P1: FPP 2nd Revised Pages Qu: 00, 00, 00, 00 Encyclopedia of Physical Science and Technology EN002C-64 May 19, 2001 20:39 Table of Contents (Subject Area: Inorganic Chemistry) Pages in the Article Authors Encyclopedia Actinide Elements Siegfried Hübener Pages 211-236 Bioinorganic Brian T. Farrer and Vincent L. Pages 117-139 Chemistry Pecoraro Herbert Beall and Donald F. Boron Hydrides Pages 301-316 Gaines Coordination R. D. Gillard Pages 739-760 Compounds L. G. Christophorou and S. J. Dielectric Gases Pages 357-371 Dale Electron Transfer Gilbert P. Haight, Jr. Pages 347-361 Reactions Halogen Chemistry Marianna Anderson Busch Pages 197-222 Inclusion (Clathrate) Jerry L. Atwood Pages 717-729 Compounds Inorganic Exotic Joel F. Liebman, Kay Severin and Pages 817-838 Molecules Thomas M. Klapötke Liquid Alkali Metals C. C. Addison Pages 661-671 Main Group Russell L. Rasmussen, Joseph G. Pages 1-30 Elements Morse and Karen W. Morse Mesoporous Robert Mokaya Pages 369-381 Materials, Synthesis Metal Cluster D. F. Shriver Pages 407-409 Chemistry Metal Hydrides Holger Kohlmann Pages 441-458 Metal Particles and Allan W. Olsen and Kenneth J. Pages 513-550 Cluster Compounds Klabunde Nano sized Inorganic Leroy Cronin, Achim Müller and Pages 303-317 Clusters Dieter Fenske Noble Metals Hubert Schmidbaur and John L. Pages 463-492 (Chemistry) Cihonski Noble-Gas Chemistry Gary J. Schrobilgen Pages 449-461 Periodic Table N. D. Epiotis and D. K. Henze Pages 671-695 (Chemistry) Rare Earth Elements Zhiping Zheng and John E. Pages 1-22 and Materials Greedan P1: FYK Revised Pages Qu: 00, 00, 00, 00 Encyclopedia of Physical Science and Technology EN001F-11 May 25, 2001 21:2 Actinide Elements Siegfried Hu¨bener Forschungszentrum Rossendorf I. Discovery, Occurrence, and Synthesis of the Actinides II. Radioactivity and Nuclear Reactions of Actinides III. Applications of Actinides IV. Actinide Metals V. Actinide Ions VI. Actinide Compounds and Complexes GLOSSARY Speciation Characterization of physical and chemical states of (actinide) species in a given (chemical) Actinyl ion Dioxo actinide cations MO2+ and MO2+. environment. Decay chain A series of nuclides in which each member Transactinide elements Artificial elements beyond the transforms into the next through nuclear decay until a actinide elements, beginning with rutherfordium (Rf), stable nuclide has been formed. element 104. The heaviest elements, synthesized until Lanthanides Fourteen elements with atomic numbers 58 now, are the elements 114, 116, and 118. At present, (cerium) to 71 (lutetium) that are a result of filling the bohrium (Bh), element 107, is the heaviest element 4 f orbitals with electrons. which has been characterized chemically; chemical Nuclear fission The division of a nucleus into two or studies of element 108, hassium (Hs), and element 112 more parts, usually accompanied by the emission of are in preparation. neutrons and γ radiation. Nuclide A species of atom characterized by its mass num- ber, atomic number, and nuclear energy state. A ra- THE ACTINIDE ELEMENTS (actinoids) comprise the dionuclide is a radioactive nuclide. 14 elements with atomic numbers 90–103, which fol- Primordial radionuclides Nuclides which were pro- low actinium in the periodic table: thorium (Th), pro- duced during element evolution and which have tactinium (Pa), uranium (U), neptunium (Np), plutonium partly survived since then due to their long half- (Pu), americium (Am), curium (Cm), berkelium (Bk), cal- lives. ifornium (Cf), einsteinium (Es), fermium (Fm), mendele- Radioactivity The property of certain nuclides of show- vium (Md), nobelium (No), and lawrencium (Lr). The ac- ing radioactive decay in which particles or γ radia- tinides constitute a unique series of elements which are tion are emitted or the nucleus undergoes spontaneous formed by the progressive filling of the 5 f electron shell. fission. Although not formally an actinide element, actinium (Ac; 211 P1: FYK Revised Pages Encyclopedia of Physical Science and Technology EN001F-11 May 7, 2001 12:19 212 Actinide Elements atomic number 89) is usually included in discussions about tained by reduction of its tetrachloride with potassium he the actinides. named thorium. (Later, in 1841, B. Peligot used the same According to the International Union of Pure and Ap- method to prepare uranium metal for the first time.) Tho- plied Chemistry (IUPAC), the name actinoid is prefer- rium constitutes 8.1 ppm of the Earth’s crust and is thus able to actinide because the ending “-ide” normally indi- as abundant as boron. Converted by neutron irradiation 233 cates a negative ion. However, owing to wide current use, to U, it could yield an amount of neutron-fissile ma- “actinide” is still allowed. terial several hundred times the amount of the naturally 235 occurring fissile uranium isotope U. The principal tho- rium ore is monazite, a mixture of rare-earth and thorium I. DISCOVERY, OCCURRENCE, AND phosphates containing up to 30% ThO2. Monazite sands SYNTHESIS OF THE ACTINIDES are widely distributed throughout the world. In Canada thorium is recovered from uranothorite (a mixed thorium- A. Naturally Occurring Actinides uranium silicate accompanied by pitchblende) as a co- All of the isotopes of the actinide elements are radioac- product of uranium. Rarer minerals thorianite (90% ThO2) tive, and only four of the primordial isotopes, 232Th, 235U, and thorite (ThSiO4; 62% thorium) have been found in the 238 244 western United States and New zealand. Natural thorium U, and Pu, have a sufficient long half-life for there to 232 is 100% Th. be any of these isotopes left in nature. Only three actinide In 1913 protactinium was discovered by K. Fajans and elements and actinium were known as late as 1940. In ad- 234m O. Go¨hring, who identified Pa as an unstable member dition to thorium and uranium, protactinium and actinium 238 of the U decay series. They named the new element bre- have been found to exist in uranium and thorium ores due 238 235 vium because of its short half-life of 1.15 min. In 1918 the to the U [Eq. (1)] and U [Eq. (2)] decay series: 231 longer-lived isotope Pa, with a half-life of 32,800 years, − − 238 U −−→α 234Th −→β 234Pa −→β 234U, (1) was identified independently by two groups, O. Hahn and 92 90 91 92 L. Meitner, and F. Soddy and J. A. Cranston, as a prod- − 235 −α 231 −β 231 −α 227 235 U −→ Th −→ Pa −→ Ac. (2) uct of U decay. Since the name brevium was obviously 92 90 91 89 inappropriate for such a long-lived radioelement, it was 244 It was not until 1971 that the existence of primordial Pu changed to protactinium, thus naming element 91 as the in nature in trace amounts was shown by D. C. Hoffman parent of actinium. Protactinium is one of the rarest of and co-workers. the naturally occurring elements. Although not worth ex- Uranium was the first actinide element to be discov- tracting from uranium ores, protactinium becomes con- ered. M. H. Klaproth showed in 1789 that pitchblende con- centrated in residues from uranium processing plants. tained a new element and named it uranium after the then Actinium was discovered by A. Debierne in 1899. Its newly discovered planet Uranus. Uranium is now known name is derived from the Greek word for beam or ray, to comprise 2.1 ppm of the Earth’s crust, which makes referring to its radioactivity. The natural occurrence of it about as abundant as arsenic or europium. It is widely 227 the longest lived actinium isotope Ac, with a half-life distributed, with the principal sources being in Australia, of 21.77 years, is entirely dependent on that of its pri- Canada, South Africa, and the United States. The two 235 227 mordial ancestor, U. The natural abundance of Ac most important oxide minerals of uranium are uraninite −10 is estimated to be 5.7 · 10 ppm. The most concentrated (U3O8; 50–90% uranium), a variety of which is called actinium sample ever prepared from a natural raw material pitchblende, and carnotite (K2(UO2)(VO4)2 · 3H2O; 54% 227 consisted of about 7 µg of Ac in less than 0.1 mg of uranium). A very common uranium mineral is autu- La2O3. nite (Ca(UO2)2(PO4)2 · nH2O, n = 8–12). Natural ura- 238 nium consists of 99.3% U and 0.72% of the fissionable 235 233 isotope U. A third important isotope, U, does not B. Synthetic Actinides occur in nature but can be produced by thermal-neutron Stimulated by the discovery of the neutron in 1932 by 232 irradiation of Th [Eq. (3)]: J. Chadwick and the first synthesis of artificial radioactive − − 232 1 233 −β 233 −β 233 nuclei using α particle-induced nuclear reactions in 1934 Th + n → Th −→ Pa −→ U. (3) 90 0 90 91 92 by F. Joliot and I. Curie, many attempts were made to This process converts thorium to fissionable fuel in a produce transuranium elements by neutron irradiation of breeder reactor. uranium. In 1934, E. Fermi and later O. Hahn, L. Meitner, Thorium was discovered by J. J. Berzelius in 1828 when and F. Strassmann reported that they had created transura- he isolated a new oxide from a Norwegian ore then known nium elements. But in 1938, O. Hahn and F. Strassmann as thorite. He named the oxide thoria, and the metal he ob- showed that the radioactive species produced by neutron P1: FYK Revised Pages Encyclopedia of Physical Science and Technology EN001F-11 May 7, 2001 12:19 Actinide Elements 213 irradiation of uranium were in fact fission fragments re- cium and curium failed, believing that they would have sulting from the nuclear fission of uranium! Thus, the early chemical properties similar to uranium, neptunium, and search for transuranium elements led to one of the greatest plutonium. Once it was recognized that these elements, discoveries of the 20th century. according to G. T. Seaborg’s actinide concept, might have The first transuranium element, neptunium, was discov- properties similar to europium and gadolinium, the use of ered in 1940 by E. M. McMillan and P. H. Abelson. They proper chemical procedures led to success. By analogy to were able to chemically separate and identify element 93 europium (named after Europe) and gadolinium (named formed in the following reaction sequences [Eq. (4)]: after Johan Gadolin, a Finnish rare-earth chemist), for el- − − ements 95 and 96 the names americium after the continent −β −β 238 1 239 239 239 U + n → U −−→ Np −−−→ Pu. (4) of America and curium to honor Pierre and Marie Curie 92 0 92 93 94 23 min 2.3 days were proposed. The elements with the atomic numbers They showed that element 93 has chemical properties sim- 97 and 98 at first could not be produced by irradiation − ilar to those of uranium and not those of an eka-rhenium as with neutrons, because β decaying isotopes of curium 241 suggested on the basis of the periodic table of that time. To were not known. By 1949 sufficient amounts of Am 242 distinguish it from uranium, element 93 was reduced by and Cm had been accumulated to make it possible to SO2 and precipitated as a fluoride. This new element was produce elements 97 and 98 in helium-ion bombardments. named neptunium after Neptune, the planet discovered af- The α particle-emitting species produced in the bombard- 237 ter Uranus. In 1952, trace amounts of Np were found ments could be identified as isotopes of elements 97 and in uranium of natural origin, formed by neutron capture 98, which were named berkelium and californium after in uranium. the city and state of discovery. It was obvious to the discoverers of neptunium that Elements 99 and 100, named einsteinium and fermium 239 Np should β decay to the isotope of element 94 with to honor Albert Einstein and Enrico Fermi, were unex- mass number 239, but they were unable to identify it. pectedly synthesized in the first U. S. thermonuclear ex- However, up to the end of 1940, G. T. Seaborg, E. M. plosion in 1952. The successive capture of numerous neu- 238 − McMillan, J. W. Kennedy, and A. C. Wahl succeeded in trons by U and subsequent β decay chains ended in 238 253 255 identifying Pu in uranium, which was bombarded with the β stable nuclides Es and Fm. From tons of coral deuterons produced in the 60-in. cyclotron at the Univer- collected at the explosion area, hundreds of atoms of the sity of California in Berkeley [Eq. (5)]: new elements could be separated and positively identi- fied. Further attempts to produce still heavier elements −β 238 2 1 238 238 U + H → 2 n + Np −−−→ Pu. (5) in underground nuclear tests or in high-flux nuclear re- 92 1 0 93 94 2.1 days 257 actors failed. Fm is the heaviest nuclide which can be Element 94 was named plutonium after the planet discov- produced using neutron-capture reactions, owing to the ered last, Pluto. In 1941, the first 0.5 µg of the fissionable very short half-lives of the heavier fermium isotopes and 239 − isotope Pu were produced by irradiating 1.2 kg of uranyl their spontaneous fission instead of β decay. To pro- 9 nitrate with cyclotron-generated neutrons. In 1948, trace duce element 101, mendelevium, only about 10 atoms of 239 253 amounts of Pu were found in nature, formed by neutron Es were made available for a bombardment with he- capture in uranium. In chemical studies, plutonium was lium ions in the Berkeley 60-in. cyclotron. For the first shown to have properties similar to uranium and not to os- time an element was discovered in “one-atom-at-a-time” mium as suggested earlier. The actinide concept advanced experiments on the basis of only 17 produced atoms re- by G. T. Seaborg, to consider the actinide elements as a coiling from the einsteinium target. The discoverers of second f transition series analogous to the lanthanides, element 101, A. Ghiorso, B. G. Harvey, G. R. Choppin, systematized the chemistry of the transuranium elements S. G. Thompson, and G. T. Seaborg, suggested the name and facilitated the search for heavier actinide elements. mendelevium in honor of the Russian chemist Dmitri I. The actinide elements americium (95) through fermium Mendeleev, who was the first to use a periodic system of (100) were produced first either via neutron or helium-ion the elements to predict the chemical properties of undis- bombardments of actinide targets in the years between covered elements. 1944 and 1955. The synthesis of element 102 was even more compli- Element 96, curium, was produced in 1944 by the bom- cated, because a fermium target to apply the bombardment 239 bardment of Pu with helium ions in the Berkeley 60-in. with helium ions was not available. In order to make use of 241 cyclotron, and soon after it was found that Pu, formed lighter target elements, heavier ions had to be accelerated. 239 from Pu by two successive neutron captures in a nuclear The discovery of element 102 was first reported in 1957 − 241 reactor, decays under β particle emission to give Am. by an international group working at the Nobel Institute Earlier attempts to produce and chemically separate ameri- of Physics in Stockholm. The name nobelium in honor of P1: FYK Revised Pages Encyclopedia of Physical Science and Technology EN001F-11 May 7, 2001 12:19 214 Actinide Elements Alfred Nobel was immediately accepted by the IUPAC. is reduced by 2, the mass number by 4. With emission − However, experiments at Berkeley and the Kurchatov of a β particle, the mass number remains unchanged, Institute in Moscow showed that the original Swedish whereas the atomic number increases by 1. As a result, claim to have prepared element 102 was in error. Attempts in these decay series the mass number can differ only by to synthesize and identify isotopes of element 102 in multiples of 4 and there are four such families, desig- heavy ion bombardments of actinide targets dragged on nated 4n + 0 (thorium series), 4n + 1 (neptunium series), for many years at the laboratories in Berkeley and Dubna, 4n + 2 (uranium or uranium-radium series), and 4n + 3 Russia. Thus, scientists from Berkeley suggested that the (actinium series). The neptunium series is missing in na- credit for the discovery should be shared. But, in 1993 the ture. It was probably present in nature for some million IUPAC-IUPAP Transfermium Working Group concluded years after the genesis of the elements, but decayed due to 237 that the Dubna laboratory finally achieved an undisputed the relatively short half-life of Np, compared with the 9 synthesis. age of the Earth (about 5 · 10 years). Each series contains a number of short-lived nuclides, and the final members Also, the discovery of element 103, the last actinide el- of each series are stable nuclides. α Decay is the domi- ement, was contested by Berkeley and Dubna for a long nant decay mode of long-lived heavy nuclei with atomic time. At Berkeley mixtures of californium isotopes were numbers Z > 83. With increasing atomic numbers spon- bombarded with boron ions, whereas at Dubna the bom- 238 taneous fission begins to compete with α decay. For U bardment of americium targets with oxygen ions was ap- −4 the probability of spontaneous fission is about 10 % of plied. Finally, both groups accepted the conclusion of the 256 that of α decay and is already about 90% for Fm. Transfermium Working Group, that full confidence was The radioactive decay is the simplest form of a nuclear built up over a decade with credit for discovery of ele- reaction according to equation [Eq. (6)]: ment 103 attaching to work in both Berkeley and Dubna. The name lawrencium after E. O. Lawrence, the inventor A → B + x + E. (6) of the cyclotron, suggested by A. Ghiorso and co-workers This is a mononuclear reaction. In nuclear science, how- from Berkeley and accepted by IUPAC, was finally rec- ever, binuclear reactions are generally understood by the ommended by IUPAC in 1997 together with the names for term “nuclear reaction.” They are described by the general the transactinide elements up to element 109. equation [Eq. (7)]: Table I summarizes the discovery or synthesis of all of the actinide elements. A + x → B + y + E, (7) where A is the target nuclide, x is the projectile, B is the product nuclide, and y is the particle or photon emitted. II. RADIOACTIVITY AND NUCLEAR Equations (3)–(5) are examples for neutron- and deuteron- REACTIONS OF ACTINIDES induced nuclear reactions. With heavy ions (heavier than α particles) as projectiles, the heaviest actinides have been All isotopes of the actinides and actinium are radioac- synthesized. Targets made from heavy actinide nuclides 248 249 tive. Table II presents data on several of the most avail- such as Cm and Bk have been used to synthesize able and important of these. The unstable, radioactive ac- several transactinide elements in heavy-ion reactions. tinide nuclei decay by emission of α particles, electrons, Nuclear fission of actinides is, without doubt, the most − + or positrons (β or β decay, respectively). Alternatively important nuclear reaction. Nuclear fission by thermal to the emission of a positron, the unstable nucleus may neutrons may be described by the general equation capture an electron of the electron shell of the atom (sym- [Eq. (8)]: bol ε). In most cases the radioactive decay leads to an A + n → B + D + νn + E. (8) excited state of the new nucleus, which gives off its excita- tion energy in the form of one or several photons (γ rays). The fission products B and D have mass numbers in the In some cases a metastable state results that decays in- range between about 70 and 160, the number of neutrons dependently of the way it was formed. Spontaneous fis- emitted is ν ≈ 2–3, and the energy set free by fission is sion (symbol sf) is another mode of radioactive decay, E ≈ 200 MeV. This energy is relatively high, because which was discovered in 1940 by G. N. Flerov and K. A. the binding energy per nucleon is higher for the fission Petrzhak. products than for the actinide nuclei. In the case of nu- The numerous radionuclides present in thorium and ura- clei with even proton and odd neutron numbers, such as 233 235 239 nium ores are members of genetic correlated radioactive U, U, and Pu, the binding energy of an additional decay series, which are represented in Fig. 1. In all of neutron is particularly high, and the barrier against fission − these decay series, only α and β decay are observed. is easily surmounted. Therefore, these nuclides have high 4 With emission of an α particle ( He), the atomic number fission yields for fission by thermal neutrons. 2 P1: FYK Revised Pages Encyclopedia of Physical Science and Technology EN001F-11 May 7, 2001 12:19 Actinide Elements 215 TABLE I Discovery or Synthesis of Actinide Elements Most Atomic Source or Isotope first stable number Element Symbol Investigators synthesis discovered isotope Source of name 227 227 89 Actinium Ac A. Debierne (1899) Uranium ore Ac Ac Greek word for ray 232 232 90 Thorium Th J. J. Berzelius (1828) Thorium ore Th Th Scandinavian god of war, Thor 234 234 91 Protactinium Pa K. Fajans, O. Go¨hring Uranium ore Pa Pa Parent of actinium (1913) concentrates 238 238 92 Uranium U M. H. Klaproth (1789) Pitchblende U U Planet Uranus 239 237 93 Neptunium Np E. M. McMillan, Bombardment of Np Np Planet Neptune P. Abelson (1940) uranium with neutrons: 238 1 U + n → 92 0 − −β 239 239 U −−→ Np 92 93 23 min 238 244 94 Plutonium Pu G. T. Seaborg, Bombardment of Pu Pu Planet Pluto E. M. McMillan, uranium with J. W. Kennedy, deuterons: 238 2 A. Wahl (1940) U + H → 92 1 1 238 2 n + Np 0 93 − −β 238 −−−→ Pu 94 2.1 days 241 243 95 Americium Am G. T. Seaborg, Bombardment of Am Am America R. A. James, plutonium with L. O. Morgan, neutrons: 239 1 A. Ghiorso (1944) Pu + 2 n → 94 0 − −β 241 241 Pu −→ Am 94 95 242 247 96 Curium Cm G. T. Seaborg, Bombardment of Cm Cm Pierre and Marie Curie R. A. James, plutonium with A. Ghiorso (1944) helium ions: 239 4 Pu + He → 94 2 242 1 Cm + n 96 0 243 247 97 Berkelium Bk S. G. Thompson, Bombardment of Bk Bk Berkeley, CA A. Ghiorso, americium with G. T. Seaborg (1949) helium ions: 241 4 Am + He 95 2 243 1 → Bk + 2 n 97 0 245 251 98 Californium Cf S. G. Thompson, Bombardment of Cf Cf California K. Street, A. Ghiorso, curium with G. T. Seaborg (1950) helium ions: 242 4 Cm + He 96 2 245 1 → Cf + n 98 0 253 252 99 Einsteinium Es Workers at Berkeley, Discovered in the Es Es Albert Einstein Argonne, and fallout of the first Los Alamos (1952) thermonuclear explosion as a result of uranium bombardment with fast neutrons: 238 1 U + 15 n → 92 0 − −7β 253 253 U −→ Es 92 99 Continues P1: FYK Revised Pages Encyclopedia of Physical Science and Technology EN001F-11 May 7, 2001 12:19 216 Actinide Elements TABLE I (continued ) Most Atomic Source or Isotope first stable number Element Symbol Investigators synthesis discovered isotope Source of name 255 257 100 Fermium Fm Workers at Berkeley, Discovered in the Fm Fm Enrico Fermi Argonne, and fallout of the first Los Alamos (1952) thermonuclear explosion as a result of uranium bombardment with fast neutrons: 238 1 U + 17 n → 92 0 −8β− 255 255 U −→ Fm 92 100 256 258 101 Mendelevium Md A. Ghiorso, Bombardment of Md Md Dimitri Mendeleev B. H. Harvey, einsteinium with G. R. Choppin, helium ions: 253 4 S. G. Thompson, Es + He 99 2 256 1 G. T. Seaborg (1955) → Md + n 101 0 254 259 102 Nobelium No E. D. Donets, Bombardment of No No Alfred Nobel V. A. Shegolev, americium with V. A. Ermakov (1966) nitrogen ions: 243 15 Am + N 95 7 254 1 → No + 4 n 102 0 258 262 103 Lawrencium Lr Workers at both Bombardments of ( Lr) Lr Ernest Lawrence Berkeley and actinide targets Dubna (1961–1971) with heavy ions III. APPLICATIONS OF ACTINIDES cial, and international problems. Technical problems are related to the safe operation of nuclear reactors, reprocess- The practical importance of the actinide elements derives ing, and waste disposal, to the prevention of environmen- mainly from their nuclear properties. The principal appli- tal contamination with radioactive and toxic substances, cation is in the production of nuclear energy. Controlled and to the prevention of the diversion of plutonium for an fission of fissile nuclides in nuclear reactors is used to uncontrolled manufacture of nuclear weapons. All these provide heat to generate electricity. The fissile nuclides technical and technological problems are soluble, but the 233 235 239 U, U, and Pu constitute an enormous, practically future of nuclear energy depends also on the solution of inexhaustible, energy source. other problems of acute global concern. Several actinide nuclides have found other applications. 238 Heat sources made from kilogram amounts of Pu have IV. ACTINIDE METALS been used to drive thermoelectric power units in space 238 vehicles. In medicine, Pu was applied as a long-lived A. Preparation of Actinide Metals compact power unit to provide energy for cardiac pace- 241 makers and artificial organs. Am has been used in neu- All of the actinide elements are metals with physical tron sources of various sizes on the basis of the (α,n) reac- and chemical properties changing along the series from tion on beryllium. The monoenergetic 59-keV γ radiation those typical of transition elements to those of the lan- 241 of Am is used in a multitude of density and thickness thanides. Several separation, purification, and preparation 252 determinations and in ionization smoke detectors. Cf techniques have been developed considering the differ- decays by both α emission and spontaneous fission. One ent properties of the actinide elements, their availability, 252 12 252 gram of Cf emits 2.4 · 10 neutrons per second. Cf and application. Powerful reducing agents are necessary thus provides an intense and compact neutron source. Neu- to produce the metals from the actinide compounds. Ac- 252 tron sources based on Cf are applied in nuclear reactor tinide metals are produced by metallothermic reduction of start-up operations and in neutron activation analysis. halides, oxides, or carbides, followed by the evaporation Nuclear energy and the application of actinide elements in vacuum or the thermal dissociation of iodides to refine in other fields may promise mankind a prosperous future; the metals. however, whether the promise becomes a reality depends The metallothermic reduction of halides was the first on the solution of numerous technological, economic, so- method to be successfully applied. Actinium metal can P1: FYK Revised Pages Encyclopedia of Physical Science and Technology EN001F-11 May 7, 2001 12:19 Actinide Elements 217 TABLE II Important Isotopes of the Actinide Elements Atomic number Element Isotope Half-life Mode of decay 227 − 89 Actinium Ac 21.7 years β (0.986), α(0.014), γ 228 − Ac 6.15 h β 232 10 90 Thorium Th 1.405 · 10 years α, 231 91 Protactinium Pa 32760 years α, γ 234 − Pa 6.70 h β 235 8 92 Uranium U 7.038 · 10 years α 238 9 U 4.468 · 10 years α 237 6 93 Neptunium Np 2.144 · 10 years α 238 94 Plutonium Pu 87.7 years α 239 4 Pu 2.411 · 10 years α 242 5 Pu 3.733 · 10 years α 244 7 Pu 8.08 · 10 years α(0.999), sf(0.001) 241 95 Americium Am 432.2 years α, γ 243 Am 7370 years α 242 96 Curium Cm 162.8 days α 244 Cm 18.10 years α 248 5 Cm 3.40 · 10 years α(0.916), sf(0.084) 247 97 Berkelium Bk 1380 years α(<100%) 249 − Bk 320 days β (0.99999), α(0.00001) 249 98 Californium Cf 351 years α 251 Cf 898 years α 252 Cf 2.645 year α(0.969), sf(0.031) 252 99 Einsteinium Es 471.7 days α(0.76, ε(0.24) 253 Es 20.47 days α 254 Es 275.7 days α 252 100 Fermium Fm 25.39 h α(0.99998), sf(0.00002) 255 Fm 20.07 h α 256 Fm 157.6 min sf(0.919), α(0.081) 255 101 Mendelevium Md 27 min ε(0.92), α(0.08) 256 Md 78.1 min ε(0.907), α(0.093) 259 102 Nobelium No 58 min α(0.75), ε(0.25) 260 103 Lawrencium Lr 3.0 min α(0.75), ε(0.25) ◦ be produced by reducing AcF3 with lithium at 1200 C. acid treatment dissolves the thorium phosphate present, Small amounts of actinium can be obtained from residues while the basic process converts the phosphates to insol- 227 of uranium processing. Gram amounts of Ac has been uble hydroxides. The separation of thorium from the ura- produced synthetically at Mol, Belgium, by neutron irra- nium and rare-earth phosphates after the acid process can 226 diation of Ra [Eq. (9)]: be carried out by selective precipitation of the thorium and rare earth phosphates and then by using a solvent extrac- − −β 226 1 227 227 Ra + n → Ra + γ −−−→ Ac. (9) tion process to remove the thorium. When the alkali open- 88 0 88 91 42.2 min ing method is used, the insoluble hydroxides are dissolved Both thorium and uranium occur to a significant extent in in nitric acid and the thorium and uranium(VI) species are nature, and industrial processes have been developed for extracted, leaving the lanthanides in the aqueous phase. the production of these elements. The thorium and uranium can then be separated by further Thorium is produced commercially from monazite solvent extraction. sands. After mining, the monazite sands are concentrated Thorium metal can be produced in several ways. In magnetically and then treated with either hot, concentrated the most common process, thorium oxide is reduced with sulfuric acid or hot, concentrated sodium hydroxide. The calcium [Eq. (10)]: P1: FYK Revised Pages Encyclopedia of Physical Science and Technology EN001F-11 May 7, 2001 12:19 218 Actinide Elements FIGURE 1 ◦ ◦ 1000 C barium vapor at 1300 C, followed by increasing the tem- ThO2 + 2Ca −−−→ Th + 2CaO. (10) ◦ perature to 1600 C to produce a bead of protactinium Ar metal. Single-crystal protactinium metal is obtained by The reaction mass is leached with water and dilute acid, a modified van Arkel process starting from the carbide. leaving thorium metal powder. Very pure thorium metal More then 150 minerals containing uranium are known. can be prepared by the van Arkel process involving the Typically, however, uranium ores contain only about 0.1% thermal decomposition of ThI4. uranium. In the commercial production of uranium metal, To obtain significant quantities of protactinium, a sep- the ore is crushed, concentrated, roasted, and in most cases aration procedure was developed for extracting protac- leached with sulfuric acid in the presence of an oxidizing tinium from the sludge that was left after the ether agent such as manganese dioxide or chlorate ions to con- extraction of uranium at the Springfields refinery. The pro- vert all of the uranium to uranyl sulfato complexes. Car- 231 cess yielded 127 g of pure Pa from 60 tons of sludge. bonate leaching is used to extract uranium from ores con- Protactinium metal can be obtained by reducing PaF4 with taining minerals such as calcite. The recovery of uranium