figure 3), e.g., A = 97 and 99. Conversely, even A masses can include even−even nuclei with 1−3 stable nuclides as minima for a single isobar (Figure 4), e.g., A = 98.34 The energy value on the ordinate could be the actual mass of the nucleus or the mass defect given in MeV. As mentioned above, each nucleus will seek its energy minimum through radioactive decay with the energy of the decay being proportional to the difference in mass. Table 5 lists the mass defects for molybdenum, technetium, and ruthenium isotopes 97−101. The most stable nuclei, those with the smallest mass defect, i.e., largest binding energy, are highlighted in bold.

(2)

Figure 2. Plot of proton number (Z) vs neutron number (N) of isotopes of Mo (Z = 42) to Ru (Z = 44) illustrating the abundance of stable isotopes of Mo and Ru surrounding technetium. Stable or very long-lived (i.e., 100 Mo) isotopes are in black. Data for the graphic has been collected from the chart of the nuclides.24 D



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Figure 4. Isobar parabola for even mass number nuclides decaying into two stable nuclides. Figure 6. Isobar A = 98 decay scheme. No excited states are shown.

Table 5. Mass Defect Values for Mo, Tc, and Ru Isotopes Atomic Weight

42

97 98 99 100 101 a

Mo, MeV

−87.54 −88.11a −85.97 −86.18 −83.51

a

43

Tc, MeV −87.22 −86.43 −87.32 −86.02 −86.34

44

Ru, MeV

−86.11 −88.22a −87.62a −89.22a −87.95a

These are stable nuclei.

The mass parabolas plotted as a function of relative decay energy (MeV) and atomic number for the isobars A = 97, 98, and 99 are shown in Figures 5, 6, and 7, respectively. As mentioned previously, for both odd A (A = 97 and 99) isobars, successive β− and β+ decays result in one stable nuclide; for A = 97, the stable nuclide is 97Mo, and for A = 99 it is 99Ru. The A = 98 is noticeably different from the other two isobar chains as there are two stable nuclides, 98Mo and 98Ru. For 97Tc and 99 Tc, the β disintegration energies (0.320 and 0.294 MeV, respectively) are lower in comparison to that of 98Tc (1.80 MeV), implying relative stability of the odd A 97 and 99 isotopes of technetium. Both 97Tc and 99Tc decay in accordance with other trending isotopes in each isobar, namely, a single decay mode. The isotope 98Tc is only reported to undergo β− decay to 98 Ru with no appreciable decay by β+ or electron capture

Figure 7. Isobar A = 99 decay scheme. No excited states are shown.

to 98Mo. However, on the basis of Figure 6, 98Tc is unstable to both β− decay and β+ or electron capture. For comparison, one may look at the similar case of 64Cu which decays 40% by β− to 64 Zn and 41% by electron capture (EC) and 19% by β+ to 64Ni. The branching ratio of electron capture to β− decay of 98Tc is less than 4%,35 with a corresponding maximum β+ or electron capture half-life of 1.1 × 108 years. The calculated energy release in β− decay for the reported decay of 98Tc is 1.792 MeV, while the corresponding energy releases for β+ and EC for this isotope to 98 Mo are 0.661 and 1.682 MeV, respectively. In summary, the mass parabola data explains the lack of stable technetium isotopes due to the relative stability of the neighboring elements.

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CONCLUSION The radioactive nature of technetium can be described using a combination of the nuclear shell model, properties of nuclear structure, and trends within the chart of the nuclides. For technetium, an odd Z element, only isotopes with odd A could possibly exhibit a stable configuration. The unique condition that the elements situated one proton above and below technetium have five consecutive stable isotopes is due to the nuclear properties exhibited by nuclei with A ≈ 100. Mattauch’s isobar rule, which states that no two adjacent isotopes in an isobar can both be stable, reflects the fact that the odd

Figure 5. Isobar A = 97 decay scheme. No excited states are shown. E



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Notes

A technetium isotopes are radioactive given the greater stability of the surrounding molybdenum and ruthenium isotopes. The binding energies of the isobars and mass parabolas of technetium with A = 97, 98, and 99 illustrate the corresponding molybdenum and ruthenium isotopes as the energetically lowest points within the valley of β stability, rendering all other neighboring nuclides within the isobar unstable. Technetium has no stable isotopes due to a combination of nuclear properties impacting relative binding energy. The characteristics of the structure of nuclei around A ≈ 100 make this region of the chart of the nuclides a likely location for encountering a radioactive element with Z < 83. Studying it also provides an opportunity to acquaint students with basic concepts of nuclear science. Understanding the nuclear properties of technetium and its isotopes is important for enhancing fundamental knowledge about radioactivity and elucidating the unique nature of this element.

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS E.V.J. would like to thank his supervisors and mentors for their relentless and invaluable help, support, and encouragement.

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APPLICATION IN THE CLASSROOM The radiochemistry of technetium has been used as a teaching example in two different undergraduate radiochemistry courses. As far as improving the understanding of basic nuclear physics and radiochemistry concepts, it seemed successful. The course presentation covered a number of topics discussed in the paper. Since technetium has an odd number of protons, a potentially stable isotope would have an even number of neutrons. The expected candidates for stable technetium isotopes would then be A = 97 or 99, based on the nature of stable isotopes, with only 4 having an odd proton and odd neutron number. In a comparison of the half-lives of technetium isotopes of A = 97, 98, and 99, it is observed that the longest half-life is for both 97Tc and 98Tc, each at 4.26 × 106 years. The discussion in the paper on observable trends for stable isotopes is used to address the half-lives of 97Tc and 99Tc in comparison with the stable isotopes of the respective isobars. The mass parabolas for A = 97, 98, and 99 were provided as examples of isobaric trends within the courses. For the A = 98 mass parabola, it was noted that multiple stable isotopes occur for a single mass. Furthermore, the ability of some isotopes to decay by multiple routes was exemplified with 98Tc and with 64Cu provided as a further, known example. Additionally, the isotopic spin and parity were compared; this was not presented in the paper. It was shown that the stable isotopes for A = 97, 98, and 99 have a lower nuclear spin than the radioactive technetium isotopes. We note that the same considerationsnuclear shell model, magic numbers, and Mattauch’s isobar rulealso explain why element 61 (promethium) has no stable isotopes. This second radioelement on the periodic table can stimulate additional classroom discussion. Finally, the mass stability was a very direct application of energy−mass relativity that was appreciated by the students. As most students are familiar with E = mc2, incorporation of this concept into terms that are readily identifiable does enhance the educational experience.

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Erik V. Johnstone: 0000-0001-7358-8085 F



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