transuranium element

Any of the chemical elements after uranium in the periodic table (with atomic numbers greater than 92).

All are radioactive (see radioactivity), with half-lives ranging from tens of millions of years to fractions of a millisecond. Only two, neptunium (93) and plutonium (94), occur in nature, and only as traces in uranium ores as a result of neutron irradiation. Transuranium elements with atomic numbers through 112, along with 114 and 116, have been produced in laboratories. Each appears to resemble the elements above it in the periodic table; in particular, the actinides, thorium (90) through lawrencium (103), are similar to the lanthanides, cerium (58) through lutetium (71). The naming of the transuranium elements has been fraught with controversy regarding which laboratory first made the discovery and should propose the name and whether elements should be named for living persons. See also Glenn Seaborg.

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▪ chemical element

Introduction

 any of the chemical elements that lie beyond uranium in the periodic table (see Figure—>)—i.e., those with atomic numbers (atomic number) greater than 92. More than 20 of these elements have been discovered and named or are awaiting confirmation of their discovery. Eleven of them, from neptunium through lawrencium, belong to the actinoid series (see actinoid element (actinide element)). The others, which have atomic numbers higher than 103, are referred to as the transactinoids. All the transuranium elements are unstable, decaying radioactively, with half-lives that range from tens of millions of years to mere fractions of a second.

      Since only two of the transuranium elements have been found in nature (neptunium and plutonium) and those only in trace amounts, the synthesis of these elements through nuclear reactions has been an important source of knowledge about them. That knowledge has expanded scientific understanding of the fundamental structure of matter and makes it possible to predict the existence and basic properties of elements much heavier than any currently known. Present theory suggests that the maximum atomic number could be found to lie somewhere between 170 and 210, if nuclear instability would not preclude the existence of such elements. All these still-unknown elements are included in the transuranium group.

Discovery of the first transuranium elements

      The first attempt to prepare a transuranium element was made in 1934 in Rome, where a team of Italian physicists headed by Enrico Fermi (Fermi, Enrico) and Emilio Segrè (Segrè, Emilio) bombarded uranium nuclei with free neutrons. Although transuranium species may have been produced, the experiment resulted in the discovery of nuclear fission rather than new elements. (The German scientists Otto Hahn, Fritz Strassman, and Lise Meitner showed that the products Fermi found were lighter, known elements formed by the splitting, or fission, of uranium.) Not until 1940 was a transuranium element first positively produced and identified, when two American physicists, Edwin Mattison McMillan (McMillan, Edwin Mattison) and Philip Hauge Abelson (Abelson, Philip Hauge), working at the University of California at Berkeley, exposed uranium oxide to neutrons from a cyclotron target. One of the resulting products was an element found to have an atomic number of 93. It was named neptunium.

      Transformations (transmutation) in atomic nuclei are represented by equations that balance all the particles of matter and the energy involved before and after the reaction. The above transformation of uranium into neptunium may be written as follows:

      In the first equation the atomic symbol of the particular isotope reacted upon, in this case U for uranium, is given with its mass number at upper left and its atomic number at lower left: ²³⁸₉₂U. The uranium-238 isotope reacts with a neutron (symbolized n, with its mass number 1 at upper left and its neutral electrical charge shown as 0 at lower left) to produce uranium-239 (²³⁹₉₂U) and the quantum of energy called a gamma ray (γ). In the next equation the arrow represents a spontaneous loss of a negative beta particle (symbolized β⁻), an electron with very high velocity, from the nucleus of uranium-239. What has happened is that a neutron within the nucleus has been transformed into a proton, with the emission of a beta particle that carries off a single negative charge; the resulting nucleus now has one more positive charge than it had before the event and thus has an atomic number of 93. Because the beta particle has negligible mass, the mass number of the nucleus has not changed, however, and is still 239. The nucleus resulting from these events is an isotope of the element neptunium, atomic number 93 and mass number 239. The above process is called negative beta-particle decay. A nucleus may also emit a positron, or positive electron, thus changing a proton into a neutron and reducing the positive charge by one (but without changing the mass number); this process is called positive beta-particle decay. In another type of beta decay a nuclear proton is transformed into a neutron when the nucleus, instead of emitting a beta particle, “captures,” or absorbs, one of the electrons orbiting the nucleus; this process of electron capture (EC decay) is preferred over positron emission in transuranium nuclei.

      The discovery of the next element after neptunium followed rapidly. In 1941 three American chemists, Glenn T. Seaborg (Seaborg, Glenn T.), Joseph W. Kennedy, and Arthur C. Wahl, produced and chemically identified element 94, named plutonium (Pu). In 1944, after further discoveries, Seaborg hypothesized that a new series of elements called the actinoid series, akin to the lanthanoid series (elements 58–71), was being produced, and that this new series began with thorium (Th), atomic number 90. Thereafter, discoveries were sought, and made, in accordance with this hypothesis.

      For further information on discoveries of transuranium elements, see Table (Discovery of the Transuranium Elements, Table).

Synthesis of transuranium elements

      The most abundant isotope of neptunium is neptunium-237. Neptunium-237 has a half-life of approximately 2 × 10⁶ years and decays by the emission of alpha particles. (Alpha particles are composed of two neutrons and two protons and are actually the very stable nucleus of helium.) Neptunium-237 is formed in kilogram quantities as a by-product of the large-scale production of plutonium in nuclear reactors. This isotope is synthesized from the reactor fuel uranium-235 by the reaction

      and from uranium-238 by

       plutonium, as the isotope plutonium-239, is produced in ton quantities in nuclear reactors by the sequence

      Because of its ability to undergo fission with neutrons of all energies, plutonium-239 has considerable practical applications as an energy source in nuclear weapons and as fuel in nuclear power reactors.

      The method of element production discussed thus far has been that of successive neutron capture resulting from the continuous, intensive irradiation with slow (low-energy) neutrons of an actinoid target. The sequence of nuclides (nuclide) that can be synthesized in nuclear reactors by this process is shown in theFigure, in which the light line indicates the principal path of neutron capture (horizontal arrows) and negative beta-particle decay (up arrows) that results in successively heavier elements and higher atomic numbers. (Down arrows represent electron-capture decay.) The heavier lines show subsidiary paths that augment the major path. The major path terminates at fermium-257 (fermium), because the short half-life of the next fermium isotope (fermium-258)—for radioactive decay by spontaneous fission (360 microseconds)—precludes its production and the production of isotopes of elements beyond fermium by this means. The heavier lines beyond indicate predictions only.

      Heavy isotopes of some transuranium elements are also produced in nuclear explosions. Typically, in such events, a uranium target is bombarded by a high number of fast (high-energy) neutrons for a small fraction of a second, a process known as rapid-neutron capture, or the -process (in contrast to the slow-neutron capture, or -process, described above). Underground detonations of nuclear explosive devices during the late 1960s resulted in the production of significant quantities of einsteinium and fermium isotopes, which were separated from rock debris by mining techniques and chemical processing. Again, the heaviest isotope found was that of fermium-257.

      An important method of synthesizing transuranium isotopes is by bombarding heavy element targets not with neutrons but with light charged particles (particle accelerator) (such as the helium nuclei mentioned above as alpha particles) from accelerators. For the synthesis of elements 101 or greater, so-called heavy ions (with atomic number greater than 2 and mass number greater than 5) have been used for the projectile nuclei. Targets and projectiles relatively rich in neutrons are required so that the resulting nuclei will have sufficiently high neutron numbers; too low a neutron number renders the nucleus extremely unstable and unobservable because of its resultantly short half-life.

      Beginning with element 106, the elements have been synthesized and identified (i.e., discovered) by the use of “cold,” or “soft,” fusion reactions. In this type of reaction, medium-weight projectiles are fused to target nuclei with protons numbering close to 82 and neutrons numbering about 126—i.e., near the doubly “magic” lead-208—resulting in a relatively “cold” compound system. Because the newly formed compound nuclei have lower excitation energies than those produced in the “hot” fusion of heavy actinoid targets and relatively light projectiles, they may emit only 1 or 2 neutrons and thus have a much higher probability of remaining intact instead of undergoing the competing prompt fission reaction. Such cold fusion reactions were first recognized as a method for the synthesis of heavy elements by Yuri Oganessian of the Joint Institute for Nuclear Research at Dubna, U.S.S.R. (now in Russia).

Nuclear properties

       Nuclear properties of selected transuranium element isotopes Nuclear properties of selected transuranium element isotopes Nuclear properties of selected transuranium element isotopesIsotopes of the transuranium elements are radioactive (radioactivity) in the usual ways: they decay by emitting alpha particles, beta particles, and gamma rays; and they also fission spontaneously. The Table (Nuclear properties of selected transuranium element isotopes) lists significant nuclear properties of certain isotopes that are useful for chemical studies. Only the principal mode of decay is given, though in many cases other modes of decay also are exhibited by the isotope. In particular, with the isotope californium-252, alpha-particle decay (alpha decay) is important because it determines the half-life, but the expected applications of the isotope exploit its spontaneous fission decay that produces an enormous neutron output. Other isotopes, such as plutonium-238, are useful because of their relatively large thermal power output during decay (given in the Table (Nuclear properties of selected transuranium element isotopes) in watts per gram). Research on the chemical and solid-state properties of these elements and their compounds obviously requires that isotopes with long half-lives be used. Isotopes of plutonium and curium, for example, are particularly desirable from this point of view. In the Table (Nuclear properties of selected transuranium element isotopes) the specific activities (a measure of the intensity of a radioactive source) are given for those elements that can be produced in nuclear reactors. Beyond element 100 the isotopes must be produced by charged-particle reactions using particle accelerators, with the result that only relatively few atoms can be made at any one time.

Nuclear structure and stability

      Although the decay properties of the transuranium elements are important with regard to the potential application of the elements, these elements have been studied largely to develop a fundamental understanding of nuclear reactions and nuclear and atomic structure. Study of the known transuranium elements also helps in predicting the properties of yet-undiscovered isotopes and elements as a guide to the researcher who can then design experiments to prepare and identify them. As shown in the Figure, the known isotopes can be represented graphically with the number of nuclear protons (Z) plotted along the left-hand axis and the number of neutrons (N) plotted on the top axis. The relative stabilities of the isotopes are indicated by their relative heights. In this metaphoric representation, the known isotopes resemble a peninsula rising above a sea of instability. The most stable isotopes, appearing as mountaintops, occur at specific values called magic numbers (magic number).

      The magic numbers derive from calculations of the energy distribution based on the theoretical structure of the nucleus. According to theory, neutrons and protons (collectively, nucleons) are arranged within the nucleus in shells (shell nuclear model) that are able to accommodate only fixed maximum numbers of them; when the shells are closed (i.e., unable to accept any more nucleons), the nucleus is much more stable than when the shells are only partially filled. The number of neutrons or protons in the closed shells yields the magic numbers. These are 2, 8, 20, 28, 50, 82, and 126. Doubly magic nuclei, such as helium-4, oxygen-16, calcium-40, calcium-48, and lead-208, which have both full proton shells and full neutron shells, are especially stable. As the proton and neutron numbers depart further and further from the magic numbers, the nuclei are relatively less stable.

      As the highest atomic numbers are reached, decay by alpha-particle emission and spontaneous fission sets in (see below). At some point the peninsula of relatively stable isotopes (i.e., with an overall half-life of at least one second) is terminated. There has been, however, considerable speculation, based on a number of theoretical calculations, that an island of stability might exist in the neighbourhood of Z = 114 and N = 184, both of which are thought to be magic numbers. Isotopes in this region, which theoretically should have significantly longer half-lives than the neighbouring known heavy isotopes, are sometimes termed superheavy elements (SHE). There is also evidence for subshells (regions of somewhat increased stability) at Z = 108 and N = 162.

Processes of nuclear decay

      The correlation and prediction of nuclear properties in the transuranium region are based on systematics (that is, extensions of observed relationships) and on the development of theoretical models of nuclear structure. The development of structural theories of the nucleus has proceeded rather rapidly, in part because valid parallels with atomic and molecular theory can be drawn.

Alpha-particle decay (alpha decay)

      A nucleus can decay to an alpha particle plus a daughter product if the mass of the nucleus is greater than the sum of the mass of the daughter product and the mass of the alpha particle—i.e., if some mass is lost during the transformation. The amount of matter defined by the difference between reacting mass and product mass is transformed into energy and is released mainly with the alpha particle. The relationship is given by Einstein's equation (Einstein's mass-energy relation) E = mc², in which the product of the mass (m) and the square of the velocity of light (c) equals the energy (E) produced by the transformation of that mass into energy. It can be shown that, because of the inequality between the mass of a nucleus and the masses of the products, most nuclei beyond about the middle of the periodic table are likely to be unstable because of the emission of alpha particles. In practice, however, because of the reaction rate, decay by ejection of an alpha particle is important only with the heavier elements. Indeed, beyond bismuth (element 83) the predominant mode of decay is by alpha-particle emission, and all the transuranium elements are alpha-unstable.

      The regularities in the alpha-particle decay energies that have been noted from experimental data can be plotted on a graph and, since the alpha-particle decay half-life depends in a regular way on the alpha-particle decay energy, the graph can be used to obtain the estimated half-lives of undiscovered elements and isotopes. Such predicted half-lives are essential for experiments designed to discover new elements and new isotopes, because the experiments must take the expected half-life into account.

Beta-particle decay (beta decay)

      In elements lighter than lead, beta-particle decay—in which a neutron is transformed into a proton or vice versa by emission of either an electron or a positron or by electron capture—is the main type of decay observed. Beta-particle decay also occurs in the transuranium elements, but only by emission of electrons or by capture of orbital electrons; positron emission has not been observed in transuranium elements. When the beta-particle decay processes are absent in transuranium isotopes, the isotopes are said to be stable to beta decay.

Decay by spontaneous fission

      The lighter actinoids such as uranium rarely decay by spontaneous fission, but at californium (element 98) spontaneous fission becomes more common (as a result of changes in energy balances) and begins to compete favourably with alpha-particle emission as a mode of decay. Regularities have been observed for this process in the very heavy element region. If the half-life of spontaneous fission is plotted against the ratio of the square of the number of protons (Z) in the nucleus divided by the mass of the nucleus (A)—i.e., the ratio Z²/A—then a regular pattern results for nuclei with even numbers of both neutrons and protons (even-even nuclei). Although this uniformity allows very rough predictions of half-lives for undiscovered isotopes, the methods actually employed are considerably more sophisticated.

      The results of study of half-life systematics for alpha-particle, negative beta-particle, and spontaneous-fission decay in the near region of undiscovered transuranium elements can be plotted in graphs for even-even nuclei, for nuclei with an odd number of protons or neutrons, and for odd-odd nuclei (those with odd numbers for both protons and neutrons). These predicted values are in the general range of experimentally determined half-lives and correctly indicate trends, but individual points may differ appreciably from known experimental data. Such graphs show that isotopes with odd numbers of neutrons or protons have longer half-lives for alpha-particle decay and for spontaneous fission than do neighbouring even-even isotopes.

Nuclear structure and shape

Nuclear models

      Several models have been used to describe nuclei and their properties. In the liquid-drop model the nucleus is treated as a uniform, charged drop of liquid. This structure does not account for certain irregularities, however, such as the increased stability found for nuclei with particular magic numbers of protons or neutrons (see above). The shell model recognized that these magic numbers resulted from the filling, or closing, of nuclear shells. Nuclei with the exact number (or close to the exact number) of neutrons and protons dictated by closed shells have spherical shapes, and their properties are successfully described by the shell theory. However, the lanthanoid and actinoid nuclei, which do not have magic numbers of nucleons, are deformed into a prolate spheroid, or football, shape, and the spherical-shell model does not adequately explain their properties. The shell model nevertheless established the fact that the neutrons and protons within a nucleus are more likely to be found inside rather than outside certain nuclear shell regions and thus showed that the interior of the nucleus is inhomogeneous. A model incorporating the shell effects to correct the ordinary homogeneous liquid-drop model was developed. This hybrid model is used, in particular, to explain spontaneous-fission half-lives.

      Since many transuranium nuclei do not have magic numbers of neutrons and protons and thus are nonspherical, considerable theoretical work has been done to describe the motions of the nucleons in their orbitals outside the spherical closed shells. These orbitals are important in explaining and predicting some of the nuclear properties of the transuranium and heavy elements.

Nuclear-shape isomers (isomer)

      The mutual interaction of fission theory and experiment brought about the discovery and interpretation of fission isomers. At Dubna, Russia, U.S.S.R., in 1962, americium-242 was produced in a new form that decayed with a spontaneous-fission half-life of 14 milliseconds, or about 10¹⁴ times shorter than the half-life of the ordinary form of that isotope. Subsequently, more than 30 other examples of this type of behaviour were found in the transuranium region. The nature of these new forms of spontaneously fissioning nuclei was believed to be explainable, in general terms at least, by the idea that the nuclei possess greatly distorted but quasi-stable nuclear shapes. The greatly distorted shapes are called isomeric states, and these new forms of nuclear matter are consequently called shape isomers. As mentioned above, calculations relating to spontaneous fission involve treating the nucleus as though it were an inhomogeneous liquid drop, and in practice this is done by incorporating a shell correction to the homogeneous liquid-drop model. In this case an apparently reasonable way to amalgamate the shell and liquid-drop energies was proposed, and the remarkable result obtained through the use of this method reveals that nuclei in the region of thorium through curium possess two energetically stable states with two different nuclear shapes. This theoretical result furnished a most natural explanation for the new form of fission, first discovered in americium-242.

      This interpretation of a new nuclear structure is of great importance, but it has significance far beyond itself because the theoretical method and other novel approaches to calculation of nuclear stability have been used to predict an island of stability beyond the point at which the peninsula in the Figure disappears into the sea of instability.

Extension of the periodic table

Transactinoid elements and their predicted properties

 The postulated nuclear island of stability is important to chemistry. The periodic (periodic law) table of the elements classifies a wealth of physical and chemical properties, and study of the chemical properties of the heavy elements would show how far the classification scheme of the table could be extended on the basis of the nuclear island of stability. Such study would shed new light on the underlying properties of electrons orbiting the nucleus because it is these properties that produce the periodic system. The positions of heavy elements in the periodic table ultimately would be determined by the characteristic energies of the electrons of their atoms, especially the valence electrons (valence electron). Complex calculations have predicted meaningful distribution of electrons in orbitals for a number of heavy elements. Results for elements 104–121 are given in the Table—>, the configurations (electronic configuration) being those that the atoms have when they are at their lowest energy level, called the ground state.

      It must be stated that these calculations are oversimplified; the actual electronic configurations are determined by complicated relativistic effects, and hence the consequent predicted chemical properties will need eventually to be modified based on additional chemical experiments on the transactinoid elements. However, the simplified predictions are accurate to a good first approximation.

The first transactinoid elements

 The first two transactinoid elements, rutherfordium (Rf) and dubnium (Db), with atomic numbers 104 (rutherfordium) and 105 (dubnium), respectively, have isotopes with half-lives sufficiently long (0.5–1.0 minute) to allow determination of chemical properties by application of specifically devised “fast chemistry” techniques. The results of these studies are consistent with the electronic structures listed in the Table—> and their position in the periodic table shown in the Figure, with some deviations that reflect the influence of relativistic effects. Seaborgium (Sg), with atomic number 106, also has isotopes that allow determination of its chemical properties. Chemical studies on still-heavier elements probably await the discovery of longer-lived isotopes (if such exist and can be synthesized). Some predictions for heavier elements—including some that will never be synthesized—follow.

Elements 113 and 114

  Some predicted properties of elements 113 and 114The calculations of electronic structure permit predictions of detailed physical and chemical properties of some superheavy elements. If, for example, the structure of the periodic system (Figure) remains predictable to higher atomic numbers, then element 113 will be in the same group of elements as boron, aluminum, gallium, indium, and thallium; and element 114 will be in the group with carbon, silicon, germanium, tin, and lead. Computer calculations of the character and energy levels of possible valence electrons in the atoms of these two superheavy elements have substantiated their placement in the expected positions. Extrapolations of properties from elements with lower numbers to elements 113 and 114 can then be made within the usual limitations of the periodic table. The attached Table (Some predicted properties of elements 113 and 114) gives the results of such extrapolations. Although, in many cases, theoretical calculations are combined with extrapolation, the fundamental method involved is to plot the value of a given property of each member of the group against the appropriate row of the periodic table. The property is then extrapolated to the seventh row, the row containing elements 113 and 114. The method is illustrated in the Figure—> for estimating the melting point of element 113.

      The bonding property (chemical bonding) of an element can be expressed by the energy required to shift a bonding, or valence, electron. This energy can be expressed in various ways, one of which is a relative value called the oxidation potential. The relative stabilities of possible oxidation states (or oxidation numbers (oxidation number)) of an element represent what is probably that element's most important chemical property. The oxidation number of the atom of an element indicates the number of its orbiting electrons available for chemical bonds or actually involved in bonds with other atoms, as in a molecule or in a crystal. When an atom is capable of several kinds of bonding arrangements, using a different number of electrons for each kind, the number of arrangements equals the number of possible oxidation states. The prediction of stable oxidation states can be illustrated with element 114, which occurs in group 14 of the periodic table. The outstanding periodic characteristic of the group 14 elements is their tendency to go from a +4, or tetrapositive, oxidation state to a +2, or dipositive, state as the atomic number increases. Thus, carbon and silicon are very stable in the tetrapositive state, germanium shows a weak dipositive state and a strong tetrapositive state, tin shows about equal stability in the tetrapositive and dipositive states, while lead is dominated by the dipositive state and shows only weak tetrapositive properties. Extrapolation in the periodic table to the seventh row, then, results in a predicted most-stable dipositive oxidation state for element 114. This result is supported by valence bond theory and by extrapolations of thermodynamic data.

Other heavy elements

      Less-detailed predictions have been made for other heavy elements. Element 117, for example, is expected to be a member of the halogen series, which is the group composed of fluorine, chlorine, bromine, iodine, and astatine. Solid element 117 should be metallic in appearance, as is astatine, but it is expected that, instead of the −1 oxidation state characteristic of the natural halogens, it will show +3, +5, and +7 oxidation states. It should also form stable interhalogen compounds with fluorine, chlorine, and bromine.

      Computer calculations suggest that element 118 should have the closed-shell electronic configuration of the noble gas elements helium, neon, argon, krypton, xenon, and radon. The element should be the most electropositive of the noble gases, and, therefore, the existence of a (partially ionized) difluoride of the element 118 is predicted. A tetrafluoride and an oxide of the type formed by xenon (XeO₄) are also expected.

      Although detailed predictions of the chemistry of element 119 have not as yet been completed, it is expected to be a typical alkali metal with a +1 oxidation state. The energetic properties of its valence electron, the 8s electron, suggest that its first ionization potential will be higher than the oxidation potential predicted by simple extrapolation, so that the element may be more like potassium than cesium in its chemistry. This higher energy will cause the metallic and ionic radii to be smaller than simple extrapolation would indicate.

      Element 120 is expected to be a typical alkaline-earth element. As with element 119, the ionization energies should be higher than the normal family trend would indicate and should make the metallic and ionic radii smaller. These changes should make the chemistry of element 120 similar to calcium and strontium. Element 121 should be similar in its chemical properties to lanthanum and actinium, but detailed properties have not been predicted.

Superactinoid series

      It is probable, in a formal sense at least, that element 122 will begin another series of elements in which each successive electron is added to a deep inner orbital, in a manner similar (see Figure) to that found in the lanthanoid and actinoid series. Such a series, which would be listed in a row below the actinoid series in the periodic table, should consist of 32 elements, ending in the neighbourhood of element 153 and resulting primarily from the filling of the 5g and 6f inner electron shells.

      Not every element of this new series would correspond to an actinoid (or lanthanoid) element on a one-to-one basis, and prediction of the chemistry of the members of the series is a complex problem. The difficulty arises partly because of uncertainty of the exact point at which the energetically similar 5g and 6f orbitals begin to fill and partly because calculations indicate that the 8p and 7d orbitals may be very close in energy to the 5g and 6f orbitals. These orbitals may all be filled, then, in a commingling fashion, resulting in a series of elements that show multiple, barely distinguishable oxidation states. The electronic basis for the periodicity shown in the Figure would then no longer be present.

      As shown, element 153 will be the last member of the superactinoid series, at least in a formal sense. The prediction of properties on the basis of an orderly extrapolation appears to be of doubtful validity, however, in this heavy-element region of the periodic table. In still higher-numbered elements, the closely spaced energy levels are expected to make multiple oxidation states the rule. The placement of the elements in the heaviest portion of the periodic table as shown in the Figure is, therefore, probably also of only formal significance.

End of the periodic table

      At some point the stability of the orbital electrons in the ordinary sense must be destroyed as more protons are added to the nucleus. There is, therefore, a critical atomic number, or range of atomic numbers, which represents the end of the periodic table. This end, it should be noted, is separate, at least philosophically, from the question of stability of the nucleus itself; i.e., nuclear stability is not the same as stability of the electron shells. The maximum atomic number, according to current theories, lies somewhere between 170 and 210. However, in a practical sense, the end of the periodic table will come much earlier than this because of nuclear instability (perhaps at or before Z = 120).

Characterization and identification

      Two important factors provided the key to the discovery and identification of many of the earliest-known transuranium elements. One was the actinoid concept, which stated that the transuranium elements were part of a series of elements that paralleled the earlier lanthanoid series. It was postulated (and subsequently demonstrated) that this actinoid series started at thorium and that its chemistry would be similar to that of the lanthanoids.

      The second factor was the technique of separating (separation and purification) elements with similar properties from a mixture by using the principle of ion exchange (see ion-exchange reaction). Ion-exchange reactions depend on the fact that some complex molecules have a charge that will attract ions of the opposite charge, hold them, and then exchange them for other ions of the same charge when brought in contact with them. Although other separation methods are possible, many of the transuranium elements have been separated and identified and their chemistries studied by the use of ion-exchange reactions that are highly specific. For example, the tripositive ions of the lanthanoids and the actinoids have been separated using a cation- (positive-ion-) exchange process. The striking similarity between the patterns of behaviour exhibited by the two groups in this process constitutes strong support for the actinoid concept. nobelium, for example, exists in aqueous solution in the dipositive oxidation state, which might be expected for the next-to-last member of the actinoid series because of the stability of the filled 5f electron shell (5f¹⁴). The tripositive state of lawrencium has also been confirmed by a very rapid solvent-exchange experiment in which the lawrencium displayed the behaviour of the tripositive actinoids and not that of the dipositive nobelium or radium, again in accord with the predictions of the actinoid concept.

      When the yields of a new element are small and its half-life is short, chemical identification and characterization are frequently not possible. In such cases the atomic number is deduced from the method of production (nuclear reaction), from the parent-daughter relationship of the new element to known elements of lower atomic number resulting from its nuclear decay, and from its nuclear-decay systematics that cannot be attributed to any known nuclides. Additionally, the variation in the yield of the new element is noted when the bombarding energy is changed or when the target or projectile or both are changed.

      Separation of the product nuclide from the target has been accomplished in the discoveries of elements 101 and heavier by a recoil collection method. When the target nucleus is struck by a heavy-ion projectile, the product nucleus recoils out of the very thin target and is either attracted to a substrate by an electrostatic potential or is swept onto a substrate by a jet of helium gas. The new element is then in a position to be observed and characterized by suitable detection techniques, essentially free of the parent isotope.

      It is desirable, though not essential, that the mass number of the new element be established by evidence related to its mode of production or to its parent-daughter relationship through radioactive decay to a radioactive isotope of known mass number. When weighable quantities of an element are available, more extensive characterization experiments can be performed. The most important of these is the preparation of the metal, frequently done by high-temperature reduction of the fluoride of the transuranium element with an alkali or alkaline-earth metal. Another method used for preparation of larger (gram) quantities of high purity is electrolytic reduction of the chloride of the transuranium element. Physical characterization of these metal samples includes determination of the density, melting point, vapour pressure, boiling point, hardness, and other properties. X-ray diffraction measurements permit the determination of the crystal structure and calculation of the metallic radius and metallic valence. Chemical characterization includes a determination of the reactivity of the metal with other substances and the chemical stability of the compounds formed. Also of importance are the oxidation states and chemical bonding properties of the element in its compounds.

Practical applications of transuranium isotopes

      More plutonium-239 has been produced than any other transuranium isotope. Like uranium-235, it is primarily used as a fuel to generate nuclear power and in nuclear weapons.

      Three other transuranium isotopes—plutonium-238, americium-241, and californium-252—have demonstrated substantial practical applications. One gram of plutonium-238 produces approximately 0.57 watt of thermal (thermal energy) power, primarily from alpha-particle decay, and this property has been used in space exploration to provide energy for small thermoelectric-power units.

      Americium-241 has a predominant gamma-ray energy (60 keV) and a long half-life (432 years) for decay by the emission of alpha particles. It is particularly useful for measuring and controlling the thickness of a wide range of industrial materials, for the diagnosis of thyroid disorders, and for smoke detectors. When mixed with beryllium, it generates neutrons at the rate of 1.0 × 10⁷ neutrons per second per gram of americium-241. The mixture is designated ²⁴¹Am-Be, and many such sources are used worldwide in oil-well operations to monitor how much oil a well produces in a given time span, such as a day.

      Californium-252 is an intense neutron source: one gram emits 2.3 × 10¹² neutrons per second. It has been used to provide neutrons for numerous applications of neutron-activation analysis, including mineral prospecting and the monitoring of oil wells. It is also used in neutron radiography, in airport neutron-activation detectors for nitrogenous materials (i.e., explosives), and for the irradiation of tumours for which gamma-ray treatment is relatively ineffective. Its most important industrial application, however, is as a start-up source (used to calibrate instrumentation) for nuclear reactors.

Glenn T. Seaborg

Additional Reading

Two articles for the lay reader are Glenn T. Seaborg, “The New Elements,” American Scientist, pp. 279–289 (May–June 1980); and Glenn T. Seaborg and Justin L. Bloom, “The Synthetic Elements: IV,” Scientific American, 220:57–67 (1969), fourth in a series that covers the field from its inception. Methods for preparing and characterizing short-lived heavy elements can be found in Peter Armbruster and Gottfried Münzenberg, “Creating Superheavy Elements,” Scientific American, 260(5):66–72 (May 1989); and Darleane C. Hoffman, “The Heaviest Elements,” Chemical & Engineering News, 72(18):24–34 (May 2, 1994). More technical works include Glenn T. Seaborg, Man-Made Transuranium Elements (1963), suitable for scientifically inclined high school students and for college students; Glenn T. Seaborg (ed.), Transuranium Elements: Products of Modern Alchemy (1978), a collection of benchmark research papers; and Glenn T. Seaborg and Walter D. Loveland, The Elements Beyond Uranium (1990). Comprehensive works on the field include Earl K. Hyde, Isadore Perlman, and Glenn T. Seaborg, The Nuclear Properties of the Heavy Elements, 3 vol. (1964, reissued 1971); and Cornelius Keller, The Chemistry of the Transuranium Elements (1971).Glenn T. Seaborg

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