Actinide

The actinide /ˈæktɪnaɪd/ or actinoid /ˈæktɪnɔɪd/ (IUPAC nomenclature) series encompasses the 15 metallic chemical elements with atomic numbers from 89 to 103, actinium through lawrencium.[2][3][4][5]

Strictly speaking, both actinium and lawrencium have been labeled as group 3 elements, but both elements are often included in any general discussion of the chemistry of the actinide elements. Actinium is the more often omitted of the two, because its placement as a group 3 element is somewhat more common in texts and for semantic reasons: since "actinide" means "like actinium", it has been argued that actinium cannot logically be an actinide, even though IUPAC acknowledges its inclusion based on common usage.[6]

The actinide series derives its name from the first element in the series, actinium. The informal chemical symbol An is used in general discussions of actinide chemistry to refer to any actinide. All but one of the actinides are f-block elements, with the exception being either actinium or lawrencium. The series mostly corresponds to the filling of the 5f electron shell, although actinium and thorium lack any f-electrons, and curium and lawrencium have the same number as the preceding element. In comparison with the lanthanides, also mostly f-block elements, the actinides show much more variable valence. They all have very large atomic and ionic radii and exhibit an unusually large range of physical properties. While actinium and the late actinides (from americium onwards) behave similarly to the lanthanides, the elements thorium, protactinium, and uranium are much more similar to transition metals in their chemistry, with neptunium and plutonium occupying an intermediate position.

All actinides are radioactive and release energy upon radioactive decay; naturally occurring uranium and thorium, and synthetically produced plutonium are the most abundant actinides on Earth. These are used in nuclear reactors and nuclear weapons. Uranium and thorium also have diverse current or historical uses, and americium is used in the ionization chambers of most modern smoke detectors.

Of the actinides, primordial thorium and uranium occur naturally in substantial quantities. The radioactive decay of uranium produces transient amounts of actinium and protactinium, and atoms of neptunium and plutonium are occasionally produced from transmutation reactions in uranium ores. The other actinides are purely synthetic elements.[2][7] Nuclear weapons tests have released at least six actinides heavier than plutonium into the environment; analysis of debris from a 1952 hydrogen bomb explosion showed the presence of americium, curium, berkelium, californium, einsteinium and fermium.[8]

In presentations of the periodic table, the lanthanides and the actinides are customarily shown as two additional rows below the main body of the table,[2] with placeholders or else a selected single element of each series (either lanthanum or lutetium, and either actinium or lawrencium, respectively) shown in a single cell of the main table, between barium and hafnium, and radium and rutherfordium, respectively. This convention is entirely a matter of aesthetics and formatting practicality; a rarely used wide-formatted periodic table inserts the lanthanide and actinide series in their proper places, as parts of the table's sixth and seventh rows (periods).

Actin­ium89Ac​[227] Thor­ium90Th232.04 Protac­tinium91Pa231.04 Ura­nium92U238.03 Neptu­nium93Np​[237] Pluto­nium94Pu​[244] Ameri­cium95Am​[243] Curium96Cm​[247] Berkel­ium97Bk​[247] Califor­nium98Cf​[251] Einstei­nium99Es​[252] Fer­mium100Fm​[257] Mende­levium101Md​[258] Nobel­ium102No​[259] Lawren­cium103Lr​[266]

Discovery, isolation and synthesis

Synthesis of transuranium elements[9][10]
Element Year Method
Neptunium 1940 Bombarding 238U by neutrons
Plutonium 1941 Bombarding 238U by deuterons
Americium 1944 Bombarding 239Pu by neutrons
Curium 1944 Bombarding 239Pu by α-particles
Berkelium 1949 Bombarding 241Am by α-particles
Californium 1950 Bombarding 242Cm by α-particles
Einsteinium 1952 As a product of nuclear explosion
Fermium 1952 As a product of nuclear explosion
Mendelevium 1955 Bombarding 253Es by α-particles
Nobelium 1965 Bombarding 243Am by 15N
or 238U with 22Ne
Lawrencium 1961
–1971
Bombarding 252Cf by 10B or 11B
and of 243Am with 18O

Like the lanthanides, the actinides form a family of elements with similar properties. Within the actinides, there are two overlapping groups: transuranium elements, which follow uranium in the periodic table—and transplutonium elements, which follow plutonium. Compared to the lanthanides, which (except for promethium) are found in nature in appreciable quantities, most actinides are rare. The majority of them do not even occur in nature, and of those that do, only thorium and uranium do so in more than trace quantities. The most abundant or easily synthesized actinides are uranium and thorium, followed by plutonium, americium, actinium, protactinium, neptunium, and curium.[11]

The existence of transuranium elements was suggested by Enrico Fermi based on his experiments in 1934.[12][13] However, even though four actinides were known by that time, it was not yet understood that they formed a family similar to lanthanides. The prevailing view that dominated early research into transuranics was that they were regular elements in the 7th period, with thorium, protactinium and uranium corresponding to 6th-period hafnium, tantalum and tungsten, respectively. Synthesis of transuranics gradually undermined this point of view. By 1944 an observation that curium failed to exhibit oxidation states above 4 (whereas its supposed 6th period homolog, platinum, can reach oxidation state of 6) prompted Glenn Seaborg to formulate a so-called "actinide hypothesis". Studies of known actinides and discoveries of further transuranic elements provided more data in support of this point of view, but the phrase "actinide hypothesis" (the implication being that a "hypothesis" is something that has not been decisively proven) remained in active use by scientists through the late 1950s.[14][15]

At present, there are two major methods of producing isotopes of transplutonium elements: (1) irradiation of the lighter elements with either neutrons or (2) accelerated charged particles. The first method is most important for applications, as only neutron irradiation using nuclear reactors allows the production of sizeable amounts of synthetic actinides; however, it is limited to relatively light elements. The advantage of the second method is that elements heavier than plutonium, as well as neutron-deficient isotopes, can be obtained, which are not formed during neutron irradiation.[16]

In 1962–1966, there were attempts in the United States to produce transplutonium isotopes using a series of six underground nuclear explosions. Small samples of rock were extracted from the blast area immediately after the test to study the explosion products, but no isotopes with mass number greater than 257 could be detected, despite predictions that such isotopes would have relatively long half-lives of α-decay. This non-observation was attributed to spontaneous fission owing to the large speed of the products and to other decay channels, such as neutron emission and nuclear fission.[17]

From actinium to uranium

Enrico Fermi 1943-49
Enrico Fermi suggested the existence of transuranium elements in 1934.

Uranium and thorium were the first actinides discovered. Uranium was identified in 1789 by the German chemist Martin Heinrich Klaproth in pitchblende ore. He named it after the planet Uranus,[7] which had been discovered only eight years earlier. Klaproth was able to precipitate a yellow compound (likely sodium diuranate) by dissolving pitchblende in nitric acid and neutralizing the solution with sodium hydroxide. He then reduced the obtained yellow powder with charcoal, and extracted a black substance that he mistook for metal.[18] Only 60 years later, the French scientist Eugène-Melchior Péligot identified it as uranium oxide. He also isolated the first sample of uranium metal by heating uranium tetrachloride with metallic potassium.[19] The atomic mass of uranium was then calculated as 120, but Dmitri Mendeleev in 1872 corrected it to 240 using his periodicity laws. This value was confirmed experimentally in 1882 by K. Zimmerman.[20][21]

Thorium oxide was discovered by Friedrich Wöhler in the mineral Thorianite, which was found in Norway (1827).[22] Jöns Jacob Berzelius characterized this material in more detail by in 1828. By reduction of thorium tetrachloride with potassium, he isolated the metal and named it thorium after the Norse god of thunder and lightning Thor.[23][24] The same isolation method was later used by Péligot for uranium.[7]

Actinium was discovered in 1899 by André-Louis Debierne, an assistant of Marie Curie, in the pitchblende waste left after removal of radium and polonium. He described the substance (in 1899) as similar to titanium[25] and (in 1900) as similar to thorium.[26] The discovery of actinium by Debierne was however questioned in 1971[27] and 2000,[28] arguing that Debierne's publications in 1904 contradicted his earlier work of 1899–1900. This view instead credits the 1902 work of Friedrich Oskar Giesel, who discovered a radioactive element named emanium that behaved similarly to lanthanum. The name actinium comes from the Greek aktis, aktinos (ακτίς, ακτίνος), meaning beam or ray. This metal was discovered not by its own radiation but by the radiation of the daughter products.[29][30] Owing to the close similarity of actinium and lanthanum and low abundance, pure actinium could only be produced in 1950. The term actinide was probably introduced by Victor Goldschmidt in 1937.[31][32]

Protactinium was possibly isolated in 1900 by William Crookes.[33] It was first identified in 1913, when Kasimir Fajans and Oswald Helmuth Göhring encountered the short-lived isotope 234mPa (half-life 1.17 minutes) during their studies of the 238U decay. They named the new element brevium (from Latin brevis meaning brief);[34][35] the name was changed to protoactinium (from Greek πρῶτος + ἀκτίς meaning "first beam element") in 1918 when two groups of scientists, led by the Austrian Lise Meitner and Otto Hahn of Germany and Frederick Soddy and John Cranston of Great Britain, independently discovered the much longer-lived 231Pa. The name was shortened to protactinium in 1949. This element was little characterized until 1960, when A. G. Maddock and his co-workers in the U.K. isolated 130 grams of protactinium from 60 tonnes of waste left after extraction of uranium from its ore.[36]

Neptunium and above

Neptunium (named for the planet Neptune, the next planet out from Uranus, after which uranium was named) was discovered by Edwin McMillan and Philip H. Abelson in 1940 in Berkeley, California.[37] They produced the 239Np isotope (half-life = 2.4 days) by bombarding uranium with slow neutrons.[36] It was the first transuranium element produced synthetically.[38]

Glenn Seaborg - 1964
Glenn T. Seaborg and his group at the University of California at Berkeley synthesized Pu, Am, Cm, Bk, Cf, Es, Fm, Md, No and element 106, which was later named seaborgium in his honor while he was still living. They also synthesized more than a hundred actinide isotopes.

Transuranium elements do not occur in sizeable quantities in nature and are commonly synthesized via nuclear reactions conducted with nuclear reactors. For example, under irradiation with reactor neutrons, uranium-238 partially converts to plutonium-239:

This synthesis reaction was used by Fermi and his collaborators in their design of the reactors located at the Hanford Site, which produced significant amounts of plutonium-239 for the nuclear weapons of the Manhattan Project and the United States' post-war nuclear arsenal.[39]

Actinides with the highest mass numbers are synthesized by bombarding uranium, plutonium, curium and californium with ions of nitrogen, oxygen, carbon, neon or boron in a particle accelerator. So, nobelium was produced by bombarding uranium-238 with neon-22 as

.

The first isotopes of transplutonium elements, americium-241 and curium-242, were synthesized in 1944 by Glenn T. Seaborg, Ralph A. James and Albert Ghiorso.[40] Curium-242 was obtained by bombarding plutonium-239 with 32-MeV α-particles

.

The americium-241 and curium-242 isotopes also were produced by irradiating plutonium in a nuclear reactor. The latter element was named after Marie Curie and her husband Pierre who are noted for discovering radium and for their work in radioactivity.[41]

Bombarding curium-242 with α-particles resulted in an isotope of californium 245Cf (1950), and a similar procedure yielded in 1949 berkelium-243 from americium-241.[42] The new elements were named after Berkeley, California, by analogy with its lanthanide homologue terbium, which was named after the village of Ytterby in Sweden.[43]

In 1945, B. B. Cunningham obtained the first bulk chemical compound of a transplutonium element, namely americium hydroxide.[44] Over the next three to four years, milligram quantities of americium and microgram amounts of curium were accumulated that allowed production of isotopes of berkelium (Thomson, 1949)[45][46] and californium (Thomson, 1950).[47][48][49] Sizeable amounts of these elements were produced only in 1958 (Burris B. Cunningham and Stanley G. Thomson),[50] and the first californium compound (0.3 µg of CfOCl) was obtained only in 1960 by B. B. Cunningham and J. C. Wallmann.[51]

Einsteinium and fermium were identified in 1952–1953 in the fallout from the "Ivy Mike" nuclear test (1 November 1952), the first successful test of a hydrogen bomb. Instantaneous exposure of uranium-238 to a large neutron flux resulting from the explosion produced heavy isotopes of uranium, including uranium-253 and uranium-255, and their β-decay yielded einsteinium-253 and fermium-255. The discovery of the new elements and the new data on neutron capture were initially kept secret on the orders of the U.S. military until 1955 due to Cold War tensions.[8][52] Nevertheless, the Berkeley team were able to prepare einsteinium and fermium by civilian means, through the neutron bombardment of plutonium-239, and published this work in 1954 with the disclaimer that it was not the first studies that had been carried out on the elements.[53][54] The "Ivy Mike" studies were declassified and published in 1955.[52] The first significant (submicrograms) amounts of einsteinium were produced in 1961 by Cunningham and colleagues, but this has not been done for fermium yet.[55]

The first isotope of mendelevium, 256Md (half-life 87 min), was synthesized by Albert Ghiorso, Glenn T. Seaborg, Gregory R. Choppin, Bernard G. Harvey and Stanley G. Thompson when they bombarded an 253Es target with alpha particles in the 60-inch cyclotron of Berkeley Radiation Laboratory; this was the first isotope of any element to be synthesized one atom at a time.[56]

There were several attempts to obtain isotopes of nobelium by Swedish (1957) and American (1958) groups, but the first reliable result was the synthesis of 256No by the Russian group (Georgy Flyorov et al.) in 1965, as acknowledged by the IUPAC in 1992. In their experiments, Flyorov et al. bombarded uranium-238 with neon-22.[9]

In 1961, Ghiorso et al. obtained the first isotope of lawrencium by irradiating californium (mostly californium-252) with boron-10 and boron-11 ions.[9] The mass number of this isotope was not clearly established (possibly 258 or 259) at the time. In 1965, 256Lr was synthesized by Flyorov et al. from 243Am and 18O. Thus IUPAC recognized the nuclear physics teams at Dubna and Berkeley as the co-discoverers of lawrencium.

Isotopes

Isotopes and half-life
Actinides have 89−103 protons and usually 117−159 neutrons.

Thirty-one isotopes of actinium and eight excited isomeric states of some of its nuclides were identified by 2010.[57] Three isotopes, 225Ac, 227Ac and 228Ac, were found in nature and the others were produced in the laboratory; only the three natural isotopes are used in applications. Actinium-225 is a member of the radioactive neptunium series;[61] it was first discovered in 1947 as a decay product of uranium-233, it is an α-emitter with a half-life of 10 days. Actinium-225 is less available than actinium-228, but is more promising in radiotracer applications.[30] Actinium-227 (half-life 21.77 years) occurs in all uranium ores, but in small quantities. One gram of uranium (in radioactive equilibrium) contains only 2×1010 gram of 227Ac.[30][57] Actinium-228 is a member of the radioactive thorium series formed by the decay of 228Ra;[61] it is a β emitter with a half-life of 6.15 hours. In one tonne of thorium there is 5×108 gram of 228Ac. It was discovered by Otto Hahn in 1906.[30]

28 isotopes of protactinium are known with mass numbers 212–239[57] as well as three excited isomeric states. Only 231Pa and 234Pa have been found in nature. All the isotopes have short lifetime, except for protactinium-231 (half-life 32,760 years). The most important isotopes are 231Pa and 233Pa, which is an intermediate product in obtaining uranium-233 and is the most affordable among artificial isotopes of protactinium. 233Pa has convenient half-life and energy of γ-radiation, and thus was used in most studies of protactinium chemistry. Protactinium-233 is a β-emitter with a half-life of 26.97 days.[57][62]

Uranium has the highest number (25) of both natural and synthetic isotopes. They have mass numbers of 215–242 (except 220 and 241),[58] and three of them, 234U, 235U and 238U, are present in appreciable quantities in nature. Among others, the most important is 233U, which is a final product of transformations of 232Th irradiated by slow neutrons. 233U has a much higher fission efficiency by low-energy (thermal) neutrons, compared e.g. with 235U. Most uranium chemistry studies were carried out on uranium-238 owing to its long half-life of 4.4×109 years.[63]

There are 23 isotopes of neptunium with mass numbers of 219 and 223–244;[58] they are all highly radioactive. The most popular among scientists are long-lived 237Np (t1/2 = 2.20×106 years) and short-lived 239Np, 238Np (t1/2 ~ 2 days).[38]

Eighteen isotopes of americium are known with mass numbers from 229 to 247 (with the exception of 231).[58] The most important are 241Am and 243Am, which are alpha-emitters and also emit soft, but intense γ-rays; both of them can be obtained in an isotopically pure form. Chemical properties of americium were first studied with 241Am, but later shifted to 243Am, which is almost 20 times less radioactive. The disadvantage of 243Am is production of the short-lived daughter isotope 239Np, which has to be considered in the data analysis.[64]

Among 19 isotopes of curium,[58] the most accessible are 242Cm and 244Cm; they are α-emitters, but with much shorter lifetime than the americium isotopes. These isotopes emit almost no γ-radiation, but undergo spontaneous fission with the associated emission of neutrons. More long-lived isotopes of curium (245–248Cm, all α-emitters) are formed as a mixture during neutron irradiation of plutonium or americium. Upon short irradiation, this mixture is dominated by 246Cm, and then 248Cm begins to accumulate. Both of these isotopes, especially 248Cm, have a longer half-life (3.48×105 years) and are much more convenient for carrying out chemical research than 242Cm and 244Cm, but they also have a rather high rate of spontaneous fission. 247Cm has the longest lifetime among isotopes of curium (1.56×107 years), but is not formed in large quantities because of the strong fission induced by thermal neutrons.

Eighteen isotopes of berkelium were identified with mass numbers 233–234, 236, and 238–252.[58] Only 249Bk is available in large quantities; it has a relatively short half-life of 330 days and emits mostly soft β-particles, which are inconvenient for detection. Its alpha radiation is rather weak (1.45×103% with respect to β-radiation), but is sometimes used to detect this isotope. 247Bk is an alpha-emitter with a long half-life of 1,380 years, but it is hard to obtain in appreciable quantities; it is not formed upon neutron irradiation of plutonium because of the β-stability of isotopes of curium isotopes with mass number below 248.[64]

Isotopes of californium with mass numbers 237–256 are formed in nuclear reactors;[58] californium-253 is a β-emitter and the rest are α-emitters. The isotopes with even mass numbers (250Cf, 252Cf and 254Cf) have a high rate of spontaneous fission, especially 254Cf of which 99.7% decays by spontaneous fission. Californium-249 has a relatively long half-life (352 years), weak spontaneous fission and strong γ-emission that facilitates its identification. 249Cf is not formed in large quantities in a nuclear reactor because of the slow β-decay of the parent isotope 249Bk and a large cross section of interaction with neutrons, but it can be accumulated in the isotopically pure form as the β-decay product of (pre-selected) 249Bk. Californium produced by reactor-irradiation of plutonium mostly consists of 250Cf and 252Cf, the latter being predominant for large neutron fluences, and its study is hindered by the strong neutron radiation.[65]

Properties of some transplutonium isotope pairs[66]
Parent
isotope
t1/2 Daughter
isotope
t1/2 Time to establish
radioactive equilibrium
243Am 7370 years 239Np 2.35 days 47.3 days
245Cm 8265 years 241Pu 14 years 129 years
247Cm 1.64×107 years 243Pu 4.95 hours 7.2 days
254Es 270 days 250Bk 3.2 hours 35.2 hours
255Es 39.8 days 255Fm 22 hours 5 days
257Fm 79 days 253Cf 17.6 days 49 days

Among the 18 known isotopes of einsteinium with mass numbers from 240 to 257,[58] the most affordable is 253Es. It is an α-emitter with a half-life of 20.47 days, a relatively weak γ-emission and small spontaneous fission rate as compared with the isotopes of californium. Prolonged neutron irradiation also produces a long-lived isotope 254Es (t1/2 = 275.5 days).[65]

Twenty isotopes of fermium are known with mass numbers of 241–260. 254Fm, 255Fm and 256Fm are α-emitters with a short half-life (hours), which can be isolated in significant amounts. 257Fm (t1/2 = 100 days) can accumulate upon prolonged and strong irradiation. All these isotopes are characterized by high rates of spontaneous fission.[65][67]

Among the 16 known isotopes of mendelevium (mass numbers from 245 to 260),[58] the most studied is 256Md, which mainly decays through the electron capture (α-radiation is ≈10%) with the half-life of 77 minutes. Another alpha emitter, 258Md, has a half-life of 53 days. Both these isotopes are produced from rare einsteinium (253Es and 255Es respectively), that therefore limits their availability.[57]

Long-lived isotopes of nobelium and isotopes of lawrencium (and of heavier elements) have relatively short half-lives. For nobelium, 11 isotopes are known with mass numbers 250–260 and 262. The chemical properties of nobelium and lawrencium were studied with 255No (t1/2 = 3 min) and 256Lr (t1/2 = 35 s). The longest-lived nobelium isotope, 259No, has a half-life of approximately 1 hour.[57]

Among all of these, the only isotopes that occur in sufficient quantities in nature to be detected in anything more than traces and have a measurable contribution to the atomic weights of the actinides are the primordial 232Th, 235U, and 238U, and three long-lived decay products of natural uranium, 230Th, 231Pa, and 234U. Natural thorium consists of 0.02(2)% 230Th and 99.98(2)% 232Th; natural protactinium consists of 100% 231Pa; and natural uranium consists of 0.0054(5)% 234U, 0.7204(6)% 235U, and 99.2742(10)% 238U.[68]

Distribution in nature

Uranium ore square
Unprocessed uranium ore

Thorium and uranium are the most abundant actinides in nature with the respective mass concentrations of 16 ppm and 4 ppm.[69] Uranium mostly occurs in the Earth's crust as a mixture of its oxides in the minerals uraninite, which is also called pitchblende because of its black color. There are several dozens of other uranium minerals such as carnotite (KUO2VO4·3H2O) and autunite (Ca(UO2)2(PO4)2·nH2O). The isotopic composition of natural uranium is 238U (relative abundance 99.2742%), 235U (0.7204%) and 234U (0.0054%); of these 238U has the largest half-life of 4.51×109 years.[70][71] The worldwide production of uranium in 2009 amounted to 50,572 tonnes, of which 27.3% was mined in Kazakhstan. Other important uranium mining countries are Canada (20.1%), Australia (15.7%), Namibia (9.1%), Russia (7.0%), and Niger (6.4%).[72]

Content of plutonium in uranium and thorium ores[73]
Ore Location Uranium
content, %
Mass ratio
239Pu/ore
Ratio
239Pu/U (×1012)
Uraninite Canada 13.5 9.1×1012 7.1
Uraninite Congo 38 4.8×1012 12
Uraninite Colorado, US 50 3.8×1012 7.7
Monazite Brazil 0.24 2.1×1014 8.3
Monazite North Carolina, US 1.64 5.9×1014 3.6
Fergusonite - 0.25 <1×1014 <4
Carnotite - 10 <4×1014 <0.4

The most abundant thorium minerals are thorianite (ThO2), thorite (ThSiO4) and monazite, ((Th,Ca,Ce)PO4). Most thorium minerals contain uranium and vice versa; and they all have significant fraction of lanthanides. Rich deposits of thorium minerals are located in the United States (440,000 tonnes), Australia and India (~300,000 tonnes each) and Canada (~100,000 tonnes).[74]

The abundance of actinium in the Earth's crust is only about 5×1015%.[62] Actinium is mostly present in uranium-containing, but also in other minerals, though in much smaller quantities. The content of actinium in most natural objects corresponds to the isotopic equilibrium of parent isotope 235U, and it is not affected by the weak Ac migration.[30] Protactinium is more abundant (10−12%) in the Earth's crust than actinium. It was discovered in the uranium ore in 1913 by Fajans and Göhring.[34] As actinium, the distribution of protactinium follows that of 235U.[62]

The half-life of the longest-lived isotope of neptunium, 237Np, is negligible compared to the age of the Earth. Thus neptunium is present in nature in negligible amounts produced as intermediate decay products of other isotopes.[38] Traces of plutonium in uranium minerals were first found in 1942, and the more systematic results on 239Pu are summarized in the table (no other plutonium isotopes could be detected in those samples). The upper limit of abundance of the longest-living isotope of plutonium, 244Pu, is 3×1020%. Plutonium could not be detected in samples of lunar soil. Owing to its scarcity in nature, most plutonium is produced synthetically.[73]

Extraction

MonaziteUSGOV
Monazite: a major thorium mineral

Owing to the low abundance of actinides, their extraction is a complex, multistep process. Fluorides of actinides are usually used because they are insoluble in water and can be easily separated with redox reactions. Fluorides are reduced with calcium, magnesium or barium:[75]

Among the actinides, thorium and uranium are the easiest to isolate. Thorium is extracted mostly from monazite: thorium pyrophosphate (ThP2O7) is reacted with nitric acid, and the produced thorium nitrate treated with tributyl phosphate. Rare-earth impurities are separated by increasing the pH in sulfate solution.[75]

In another extraction method, monazite is decomposed with a 45% aqueous solution of sodium hydroxide at 140 °C. Mixed metal hydroxides are extracted first, filtered at 80 °C, washed with water and dissolved with concentrated hydrochloric acid. Next, the acidic solution is neutralized with hydroxides to pH = 5.8 that results in precipitation of thorium hydroxide (Th(OH)4) contaminated with ~3% of rare-earth hydroxides; the rest of rare-earth hydroxides remains in solution. Thorium hydroxide is dissolved in an inorganic acid and then purified from the rare earth elements. An efficient method is the dissolution of thorium hydroxide in nitric acid, because the resulting solution can be purified by extraction with organic solvents:[75]

Plutonium and uranium extraction from nuclear fuel-eng
Separation of uranium and plutonium from nuclear fuel[76]
Th(OH)4 + 4 HNO3 → Th(NO3)4 + 4 H2O

Metallic thorium is separated from the anhydrous oxide, chloride or fluoride by reacting it with calcium in an inert atmosphere:[77]

ThO2 + 2 Ca → 2 CaO + Th

Sometimes thorium is extracted by electrolysis of a fluoride in a mixture of sodium and potassium chloride at 700–800 °C in a graphite crucible. Highly pure thorium can be extracted from its iodide with the crystal bar process.[78]

Uranium is extracted from its ores in various ways. In one method, the ore is burned and then reacted with nitric acid to convert uranium into a dissolved state. Treating the solution with a solution of tributyl phosphate (TBP) in kerosene transforms uranium into an organic form UO2(NO3)2(TBP)2. The insoluble impurities are filtered and the uranium is extracted by reaction with hydroxides as (NH4)2U2O7 or with hydrogen peroxide as UO4·2H2O.[75]

When the uranium ore is rich in such minerals as dolomite, magnesite, etc., those minerals consume much acid. In this case, the carbonate method is used for uranium extraction. Its main component is an aqueous solution of sodium carbonate, which converts uranium into a complex [UO2(CO3)3]4−, which is stable in aqueous solutions at low concentrations of hydroxide ions. The advantages of the sodium carbonate method are that the chemicals have low corrosivity (compared to nitrates) and that most non-uranium metals precipitate from the solution. The disadvantage is that tetravalent uranium compounds precipitate as well. Therefore, the uranium ore is treated with sodium carbonate at elevated temperature and under oxygen pressure:

2 UO2 + O2 + 6 CO2−
3
→ 2 [UO2(CO3)3]4−

This equation suggests that the best solvent for the uranium carbonate processing is a mixture of carbonate with bicarbonate. At high pH, this results in precipitation of diuranate, which is treated with hydrogen in the presence of nickel yielding an insoluble uranium tetracarbonate.[75]

Another separation method uses polymeric resins as a polyelectrolyte. Ion exchange processes in the resins result in separation of uranium. Uranium from resins is washed with a solution of ammonium nitrate or nitric acid that yields uranyl nitrate, UO2(NO3)2·6H2O. When heated, it turns into UO3, which is converted to UO2 with hydrogen:

UO3 + H2 → UO2 + H2O

Reacting uranium dioxide with hydrofluoric acid changes it to uranium tetrafluoride, which yields uranium metal upon reaction with magnesium metal:[77]

4 HF + UO2 → UF4 + 2 H2O

To extract plutonium, neutron-irradiated uranium is dissolved in nitric acid, and a reducing agent (FeSO4, or H2O2) is added to the resulting solution. This addition changes the oxidation state of plutonium from +6 to +4, while uranium remains in the form of uranyl nitrate (UO2(NO3)2). The solution is treated with a reducing agent and neutralized with ammonium carbonate to pH = 8 that results in precipitation of Pu4+ compounds.[75]

In another method, Pu4+ and UO2+
2
are first extracted with tributyl phosphate, then reacted with hydrazine washing out the recovered plutonium.[75]

The major difficulty in separation of actinium is the similarity of its properties with those of lanthanum. Thus actinium is either synthesized in nuclear reactions from isotopes of radium or separated using ion-exchange procedures.[30]

Properties

Actinides have similar properties to lanthanides. The 6d and 7s electronic shells are filled in actinium and thorium, and the 5f shell is being filled with further increase in atomic number; the 4f shell is filled in the lanthanides. The first experimental evidence for the filling of the 5f shell in actinides was obtained by McMillan and Abelson in 1940.[79] As in lanthanides (see lanthanide contraction), the ionic radius of actinides monotonically decreases with atomic number (see also Aufbau principle).[80]

Physical properties

ActinidesLattice ACTIION
Major crystal structures of some actinides vs. temperature Metallic and ionic radii of actinides[81]
Radioisotope thermoelectric generator plutonium pellet
A pellet of 238PuO2 to be used in a radioisotope thermoelectric generator for either the Cassini or Galileo mission. The pellet produces 62 watts of heat and glows because of the heat generated by the radioactive decay (primarily α). Photo is taken after insulating the pellet under a graphite blanket for minutes and removing the blanket.

Actinides are typical metals. All of them are soft and have a silvery color (but tarnish in air),[85] relatively high density and plasticity. Some of them can be cut with a knife. Their electrical resistivity varies between 15 and 150 µOhm·cm.[81] The hardness of thorium is similar to that of soft steel, so heated pure thorium can be rolled in sheets and pulled into wire. Thorium is nearly half as dense as uranium and plutonium, but is harder than either of them. All actinides are radioactive, paramagnetic, and, with the exception of actinium, have several crystalline phases: plutonium has seven, and uranium, neptunium and californium three. The crystal structures of protactinium, uranium, neptunium and plutonium do not have clear analogs among the lanthanides and are more similar to those of the 3d-transition metals.[71]

All actinides are pyrophoric, especially when finely divided, that is, they spontaneously ignite upon reaction with air.[85] The melting point of actinides does not have a clear dependence on the number of f-electrons. The unusually low melting point of neptunium and plutonium (~640 °C) is explained by hybridization of 5f and 6d orbitals and the formation of directional bonds in these metals.[71]

Chemical properties

Like the lanthanides, all actinides are highly reactive with halogens and chalcogens; however, the actinides react more easily. Actinides, especially those with a small number of 5f-electrons, are prone to hybridization. This is explained by the similarity of the electron energies at the 5f, 7s and 6d shells. Most actinides exhibit a larger variety of valence states, and the most stable are +6 for uranium, +5 for protactinium and neptunium, +4 for thorium and plutonium and +3 for actinium and other actinides.[87]

Chemically, actinium is similar to lanthanum, which is explained by their similar ionic radii and electronic structure. Like lanthanum, actinium almost always has an oxidation state of +3 in compounds, but it is less reactive and has more pronounced basic properties. Among other trivalent actinides Ac3+ is least acidic, i.e. has the weakest tendency to hydrolyze in aqueous solutions.[30][71]

Thorium is rather active chemically. Owing to lack of electrons on 6d and 5f orbitals, the tetravalent thorium compounds are colorless. At pH < 3, the solutions of thorium salts are dominated by the cations [Th(H2O)8]4+. The Th4+ ion is relatively large, and depending on the coordination number can have a radius between 0.95 and 1.14 Å. As a result, thorium salts have a weak tendency to hydrolyse. The distinctive ability of thorium salts is their high solubility, not only in water, but also in polar organic solvents.[71]

Protactinium exhibits two valence states; the +5 is stable, and the +4 state easily oxidizes to protactinium(V). Thus tetravalent protactinium in solutions is obtained by the action of strong reducing agents in a hydrogen atmosphere. Tetravalent protactinium is chemically similar to uranium(IV) and thorium(IV). Fluorides, phosphates, hypophosphate, iodate and phenylarsonates of protactinium(IV) are insoluble in water and dilute acids. Protactinium forms soluble carbonates. The hydrolytic properties of pentavalent protactinium are close to those of tantalum(V) and niobium(V). The complex chemical behavior of protactinium is a consequence of the start of the filling of the 5f shell in this element.[62]

Uranium has a valence from 3 to 6, the last being most stable. In the hexavalent state, uranium is very similar to the group 6 elements. Many compounds of uranium(IV) and uranium(VI) are non-stoichiometric, i.e. have variable composition. For example, the actual chemical formula of uranium dioxide is UO2+x, where x varies between −0.4 and 0.32. Uranium(VI) compounds are weak oxidants. Most of them contain the linear "uranyl" group, UO2+
2
. Between 4 and 6 ligands can be accommodated in an equatorial plane perpendicular to the uranyl group. The uranyl group acts as a hard acid and forms stronger complexes with oxygen-donor ligands than with nitrogen-donor ligands. NpO2+
2
and PuO2+
2
are also the common form of Np and Pu in the +6 oxidation state. Uranium(IV) compounds exhibit reducing properties, e.g., they are easily oxidized by atmospheric oxygen. Uranium(III) is a very strong reducing agent. Owing to the presence of d-shell, uranium (as well as many other actinides) forms organometallic compounds, such as UIII(C5H5)3 and UIV(C5H5)4.[71][88]

Neptunium has valence states from 3 to 7, which can be simultaneously observed in solutions. The most stable state in solution is +5, but the valence +4 is preferred in solid neptunium compounds. Neptunium metal is very reactive. Ions of neptunium are prone to hydrolysis and formation of coordination compounds.[38]

Plutonium also exhibits valence states between 3 and 7 inclusive, and thus is chemically similar to neptunium and uranium. It is highly reactive, and quickly forms an oxide film in air. Plutonium reacts with hydrogen even at temperatures as low as 25–50 °C; it also easily forms halides and intermetallic compounds. Hydrolysis reactions of plutonium ions of different oxidation states are quite diverse. Plutonium(V) can enter polymerization reactions.[89][90]

The largest chemical diversity among actinides is observed in americium, which can have valence between 2 and 6. Divalent americium is obtained only in dry compounds and non-aqueous solutions (acetonitrile). Oxidation states +3, +5 and +6 are typical for aqueous solutions, but also in the solid state. Tetravalent americium forms stable solid compounds (dioxide, fluoride and hydroxide) as well as complexes in aqueous solutions. It was reported that in alkaline solution americium can be oxidized to the heptavalent state, but these data proved erroneous. The most stable valence of americium is 3 in the aqueous solutions and 3 or 4 in solid compounds.[91]

Valence 3 is dominant in all subsequent elements up to lawrencium (with the exception of nobelium). Curium can be tetravalent in solids (fluoride, dioxide). Berkelium, along with a valence of +3, also shows the valence of +4, more stable than that of curium; the valence 4 is observed in solid fluoride and dioxide. The stability of Bk4+ in aqueous solution is close to that of Ce4+.[92] Only valence 3 was observed for californium, einsteinium and fermium. The divalent state is proven for mendelevium and nobelium, and in nobelium it is more stable than the trivalent state. Lawrencium shows valence 3 both in solutions and solids.[91]

The redox potential increases from −0.32 V in uranium, through 0.34 V (Np) and 1.04 V (Pu) to 1.34 V in americium revealing the increasing reduction ability of the An4+ ion from americium to uranium. All actinides form AnH3 hydrides of black color with salt-like properties. Actinides also produce carbides with the general formula of AnC or AnC2 (U2C3 for uranium) as well as sulfides An2S3 and AnS2.[87]

Uranyl Nitrate

Uranyl nitrate (UO2(NO3)2)

U Oxstufen

Aqueous solutions of uranium III, IV, V, VI salts

Np ox st

Aqueous solutions of neptunium III, IV, V, VI, VII salts

Plutonium in solution

Aqueous solutions of plutonium III, IV, V, VI, VII salts

Yellowcake

U3O8 (yellowcake)

Compounds

Oxides and hydroxides

An – actinide
**Depending on the isotopes

Some actinides can exist in several oxide forms such as An2O3, AnO2, An2O5 and AnO3. For all actinides, oxides AnO3 are amphoteric and An2O3, AnO2 and An2O5 are basic, they easily react with water, forming bases:[87]

An2O3 + 3 H2O → 2 An(OH)3.

These bases are poorly soluble in water and by their activity are close to the hydroxides of rare-earth metals.[87] Np(OH)3 has not yet been synthesized, Pu(OH)3 has a blue color while Am(OH)3 is pink and curium hydroxide Cm(OH)3 is colorless.[99] Bk(OH)3 and Cf(OH)3 are also known, as are tetravalent hydroxides for Np, Pu and Am and pentavalent for Np and Am.[99]

The strongest base is of actinium. All compounds of actinium are colorless, except for black actinium sulfide (Ac2S3).[87] Dioxides of tetravalent actinides crystallize in the cubic system, same as in calcium fluoride.

Thorium reacting with oxygen exclusively forms the dioxide:

Thorium dioxide is a refractory material with the highest melting point among any known oxide (3390 °C).[97] Adding 0.8–1% ThO2 to tungsten stabilizes its structure, so the doped filaments have better mechanical stability to vibrations. To dissolve ThO2 in acids, it is heated to 500–600 °C; heating above 600 °C produces a very resistant to acids and other reagents form of ThO2. Small addition of fluoride ions catalyses dissolution of thorium dioxide in acids.

Two protactinium oxides have been obtained: PaO2 (black) and Pa2O5 (white); the former is isomorphic with ThO2 and the latter is easier to obtain. Both oxides are basic, and Pa(OH)5 is a weak, poorly soluble base.[87]

Decomposition of certain salts of uranium, for example UO2(NO3)·6H2O in air at 400 °C, yields orange or yellow UO3.[97] This oxide is amphoteric and forms several hydroxides, the most stable being uranyl hydroxide UO2(OH)2. Reaction of uranium(VI) oxide with hydrogen results in uranium dioxide, which is similar in its properties with ThO2. This oxide is also basic and corresponds to the uranium hydroxide (U(OH)4).[87]

Plutonium, neptunium and americium form two basic oxides: An2O3 and AnO2. Neptunium trioxide is unstable; thus, only Np3O8 could be obtained so far. However, the oxides of plutonium and neptunium with the chemical formula AnO2 and An2O3 are well characterized.[87]

Salts

*An – actinide
**Depending on the isotopes
Einsteinium triiodide by transmitted light
Einsteinium triiodide glowing in the dark

Actinides easily react with halogens forming salts with the formulas MX3 and MX4 (X = halogen). So the first berkelium compound, BkCl3, was synthesized in 1962 with an amount of 3 nanograms. Like the halogens of rare earth elements, actinide chlorides, bromides, and iodides are water-soluble, and fluorides are insoluble. Uranium easily yields a colorless hexafluoride, which sublimates at a temperature of 56.5 °C; because of its volatility, it is used in the separation of uranium isotopes with gas centrifuge or gaseous diffusion. Actinide hexafluorides have properties close to anhydrides. They are very sensitive to moisture and hydrolyze forming AnO2F2.[103] The pentachloride and black hexachloride of uranium were synthesized, but they are both unstable.[87]

Action of acids on actinides yields salts, and if the acids are non-oxidizing then the actinide in the salt is in low-valence state:

U + 2H2SO4 → U(SO4)2 + 2H2
2Pu + 6HCl → 2PuCl3 + 3H2

However, in these reactions the regenerating hydrogen can react with the metal, forming the corresponding hydride. Uranium reacts with acids and water much more easily than thorium.[87]

Actinide salts can also be obtained by dissolving the corresponding hydroxides in acids. Nitrates, chlorides, sulfates and perchlorates of actinides are water-soluble. When crystallizing from aqueous solutions, these salts forming a hydrates, such as Th(NO3)4·6H2O, Th(SO4)2·9H2O and Pu2(SO4)3·7H2O. Salts of high-valence actinides easily hydrolyze. So, colorless sulfate, chloride, perchlorate and nitrate of thorium transform into basic salts with formulas Th(OH)2SO4 and Th(OH)3NO3. The solubility and insolubility of trivalent and tetravalent actinides is like that of lanthanide salts. So phosphates, fluorides, oxalates, iodates and carbonates of actinides are weakly soluble in water; they precipitate as hydrates, such as ThF4·3H2O and Th(CrO4)2·3H2O.[87]

Actinides with oxidation state +6, except for the AnO22+-type cations, form [AnO4]2−, [An2O7]2− and other complex anions. For example, uranium, neptunium and plutonium form salts of the Na2UO4 (uranate) and (NH4)2U2O7 (diuranate) types. In comparison with lanthanides, actinides more easily form coordination compounds, and this ability increases with the actinide valence. Trivalent actinides do not form fluoride coordination compounds, whereas tetravalent thorium forms K2ThF6, KThF5, and even K5ThF9 complexes. Thorium also forms the corresponding sulfates (for example Na2SO4·Th(SO4)2·5H2O), nitrates and thiocyanates. Salts with the general formula An2Th(NO3)6·nH2O are of coordination nature, with the coordination number of thorium equal to 12. Even easier is to produce complex salts of pentavalent and hexavalent actinides. The most stable coordination compounds of actinides – tetravalent thorium and uranium – are obtained in reactions with diketones, e.g. acetylacetone.[87]

Applications

InsideSmokeDetector
Interior of a smoke detector containing americium-241.

While actinides have some established daily-life applications, such as in smoke detectors (americium)[104][105] and gas mantles (thorium),[77] they are mostly used in nuclear weapons and use as a fuel in nuclear reactors.[77] The last two areas exploit the property of actinides to release enormous energy in nuclear reactions, which under certain conditions may become self-sustaining chain reaction.

Cerenkov Effect
Self-illumination of a nuclear reactor by Cherenkov radiation.

The most important isotope for nuclear power applications is uranium-235. It is used in the thermal reactor, and its concentration in natural uranium does not exceed 0.72%. This isotope strongly absorbs thermal neutrons releasing much energy. One fission act of 1 gram of 235U converts into about 1 MW·day. Of importance, is that 235
92
U
emits more neutrons than it absorbs;[106] upon reaching the critical mass, 235
92
U
enters into a self-sustaining chain reaction.[71] Typically, uranium nucleus is divided into two fragments with the release of 2–3 neutrons, for example:

235
92
U
+ 1
0
n
115
45
Rh
+ 118
47
Ag
+ 31
0
n

Other promising actinide isotopes for nuclear power are thorium-232 and its product from the thorium fuel cycle, uranium-233.

Nuclear reactor[71][107][108]
The core of most Generation II nuclear reactors contains a set of hollow metal rods, usually made of zirconium alloys, filled with solid nuclear fuel pellets – mostly oxide, carbide, nitride or monosulfide of uranium, plutonium or thorium, or their mixture (the so-called MOX fuel). The most common fuel is oxide of uranium-235.
Nuclear reactor scheme

Fast neutrons are slowed by moderators, which contain water, carbon, deuterium, or beryllium, as thermal neutrons to increase the efficiency of their interaction with uranium-235. The rate of nuclear reaction is controlled by introducing additional rods made of boron or cadmium or a liquid absorbent, usually boric acid. Reactors for plutonium production are called breeder reactor or breeders; they have a different design and use fast neutrons.

Emission of neutrons during the fission of uranium is important not only for maintaining the nuclear chain reaction, but also for the synthesis of the heavier actinides. Uranium-239 converts via β-decay into plutonium-239, which, like uranium-235, is capable of spontaneous fission. The world's first nuclear reactors were built not for energy, but for producing plutonium-239 for nuclear weapons.

About half of the produced thorium is used as the light-emitting material of gas mantles.[77] Thorium is also added into multicomponent alloys of magnesium and zinc. So the Mg-Th alloys are light and strong, but also have high melting point and ductility and thus are widely used in the aviation industry and in the production of missiles. Thorium also has good electron emission properties, with long lifetime and low potential barrier for the emission.[106] The relative content of thorium and uranium isotopes is widely used to estimate the age of various objects, including stars (see radiometric dating).[109]

The major application of plutonium has been in nuclear weapons, where the isotope plutonium-239 was a key component due to its ease of fission and availability. Plutonium-based designs allow reducing the critical mass to about a third of that for uranium-235.[110] The "Fat Man"-type plutonium bombs produced during the Manhattan Project used explosive compression of plutonium to obtain significantly higher densities than normal, combined with a central neutron source to begin the reaction and increase efficiency. Thus only 6.2 kg of plutonium was needed for an explosive yield equivalent to 20 kilotons of TNT.[111] (See also Nuclear weapon design.) Hypothetically, as little as 4 kg of plutonium—and maybe even less—could be used to make a single atomic bomb using very sophisticated assembly designs.[112]

Plutonium-238 is potentially more efficient isotope for nuclear reactors, since it has smaller critical mass than uranium-235, but it continues to release much thermal energy (0.56 W/g)[105][113] by decay even when the fission chain reaction is stopped by control rods. Its application is limited by the high price (about US$1000/g). This isotope has been used in thermopiles and water distillation systems of some space satellites and stations. So Galileo and Apollo spacecraft (e.g. Apollo 14[114]) had heaters powered by kilogram quantities of plutonium-238 oxide; this heat is also transformed into electricity with thermopiles. The decay of plutonium-238 produces relatively harmless alpha particles and is not accompanied by gamma-irradiation. Therefore, this isotope (~160 mg) is used as the energy source in heart pacemakers where it lasts about 5 times longer than conventional batteries.[105]

Actinium-227 is used as a neutron source. Its high specific energy (14.5 W/g) and the possibility of obtaining significant quantities of thermally stable compounds are attractive for use in long-lasting thermoelectric generators for remote use. 228Ac is used as an indicator of radioactivity in chemical research, as it emits high-energy electrons (2.18 MeV) that can be easily detected. 228Ac-228Ra mixtures are widely used as an intense gamma-source in industry and medicine.[30]

Development of self-glowing actinide-doped materials with durable crystalline matrices is a new area of actinide utilization as the addition of alpha-emitting radionuclides to some glasses and crystals may confer luminescence.[115]

Toxicity

Alfa beta gamma radiation penetration
Schematic illustration of penetration of radiation through sheets of paper, aluminium and lead brick
Periodic Table Radioactivity
Periodic table with elements colored according to the half-life of their most stable isotope.
  Elements which contain at least one stable isotope.
  Slightly radioactive elements: the most stable isotope is very long-lived, with a half-life of over two million years.
  Significantly radioactive elements: the most stable isotope has half-life between 800 and 34,000 years.
  Radioactive elements: the most stable isotope has half-life between one day and 130 years.
  Highly radioactive elements: the most stable isotope has half-life between several minutes and one day.
  Extremely radioactive elements: the most stable isotope has half-life less than several minutes.

Radioactive substances can harm human health via (i) local skin contamination, (ii) internal exposure due to ingestion of radioactive isotopes, and (iii) external overexposure by β-activity and γ-radiation. Together with radium and transuranium elements, actinium is one of the most dangerous radioactive poisons with high specific α-activity. The most important feature of actinium is its ability to accumulate and remain in the surface layer of skeletons. At the initial stage of poisoning, actinium accumulates in the liver. Another danger of actinium is that it undergoes radioactive decay faster than being excreted. Adsorption from the digestive tract is much smaller (~0.05%) for actinium than radium.[30]

Protactinium in the body tends to accumulate in the kidneys and bones. The maximum safe dose of protactinium in the human body is 0.03 µCi that corresponds to 0.5 micrograms of 231Pa. This isotope, which might be present in the air as aerosol, is 2.5×108 times more toxic than hydrocyanic acid.[62]

Plutonium, when entering the body through air, food or blood (e.g. a wound), mostly settles in the lungs, liver and bones with only about 10% going to other organs, and remains there for decades. The long residence time of plutonium in the body is partly explained by its poor solubility in water. Some isotopes of plutonium emit ionizing α-radiation, which damages the surrounding cells. The median lethal dose (LD50) for 30 days in dogs after intravenous injection of plutonium is 0.32 milligram per kg of body mass, and thus the lethal dose for humans is approximately 22 mg for a person weighing 70 kg; the amount for respiratory exposure should be approximately four times greater. Another estimate assumes that plutonium is 50 times less toxic than radium, and thus permissible content of plutonium in the body should be 5 µg or 0.3 µCi. Such amount is nearly invisible in under microscope. After trials on animals, this maximum permissible dose was reduced to 0.65 µg or 0.04 µCi. Studies on animals also revealed that the most dangerous plutonium exposure route is through inhalation, after which 5–25% of inhaled substances is retained in the body. Depending on the particle size and solubility of the plutonium compounds, plutonium is localized either in the lungs or in the lymphatic system, or is absorbed in the blood and then transported to the liver and bones. Contamination via food is the least likely way. In this case, only about 0.05% of soluble 0.01% insoluble compounds of plutonium absorbs into blood, and the rest is excreted. Exposure of damaged skin to plutonium would retain nearly 100% of it.[89]

Using actinides in nuclear fuel, sealed radioactive sources or advanced materials such as self-glowing crystals has many potential benefits. However, a serious concern is the extremely high radiotoxicity of actinides and their migration in the environment.[116] Use of chemically unstable forms of actinides in MOX and sealed radioactive sources is not appropriate by modern safety standards. There is a challenge to develop stable and durable actinide-bearing materials, which provide safe storage, use and final disposal. A key need is application of actinide solid solutions in durable crystalline host phases.[115]

Nuclear properties

Half-lives and branching fractions for actinides and natural decay products[117]
Nuclide Half-life Decay mode Branching fraction Source
206
81
Tl
4.202 ± 0.011 m β- 1.0 LNHB
208
81
Tl
3.060 ± 0.008 m β- 1.0 BIPM-5
210
82
Pb
22.20 ± 0.22 y β- 1.0 ENSDF
α ( 1.9 ± 0.4 ) x 10−8
211
82
Pb
36.1 ± 0.2 m β- 1.0 ENSDF
212
82
Pb
10.64 ± 0.01 h β- 1.0 BIPM-5
214
82
Pb
26.8 ± 0.9 m β- 1.0 ENSDF
211
83
Bi
2.14 ± 0.02 m β- 0.00276 ± 0.00004 ENSDF
α 0.99724 ± 0.00004
212
83
Bi
60.54 ± 0.06 m α 0.3593 ± 0.0007 BIPM-5
β- 0.6407 ± 0.0007
214
83
Bi
19.9 ± 0.4 m α 0.00021 ± 0.00001 ENSDF
β- 0.99979 ± 0.00001
210
84
Po
138.376 ± 0.002 d α 1.0 ENSDF
219
86
Rn
3.96 ± 0.01 s α 1.0 ENSDF
220
86
Rn
55.8 ± 0.3 s α 1.0 BIPM-5
221
87
Fr
4.9 ± 0.2 m β- 0.00005 ± 0.00003 ENSDF
α 0.99995 ± 0.00003
223
88
Ra
11.43 ± 0.05 d α 1.0 ENSDF
14C ( 8.9 ± 0.4 ) x 10−10
224
88
Ra
3.627 ± 0.007 d α 1.0 BIPM-5
225
88
Ra
14.9 ± 0.2 d β- 1.0 ENSDF
226
88
Ra
( 1.600 ± 0.007 ) x 103 y α 1.0 BIPM-5
228
88
Ra
5.75 ± 0.03 y β- 1.0 ENSDF
224
89
Ac
2.78 ± 0.17 h α 0.091 +0.020 -0.014 ENSDF
EC 0.909 +0.014 -0.020
225
89
Ac
10.0 ± 0.1 d α 1.0 ENSDF
227
89
Ac
21.772 ± 0.003 y α 0.01380 ± 0.00004 ENSDF
β- 0.98620 ± 0.00004
228
89
Ac
6.15 ± 0.02 h β- 1.0 ENSDF
227
90
Th
18.718 ± 0.005 d α 1.0 BIPM-5
228
90
Th
698.60 ± 0.23 d α 1.0 BIPM-5
229
90
Th
( 7.34 ± 0.16 ) x 103 y α 1.0 ENSDF
230
90
Th
( 7.538 ± 0.030 ) x 104 y α 1.0 ENSDF
SF ≤ 4 x 10−13
231
90
Th
25.52 ± 0.01 h β- 1.0 ENSDF
α ~ 4 x 10−13
232
90
Th
( 1.405 ± 0.006 ) x 1010 y α 1.0 ENSDF
SF ( 1.1 ± 0.4 ) x 10−11
233
90
Th
22.15 ± 0.15 m β- 1.0 LNHB
234
90
Th
24.10 ± 0.03 d β- 1.0 ENSDF
231
91
Pa
( 3.276 ± 0.011 ) x 104 y α 1.0 ENSDF
SF ≤ 3 x 10−12
232
91
Pa
1.32 ± 0.02 d EC 0.00003 ± 0.00001 ENSDF
β- 0.99997 ± 0.00001
233
91
Pa
26.98 ± 0.02 d β- 1.0 LNHB
234
91
Pa
6.70 ± 0.05 h β- 1.0 ENSDF
234m
91
Pa
1.159 ± 0.016 m IT 0.0016 ± 0.0002 IAEA-CRP-XG
β- 0.9984 ± 0.0002
232
92
U
68.9 ± 0.4 y α 1.0 ENSDF
SF
233
92
U
( 1.592 ± 0.002 ) x 105 y α 1.0 ENSDF
SF
234
92
U
( 2.455 ± 0.006 ) x 105 y α 1.0 LNHB
SF ( 1.6 ± 0.2 ) x 10−11
235m
92
U
26 ± 1 m IT 1.0 ENSDF
235
92
U
( 7.038 ± 0.005 ) x 108 y α 1.0 ENSDF
SF ( 7 ± 2 ) x 10−11
236
92
U
( 2.342 ± 0.004 ) x 107 y α 1.0 ENSDF
SF ( 9.4 ± 0.4 ) x 10−10
237
92
U
6.749 ± 0.016 d β- 1.0 LNHB
238
92
U
( 4.468 ± 0.005 ) x 109 y α 1.0 LNHB
SF ( 5.45 ± 0.04 ) x 10−7
239
92
U
23.45 ± 0.02 m β- 1.0 ENSDF
236
93
Np
( 1.55 ± 0.08 ) x 105 y α 0.0016 ± 0.0006 LNHB
β- 0.120 ± 0.006
EC 0.878 ± 0.006
236m
93
Np
22.5 ± 0.4 h β- 0.47 ± 0.01 LNHB
EC 0.53 ± 0.01
237
93
Np
( 2.144 ± 0.007 ) x 106 y α 1.0 ENSDF
SF
238
93
Np
2.117 ± 0.002 d β- 1.0 ENSDF
239
93
Np
2.356 ± 0.003 d β- 1.0 ENSDF
236
94
Pu
2.858 ± 0.008 y α 1.0 ENSDF
References
LNHB Laboratoire National Henri Becquerel, Recommended Data,

http://www.nucleide.org/DDEP_WG/DDEPdata.htm, 3 October 2006.

BIPM-5 M.-M. Bé, V. Chisté, C. Dulieu, E. Browne, V. Chechev, N. Kuzmenko, R. Helmer,

A. Nichols, E. Schönfeld, R. Dersch, Monographie BIPM-5, Table of Radionuclides, Vol. 2 - A = 151 to 242, 2004.

ENSDF Evaluated Nuclear Structure Data File, http://www-nds.iaea.org/ensdf/,

15 November 2006.

IAEA-CRP-XG M.-M. Bé, V. P. Chechev, R. Dersch, O. A. M. Helene, R. G. Helmer, M. Herman,

S. Hlavác, A. Marcinkowski, G. L. Molnár, A. L. Nichols, E. Schönfeld, V. R. Vanin, M. J. Woods, IAEA CRP "Update of X Ray and Gamma Ray Decay Data Standards for Detector Calibration and Other Applications", IAEA Scientific and Technical Information report STI/PUB/1287, May 2007, International Atomic Energy Agency, Vienna, Austria, ISBN 92-0-113606-4.

See also

References and notes

  1. ^ The Manhattan Project. An Interactive History. US Department of Energy
  2. ^ a b c Theodore Gray (2009). The Elements: A Visual Exploration of Every Known Atom in the Universe. New York: Black Dog & Leventhal Publishers. p. 240. ISBN 978-1-57912-814-2.
  3. ^ Actinide element, Encyclopædia Britannica on-line
  4. ^ Although "actinoid" (rather than "actinide") means "actinium-like" and therefore should exclude actinium, that element is usually included in the series.
  5. ^ Neil G. Connelly; et al. (2005). "Elements". Nomenclature of Inorganic Chemistry. London: Royal Society of Chemistry. p. 52. ISBN 978-0-85404-438-2.
  6. ^ Greenwood, Norman N.; Earnshaw, Alan (1997). Chemistry of the Elements (2nd ed.). Butterworth-Heinemann. pp. 1230–1242. ISBN 978-0-08-037941-8.
  7. ^ a b c Greenwood, p. 1250
  8. ^ a b Fields, P.; Studier, M.; Diamond, H.; Mech, J.; Inghram, M.; Pyle, G.; Stevens, C.; Fried, S.; Manning, W.; et al. (1956). "Transplutonium Elements in Thermonuclear Test Debris". Physical Review. 102 (1): 180–182. Bibcode:1956PhRv..102..180F. doi:10.1103/PhysRev.102.180.
  9. ^ a b c Greenwood, p. 1252
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Bibliography

External links

Actinide chemistry

Actinide chemistry (or actinoid chemistry) is one of the main branches of nuclear chemistry that investigates the processes and molecular systems of the actinides. The actinides derive their name from the group 3 element actinium. The informal chemical symbol An is used in general discussions of actinide chemistry to refer to any actinide. All but one of the actinides are f-block elements, corresponding to the filling of the 5f electron shell; lawrencium, a d-block element, is also generally considered an actinide. In comparison with the lanthanides, also mostly f-block elements, the actinides show much more variable valence. The actinide series encompasses the 15 metallic chemical elements with atomic numbers from 89 to 103, actinium through lawrencium.

Actinocene

An actinocene is a type of metallocene compound that contains an element from the actinide series. The typical structure is a sandwich compound, with two cyclooctatetraenyl dianions (cot, which is C8H2−8) bound to an actinide-metal center (An) in the oxidation state IV, with the resulting general formula An(C8H8)2The most studied actinocene is uranocene, U(C8H8)2.The actinide-cyclooctatetraenyl bonding was shown by computational chemistry to be mainly due to mixing of actinide 6d orbitals into ligand π-orbitals and therefore donation of electronic charge to the actinide, with a smaller such interaction involving the actinide 5f orbitals. Other actinocenes include protactinocene (Pa(C8H8)2), thorocene (Th(C8H8)2), neptunocene (Np(C8H8)2), and plutonocene (Pu(C8H8)2).

Sandwiched M(C8H8)2 compounds (actinocenes and lanthanocenes) exist for M = Nd, Tb, Pu, Pa, Np, Th, U and Yb.

Berkelium

Berkelium is a transuranic radioactive chemical element with symbol Bk and atomic number 97. It is a member of the actinide and transuranium element series. It is named after the city of Berkeley, California, the location of the Lawrence Berkeley National Laboratory (then the University of California Radiation Laboratory) where it was discovered in December 1949. Berkelium was the fifth transuranium element discovered after neptunium, plutonium, curium and americium.

The major isotope of berkelium, 249Bk, is synthesized in minute quantities in dedicated high-flux nuclear reactors, mainly at the Oak Ridge National Laboratory in Tennessee, USA, and at the Research Institute of Atomic Reactors in Dimitrovgrad, Russia. The production of the second-most important isotope 247Bk involves the irradiation of the rare isotope 244Cm with high-energy alpha particles.

Just over one gram of berkelium has been produced in the United States since 1967. There is no practical application of berkelium outside scientific research which is mostly directed at the synthesis of heavier transuranic elements and transactinides. A 22 milligram batch of berkelium-249 was prepared during a 250-day irradiation period and then purified for a further 90 days at Oak Ridge in 2009. This sample was used to synthesize the new element tennessine for the first time in 2009 at the Joint Institute for Nuclear Research, Russia, after it was bombarded with calcium-48 ions for 150 days. This was the culmination of the Russia–US collaboration on the synthesis of the heaviest elements on the periodic table.

Berkelium is a soft, silvery-white, radioactive metal. The berkelium-249 isotope emits low-energy electrons and thus is relatively safe to handle. It decays with a half-life of 330 days to californium-249, which is a strong emitter of ionizing alpha particles. This gradual transformation is an important consideration when studying the properties of elemental berkelium and its chemical compounds, since the formation of californium brings not only chemical contamination, but also free-radical effects and self-heating from the emitted alpha particles.

Californium

Californium is a radioactive chemical element with symbol Cf and atomic number 98. The element was first synthesized in 1950 at the Lawrence Berkeley National Laboratory (then the University of California Radiation Laboratory), by bombarding curium with alpha particles (helium-4 ions). It is an actinide element, the sixth transuranium element to be synthesized, and has the second-highest atomic mass of all the elements that have been produced in amounts large enough to see with the unaided eye (after einsteinium). The element was named after the university and the state of California.

Two crystalline forms exist for californium under normal pressure: one above and one below 900 °C (1,650 °F). A third form exists at high pressure. Californium slowly tarnishes in air at room temperature. Compounds of californium are dominated by the +3 oxidation state. The most stable of californium's twenty known isotopes is californium-251, which has a half-life of 898 years. This short half-life means the element is not found in significant quantities in the Earth's crust. Californium-252, with a half-life of about 2.645 years, is the most common isotope used and is produced at the Oak Ridge National Laboratory in the United States and the Research Institute of Atomic Reactors in Russia.

Californium is one of the few transuranium elements that have practical applications. Most of these applications exploit the property of certain isotopes of californium to emit neutrons. For example, californium can be used to help start up nuclear reactors, and it is employed as a source of neutrons when studying materials using neutron diffraction and neutron spectroscopy. Californium can also be used in nuclear synthesis of higher mass elements; oganesson (element 118) was synthesized by bombarding californium-249 atoms with calcium-48 ions. Users of californium must take into account radiological concerns and the element's ability to disrupt the formation of red blood cells by bioaccumulating in skeletal tissue.

Curium

Curium is a transuranic radioactive chemical element with symbol Cm and atomic number 96. This element of the actinide series was named after Marie and Pierre Curie – both were known for their research on radioactivity. Curium was first intentionally produced and identified in July 1944 by the group of Glenn T. Seaborg at the University of California, Berkeley. The discovery was kept secret and only released to the public in November 1947. Most curium is produced by bombarding uranium or plutonium with neutrons in nuclear reactors – one tonne of spent nuclear fuel contains about 20 grams of curium.

Curium is a hard, dense, silvery metal with a relatively high melting point and boiling point for an actinide. Whereas it is paramagnetic at ambient conditions, it becomes antiferromagnetic upon cooling, and other magnetic transitions are also observed for many curium compounds. In compounds, curium usually exhibits valence +3 and sometimes +4, and the +3 valence is predominant in solutions. Curium readily oxidizes, and its oxides are a dominant form of this element. It forms strongly fluorescent complexes with various organic compounds, but there is no evidence of its incorporation into bacteria and archaea. When introduced into the human body, curium accumulates in the bones, lungs and liver, where it promotes cancer.

All known isotopes of curium are radioactive and have a small critical mass for a sustained nuclear chain reaction. They predominantly emit α-particles, and the heat released in this process can serve as a heat source in radioisotope thermoelectric generators, but this application is hindered by the scarcity and high cost of curium isotopes. Curium is used in production of heavier actinides and of the 238Pu radionuclide for power sources in artificial pacemakers. It served as the α-source in the alpha particle X-ray spectrometers installed on several space probes, including the Sojourner, Spirit, Opportunity and Curiosity Mars rovers and the Philae lander on comet 67P/Churyumov–Gerasimenko, to analyze the composition and structure of the surface.

Einsteinium

Einsteinium is a synthetic element with symbol Es and atomic number 99. A member of the actinide series, it is the seventh transuranic element.

Einsteinium was discovered as a component of the debris of the first hydrogen bomb explosion in 1952, and named after Albert Einstein. Its most common isotope einsteinium-253 (half-life 20.47 days) is produced artificially from decay of californium-253 in a few dedicated high-power nuclear reactors with a total yield on the order of one milligram per year. The reactor synthesis is followed by a complex process of separating einsteinium-253 from other actinides and products of their decay. Other isotopes are synthesized in various laboratories, but at much smaller amounts, by bombarding heavy actinide elements with light ions. Owing to the small amounts of produced einsteinium and the short half-life of its most easily produced isotope, there are currently almost no practical applications for it outside basic scientific research. In particular, einsteinium was used to synthesize, for the first time, 17 atoms of the new element mendelevium in 1955.

Einsteinium is a soft, silvery, paramagnetic metal. Its chemistry is typical of the late actinides, with a preponderance of the +3 oxidation state; the +2 oxidation state is also accessible, especially in solids. The high radioactivity of einsteinium-253 produces a visible glow and rapidly damages its crystalline metal lattice, with released heat of about 1000 watts per gram. Difficulty in studying its properties is due to einsteinium-253's decay to berkelium-249 and then californium-249 at a rate of about 3% per day. The isotope of einsteinium with the longest half-life, einsteinium-252 (half-life 471.7 days) would be more suitable for investigation of physical properties, but it has proven far more difficult to produce and is available only in minute quantities, and not in bulk. Einsteinium is the element with the highest atomic number which has been observed in macroscopic quantities in its pure form, and this was the common short-lived isotope einsteinium-253.Like all synthetic transuranic elements, isotopes of einsteinium are very radioactive and are considered highly dangerous to health on ingestion.

Fermium

Fermium is a synthetic element with symbol Fm and atomic number 100. It is an actinide and the heaviest element that can be formed by neutron bombardment of lighter elements, and hence the last element that can be prepared in macroscopic quantities, although pure fermium metal has not yet been prepared. A total of 19 isotopes are known, with 257Fm being the longest-lived with a half-life of 100.5 days.

It was discovered in the debris of the first hydrogen bomb explosion in 1952, and named after Enrico Fermi, one of the pioneers of nuclear physics. Its chemistry is typical for the late actinides, with a preponderance of the +3 oxidation state but also an accessible +2 oxidation state. Owing to the small amounts of produced fermium and all of its isotopes having relatively short half-lives, there are currently no uses for it outside basic scientific research.

Ferromagnetism

Ferromagnetism is the basic mechanism by which certain materials (such as iron) form permanent magnets, or are attracted to magnets. In physics, several different types of magnetism are distinguished. Ferromagnetism (along with the similar effect ferrimagnetism) is the strongest type and is responsible for the common phenomena of magnetism in magnets encountered in everyday life. Substances respond weakly to magnetic fields with three other types of magnetism, paramagnetism, diamagnetism, and antiferromagnetism, but the forces are usually so weak that they can only be detected by sensitive instruments in a laboratory. An everyday example of ferromagnetism is a refrigerator magnet used to hold notes on a refrigerator door. The attraction between a magnet and ferromagnetic material is "the quality of magnetism first apparent to the ancient world, and to us today".Permanent magnets (materials that can be magnetized by an external magnetic field and remain magnetized after the external field is removed) are either ferromagnetic or ferrimagnetic, as are the materials that are noticeably attracted to them. Only a few substances are ferromagnetic. The common ones are iron, nickel, cobalt and most of their alloys, and some compounds of rare earth metals.

Ferromagnetism is very important in industry and modern technology, and is the basis for many electrical and electromechanical devices such as electromagnets, electric motors, generators, transformers, and magnetic storage such as tape recorders, and hard disks, and nondestructive testing of ferrous materials.

Integrated Nuclear Fuel Cycle Information System

Integrated Nuclear Fuel Cycle Information System (iNFCIS) is a set of databases related to the nuclear fuel cycle maintained by the International Atomic Energy Agency (IAEA). The main objective of iNFCIS is to provide information on all aspects of nuclear fuel cycle to various researchers, analysts, energy planners, academicians, students and the general public. Presently iNFCIS includes several modules. iNFCIS requires free registration for on-line access.

Lawrencium

Lawrencium is a synthetic chemical element with symbol Lr (formerly Lw) and atomic number 103. It is named in honor of Ernest Lawrence, inventor of the cyclotron, a device that was used to discover many artificial radioactive elements. A radioactive metal, lawrencium is the eleventh transuranic element and is also the final member of the actinide series. Like all elements with atomic number over 100, lawrencium can only be produced in particle accelerators by bombarding lighter elements with charged particles. Twelve isotopes of lawrencium are currently known; the most stable is 266Lr with a half-life of 11 hours, but the shorter-lived 260Lr (half-life 2.7 minutes) is most commonly used in chemistry because it can be produced on a larger scale.

Chemistry experiments have confirmed that lawrencium behaves as a heavier homolog to lutetium in the periodic table, and is a trivalent element. It thus could also be classified as the first of the 7th-period transition metals: however, its electron configuration is anomalous for its position in the periodic table, having an s2p configuration instead of the s2d configuration of its homolog lutetium. This means that lawrencium may be more volatile than expected for its position in the periodic table and have a volatility comparable to that of lead.

In the 1950s, 1960s, and 1970s, many claims of the synthesis of lawrencium of varying quality were made from laboratories in the Soviet Union and the United States. The priority of the discovery and therefore the naming of the element was disputed between Soviet and American scientists, and while the International Union of Pure and Applied Chemistry (IUPAC) initially established lawrencium as the official name for the element and gave the American team credit for the discovery, this was reevaluated in 1997, giving both teams shared credit for the discovery but not changing the element's name.

Major actinide

Major actinides is a term used in the nuclear power industry that refers to the plutonium and uranium present in used nuclear fuel, as opposed to the minor actinides neptunium, americium, curium, berkelium, and californium.

Mendelevium

Mendelevium is a synthetic element with chemical symbol Md (formerly Mv) and atomic number 101. A metallic radioactive transuranic element in the actinide series, it is the first element that currently cannot be produced in macroscopic quantities through neutron bombardment of lighter elements. It is the third-to-last actinide and the ninth transuranic element. It can only be produced in particle accelerators by bombarding lighter elements with charged particles. A total of sixteen mendelevium isotopes are known, the most stable being 258Md with a half-life of 51 days; nevertheless, the shorter-lived 256Md (half-life 1.17 hours) is most commonly used in chemistry because it can be produced on a larger scale.

Mendelevium was discovered by bombarding einsteinium with alpha particles in 1955, the same method still used to produce it today. It was named after Dmitri Mendeleev, father of the periodic table of the chemical elements. Using available microgram quantities of the isotope einsteinium-253, over a million mendelevium atoms may be produced each hour. The chemistry of mendelevium is typical for the late actinides, with a preponderance of the +3 oxidation state but also an accessible +2 oxidation state. Owing to the small amounts of produced mendelevium and all of its isotopes having relatively short half-lives, there are currently no uses for it outside basic scientific research.

Minor actinide

The minor actinides are the actinide elements in used nuclear fuel other than uranium and plutonium, which are termed the major actinides. The minor actinides include neptunium (element 93), americium (element 95), curium (element 96), berkelium (element 97), californium (element 98), einsteinium (element 99), and fermium (element 100). The most important isotopes in spent nuclear fuel are neptunium-237, americium-241, americium-243, curium-242 through -248, and californium-249 through -252.

Plutonium and the minor actinides will be responsible for the bulk of the radiotoxicity and heat generation of used nuclear fuel in the medium term (300 to 20,000 years in the future).The plutonium from a power reactor tends to have a greater amount of Pu-241 than the plutonium generated by the lower burnup operations designed to create weapons-grade plutonium. Because the reactor-grade plutonium contains so much Pu-241 the presence of americium-241 makes the plutonium less suitable for making a nuclear weapon. The ingrowth of americium in plutonium is one of the methods for identifying the origin of an unknown sample of plutonium and the time since it was last separated chemically from the americium.

Americium is commonly used in industry as both an alpha particle and as a low photon energy gamma radiation source. For instance it is used in many smoke detectors. Americium can be formed by neutron capture of Pu-239 and Pu-240 forming Pu-241 which then beta decays to Am-241. In general, as the energy of the neutrons increases, the ratio of the fission cross section to the neutron capture cross section changes in favour of fission. Hence if MOX is used in a thermal reactor such as a boiling water reactor (BWR) or pressurized water reactor (PWR) then more americium can be expected in the used fuel than that from a fast neutron reactor.Some of the minor actinides have been found in fallout from bomb tests. See Actinides in the environment for details.

Neptunium

Neptunium is a chemical element with symbol Np and atomic number 93. A radioactive actinide metal, neptunium is the first transuranic element. Its position in the periodic table just after uranium, named after the planet Uranus, led to it being named after Neptune, the next planet beyond Uranus. A neptunium atom has 93 protons and 93 electrons, of which seven are valence electrons. Neptunium metal is silvery and tarnishes when exposed to air. The element occurs in three allotropic forms and it normally exhibits five oxidation states, ranging from +3 to +7. It is radioactive, poisonous, pyrophoric, and can accumulate in bones, which makes the handling of neptunium dangerous.

Although many false claims of its discovery were made over the years, the element was first synthesized by Edwin McMillan and Philip H. Abelson at the Berkeley Radiation Laboratory in 1940. Since then, most neptunium has been and still is produced by neutron irradiation of uranium in nuclear reactors. The vast majority is generated as a by-product in conventional nuclear power reactors. While neptunium itself has no commercial uses at present, it is used as a precursor for the formation of plutonium-238, used in radioisotope thermal generators to provide electricity for spacecraft. Neptunium has also been used in detectors of high-energy neutrons.

The most stable isotope of neptunium, neptunium-237, is a by-product of nuclear reactors and plutonium production. It, and the isotope neptunium-239, are also found in trace amounts in uranium ores due to neutron capture reactions and beta decay.

Neptunocene

Neptunocene, Np(C8H8)2, is an organoneptunium compound composed of a neptunium atom sandwiched between two cyclooctatetraenide rings. It was one of the first organoneptunium compounds to be synthesized. Neptunocene, a member of the "actinocenes," a group of metallocenes incorporating elements from the actinide series.

Nobelium

Nobelium is a synthetic chemical element with symbol No and atomic number 102. It is named in honor of Alfred Nobel, the inventor of dynamite and benefactor of science. A radioactive metal, it is the tenth transuranic element and is the penultimate member of the actinide series. Like all elements with atomic number over 100, nobelium can only be produced in particle accelerators by bombarding lighter elements with charged particles. A total of twelve nobelium isotopes are known to exist; the most stable is 259No with a half-life of 58 minutes, but the shorter-lived 255No (half-life 3.1 minutes) is most commonly used in chemistry because it can be produced on a larger scale.

Chemistry experiments have confirmed that nobelium behaves as a heavier homolog to ytterbium in the periodic table. The chemical properties of nobelium are not completely known: they are mostly only known in aqueous solution. Before nobelium's discovery, it was predicted that it would show a stable +2 oxidation state as well as the +3 state characteristic of the other actinides: these predictions were later confirmed, as the +2 state is much more stable than the +3 state in aqueous solution and it is difficult to keep nobelium in the +3 state.

In the 1950s and 1960s, many claims of the discovery of nobelium were made from laboratories in Sweden, the Soviet Union, and the United States. Although the Swedish scientists soon retracted their claims, the priority of the discovery and therefore the naming of the element was disputed between Soviet and American scientists, and it was not until 1997 that International Union of Pure and Applied Chemistry (IUPAC) credited the Soviet team with the discovery, but retained nobelium, the Swedish proposal, as the name of the element due to its long-standing use in the literature.

Period 7 element

A period 7 element is one of the chemical elements in the seventh row (or period) of the periodic table of the chemical elements. The periodic table is laid out in rows to illustrate recurring (periodic) trends in the chemical behaviour of the elements as their atomic number increases: a new row is begun when chemical behaviour begins to repeat, meaning that elements with similar behaviour fall into the same vertical columns. The seventh period contains 32 elements, tied for the most with period 6, beginning with francium and ending with oganesson, the heaviest element currently discovered. As a rule, period 7 elements fill their 7s shells first, then their 5f, 6d, and 7p shells, in that order; however, there are exceptions, such as plutonium.

Periodic table (detailed cells)

The periodic table is a tabular method of displaying the chemical elements. It can show much information, after name, symbol and atomic number. Also, for each element mean atomic mass value for the natural isotopic composition of each element can be noted.

The two layout forms originate from two graphic forms of presentation of the same periodic table. Historically, when the f-block was identified it was drawn below the existing table, with markings for its in-table location (this page uses dots or asterisks). Also, a common presentation is to put all 15 lanthanide and actinide columns below, while the f-block only has 14 columns. One lanthanide and actinide each are d-block elements, belonging to group 3 with scandium and yttrium, though whether these are the first of each series (lanthanum and actinium) or the last (lutetium and lawrencium) has been disputed. The tables below show lanthanum and actinium as group 3 elements, as this is the more common form in the literature.

Although precursors to this table exist, its invention is generally credited to Russian chemist Dmitri Mendeleev in 1869. Mendeleev invented the table to illustrate recurring ("periodic") trends in the properties of the elements. The layout of the table has been refined and extended over time, as new elements have been discovered, and new theoretical models have been developed to explain chemical behavior.

Transactinide element

In chemistry, transactinide elements (also transactinides, superheavy elements, or super-heavy elements) are the chemical elements with atomic numbers from 104 to 120. Their atomic numbers are immediately greater than those of the actinides, the heaviest of which is lawrencium (atomic number 103).

Glenn T. Seaborg first proposed the actinide concept, which led to the acceptance of the actinide series. He also proposed the transactinide series ranging from element 104 to 121 and the superactinide series approximately spanning elements 122 to 153. The transactinide seaborgium was named in his honor.By definition, transactinide elements are also transuranic elements, i.e. have an atomic number greater than uranium (92).

The transactinide elements all have electrons in the 6d subshell in their ground state. Except for rutherfordium and dubnium, even the longest-lasting isotopes of transactinide elements have extremely short half-lives of minutes or less. The element naming controversy involved the first five or six transactinide elements. These elements thus used systematic names for many years after their discovery had been confirmed. (Usually the systematic names are replaced with permanent names proposed by the discoverers relatively shortly after a discovery has been confirmed.)

Transactinides are radioactive and have only been obtained synthetically in laboratories. None of these elements have ever been collected in a macroscopic sample. Transactinide elements are all named after physicists and chemists or important locations involved in the synthesis of the elements.

IUPAC defines an element to exist if its lifetime is longer than 10−14 seconds, which is the time it takes for the nucleus to form an electron cloud.

Nuclear properties of isotopes of the most important transplutonium isotopes[57][58][59]
Isotope Half-life Probability of
spontaneous
fission
in %
Emission energy
(MeV) (yield in %)
Specific activity (Bq/kg)[60] of
α γ α, β-particles fission
241Am 432.2(7) y 4.3(18)×10−10 5.485 (84.8)
5.442 (13.1)
5.388 (1.66)
0.059 (35.9)
0.026 (2.27)
1.27×1014 546.1
243Am 7.37(4)×103 y 3.7(2)×10−9 5.275 (87.1)
5.233 (11.2)
5.181 (1.36)
0.074 (67.2)
0.043 (5.9)
7.39×1012 273.3
242Cm 162.8(2) d 6.2(3)×10−6 6.069 (25.92)
6.112 (74.08)
0.044 (0.04)
0.102 (4×10−3)
1.23×1017 7.6×109
244Cm 18.10(2) y 1.37(3)×10−4 5.762 (23.6)
5.804 (76.4)
0.043 (0.02)
0.100 (1.5×10−3)
2.96×1015 4.1×109
245Cm 8.5(1)×103 y 6.1(9)×10−7 5.529 (0.58)
5.488 (0.83)
5.361 (93.2)
0.175 (9.88)
0.133 (2.83)
6.35×1012 3.9×104
246Cm 4.76(4)×103 y 0.02615(7) 5.343 (17.8)
5.386 (82.2)
0.045 (19) 1.13×1013 2.95×109
247Cm 1.56(5)×107 y 5.267 (13.8)
5.212 (5.7)
5.147 (1.2)
0.402 (72)
0.278 (3.4)
3.43×109
248Cm 3.48(6)×105 y 8.39(16) 5.034 (16.52)
5.078 (75)
1.40×1011 1.29×1010
249Bk 330(4) d 4.7(2)×10−8 5.406 (1×10−3)
5.378 (2.6×10−4)
0.32 (5.8×10−5) 5.88×1016 2.76×107
249Cf 351(2) y 5.0(4)×10−7 6.193 (2.46)
6.139 (1.33)
5.946 (3.33)
0.388 (66)
0.333 (14.6)
1.51×1014 7.57×105
250Cf 13.08(9) y 0.077(3) 5.988 (14.99)
6.030 (84.6)
0.043 4.04×1015 3.11×1012
251Cf 900(40) y ? 6.078 (2.6)
5.567 (0.9)
5.569 (0.9)
0.177 (17.3)
0.227 (6.8)
5.86×1013
252Cf 2.645(8) y 3.092(8) 6.075 (15.2)
6.118 (81.6)
0.042 (1.4×10−2)
0.100 (1.3×10−2)
1.92×1016 6.14×1014
254Cf 60.5(2) d ≈100 5.834 (0.26)
5.792 (5.3×10−2)
9.75×1014 3.13×1017
253Es 20.47(3) d 8.7(3)×10−6 6.540 (0.85)
6.552 (0.71)
6.590 (6.6)
0.387 (0.05)
0.429 (8×10−3)
9.33×1017 8.12×1010
254Es 275.7(5) d < 3×10−6 6.347 (0.75)
6.358 (2.6)
6.415 (1.8)
0.042 (100)
0.034 (30)
6.9×1016
255Es 39.8(12) d 0.0041(2) 6.267 (0.78)
6.401 (7)
4.38×1017(β)
3.81×1016(α)
1.95×1013
255Fm 20.07(7) h 2.4(10)×10−5 7.022 (93.4)
6.963 (5.04)
6.892 (0.62)
0.00057 (19.1)
0.081 (1)
2.27×1019 5.44×1012
256Fm 157.6(13) min 91.9(3) 6.872 (1.2)
6.917 (6.9)
1.58×1020 1.4×1019
257Fm 100.5(2) d 0.210(4) 6.752 (0.58)
6.695 (3.39)
6.622 (0.6)
0.241 (11)
0.179 (8.7)
1.87×1017 3.93×1014
256Md 77(2) min 7.142 (1.84)
7.206 (5.9)
3.53×1020
257Md 5.52(5) h 7.074 (14) 0.371 (11.7)
0.325 (2.5)
8.17×1019
258Md 51.5(3) d 6.73 3.64×1017
255No 3.1(2) min 8.312 (1.16)
8.266 (2.6)
8.121 (27.8)
0.187 (3.4) 8.78×1021
259No 58(5) min 7.455 (9.8)
7.500 (29.3)
7.533 (17.3)
4.63×1020
256Lr 27(3) s < 0.03 8.319 (5.4)
8.390 (16)
8.430 (33)
5.96×1022
257Lr 646(25) ms 8.796 (18)
8.861 (82)
1.54×1024
Properties of actinides (the mass of the most long-lived isotope is in square brackets)[71][81]
Property Ac Th Pa U Np Pu Am Cm Bk Cf Es Fm Md No Lr
Core charge 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103
atomic mass [227] 232.0377(4) 231.03588(2) 238.02891(3) [237] [244] [243] [247] [247] [251] [252] [257] [258] [259] [266]
Number of natural isotopes[82] 3 7 3 8 3 4 0 0 0 0 0 0 0 0 0
Natural isotopes[82][83] 225, 227–228 227–232, 234 231, 233–234 233–240 237, 239–240 238–240, 244
Natural quantity isotopes 230, 232 231 234, 235, 238
Longest-lived isotope 227 232 231 238 237 244 243 247 247 251 252 257 258 259 266
Half-life of the longest-lived isotope 21.8 years 14 billion years 32,500 years 4.47 billion years 2.14 million years 80.8 million years 7,370 years 15.6 million years 1,380 years 900 years 1.29 years 100.5 days 52 days 58 min 11 hours
Most common isotope 227 232 231 238 237 239 241 244 249 252 253 255 256 255 260
Half-life of the most common isotope 21.8 years 14 billion years 32,500 years 4.47 billion years 2.14 million years 24,100 years 433 years 18.1 years 320 days 2.64 years 20.47 days 20.07 hours 78 min 3.1 min 2.7 min
Electronic configuration in
the ground state (gas phase)
6d17s2 6d27s2 5f26d17s2 or
5f16d27s2
5f36d17s2 5f46d17s2 or
5f57s2
5f67s2 5f77s2 5f76d17s2 5f97s2 or
5f86d17s2
5f107s2 5f117s2 5f127s2 5f137s2 5f147s2 5f147s27p1
Electronic configuration in
the ground state (solid phase)
6d17s2 5f0.56d1.57s2 5f1.76d1.37s2 5f2.96d1.17s2 5f46d17s2 5f56d17s2 5f66d17s2 5f76d17s2 5f86d17s2 5f96d17s2 5f117s2 5f127s2 5f137s2 5f147s2 5f146d17s2
Oxidation states 2, 3 2, 3, 4 2, 3, 4, 5 2, 3, 4, 5, 6 3, 4, 5, 6, 7 3, 4, 5, 6, 7 2, 3, 4, 5, 6, 7 2, 3, 4, 6 2, 3, 4 2, 3, 4 2, 3, 4 2, 3 2, 3 2, 3 3
Metallic radius, nm 0.203 0.180 0.162 0.153 0.150 0.162 0.173 0.174 0.170 0.186 0.186 ? 0.198 ? 0.194 ? 0.197 ? 0.171
Ionic radius, nm:
An4+
An3+


0.126

0.114

0.104
0.118

0.103
0.118

0.101
0.116

0.100
0.115

0.099
0.114

0.099
0.112

0.097
0.110

0.096
0.109

0.085
0.098

0.084
0.091

0.084
0.090

0.084
0.095

0.083
0.088
Temperature, °C:
melting
boiling

1050
3198

1842
4788

1568
? 4027

1132.2
4131

639
? 4174

639.4
3228

1176
? 2607

1340
3110

986
2627

900
? 1470

860
? 996

1530

830

830

1630
Density, g/cm3 10.07 11.78 15.37 19.06 20.45 19.84 11.7 13.51 14.78 15.1 8.84 ? 9.7 ? 10.3 ? 9.9 ? 15.6
Standard electrode potential, V:
E° (An4+/An0)
E° (An3+/An0)


−2.13

−1.83

−1.47

−1.38
−1.66

−1.30
−1.79

−1.25
−2.00

−0.90
−2.07

−0.75
−2.06

−0.55
−1.96

−0.59
−1.97

−0.36
−1.98

−0.29
−1.96


−1.74


−1.20


−2.10
Color
[M(H2O)n]4+
[M(H2O)n]3+


Colorless

Colorless
Blue

Yellow
Dark blue

Green
Purple

Yellow-green
Purple

Brown
Violet

Red
Rose

Yellow
Colorless

Beige
Yellow-green

Green
Green


Pink








Approximate colors of actinide ions in aqueous solution. Colors for the last four actinides are unknown as sufficient quantities have not yet been synthesized.[84]
Oxidation state 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103
+2 Fm2+ Md2+ No2+
+3 Ac3+ Th3+ Pa3+ U3+ Np3+ Pu3+ Am3+ Cm3+ Bk3+ Cf3+ Es3+ Fm3+ Md3+ No3+ Lr3+
+4 Th4+ Pa4+ U4+ Np4+ Pu4+ Am4+ Cm4+ Bk4+ Cf4+
+5 PaO+
2
UO+
2
NpO+
2
PuO+
2
AmO+
2
+6 UO2+
2
NpO2+
2
PuO2+
2
AmO2+
2
+7 NpO3+
2
PuO3+
2
AmO3−
5
Comparison of ionic radii of lanthanides and actinides[86]
Lanthanides Ln3+, Å Actinides An3+, Å An4+, Å
Lanthanum 1.061 Actinium 1.11
Cerium 1.034 Thorium 1.08 0.99
Praseodymium 1.013 Protactinium 1.05 0.93
Neodymium 0.995 Uranium 1.03 0.93
Promethium 0.979 Neptunium 1.01 0.92
Samarium 0.964 Plutonium 1.00 0.90
Europium 0.950 Americium 0.99 0.89
Gadolinium 0.938 Curium 0.98 0.88
Terbium 0.923 Berkelium
Dysprosium 0.908 Californium
Holmium 0.894 Einsteinium
Erbium 0.881 Fermium
Thulium 0.869 Mendelevium
Ytterbium 0.858 Nobelium
Lutetium 0.848 Lawrencium
Oxides of actinides[30][38][62][93][94]
Compound Color Crystal symmetry, type Lattice constants, Å Density, g/cm3 Temperature, °C
a b c
Ac2O3 White Hexagonal, La2O3 4.07 - 6.29 9.19
PaO2 - Cubic, CaF2 5.505 - - - -
Pa2O5 White cubic, CaF2
Cubic
Tetragonal
Hexagonal
Rhombohedral
Orthorhombic
5.446
10.891
5.429
3.817
5.425
6.92
-
-
-
-
-
4.02
-
10.992
5.503
13.22
-
4. 18
- 700
700–1100
1000
1000–1200
1240–1400
ThO2 Colorless Cubic 5.59 - - 9.87
UO2 Black-brown Cubic 5.47 - - 10.9
NpO2 Greenish-brown Cubic, CaF2 5.424 - - 11.1
PuO Black Cubic, NaCl 4.96 - - 13.9
PuO2 Olive green Cubic 5.39 - - 11.44
Am2O3 Red-brown
Red-brown
Cubic, Mn2O3
Hexagonal, La2O3
11.03
3.817
- -
5.971
10.57
11.7
AmO2 Black Cubic, CaF2 5.376 - - - -
Cm2O3 White[95]
-
-
Cubic, Mn2O2
Hexagonal, LaCl3
Monoclinic, Sm2O3
11.01
3.80
14.28
-
-
3.65
-
6
8.9
11.7
CmO2 Black Cubic, CaF2 5.37 - - - -
Bk2O3 Light brown Cubic, Mn2O3 10.886 - - - -
BkO2 Red-brown Cubic, CaF2 5.33 - - - -
Cf2O3[96] Colorless
Yellowish
-
Cubic, Mn2O3
Monoclinic, Sm2O3
Hexagonal, La2O3
10.79
14.12
3.72
-
3.59
-
-
8.80
5.96
- -
CfO2 Black Cubic 5.31 - - - -
Es2O3 - Cubic, Mn2O3
Monoclinic
Hexagonal, La2O3
10.07
14.1
3.7
-
3.59
-
-
8.80
6
- -
Approximate colors of actinide oxides
(most stable are bolded)[97]
Oxidation state 89 90 91 92 93 94 95 96 97 98 99
+3 Pu2O3 Am2O3 Cm2O3 Bk2O3 Cf2O3 Es2O3
+4 ThO2 PaO2 UO2 NpO2 PuO2 AmO2 CmO2 BkO2 CfO2
+5 Pa2O5 U2O5 Np2O5
+6 U3O8
UO3
Dioxides of some actinides
Chemical formula ThO2 PaO2 UO2 NpO2 PuO2 AmO2 CmO2 BkO2 CfO2
CAS-number 1314-20-1 12036-03-2 1344-57-6 12035-79-9 12059-95-9 12005-67-3 12016-67-0 12010-84-3 12015-10-0
Molar mass 264.04 263.035 270.03 269.047 276.063 275.06 270–284** 279.069 283.078
Melting point[98] 3390 °C 2865 °C 2547 °C 2400 °C 2175 °C
Crystal structure CaF2 polyhedra
An4+: __  /  O2−: __
Space group Fm3m
Coordination number An[8], O[4]
Trichlorides of some actinides[100]
Chemical formula AcCl3 UCl3 NpCl3 PuCl3 AmCl3 CmCl3 BkCl3 CfCl3
CAS-number 22986-54-5 10025-93-1 20737-06-8 13569-62-5 13464-46-5 13537-20-7 13536-46-4 13536-90-8
Molar mass 333.386 344.387 343.406 350.32 349.42 344–358** 353.428 357.438
Melting point 837 °C 800 °C 767 °C 715 °C 695 °C 603 °C 545 °C
Boiling point 1657 °C 1767 °C 850 °C
Crystal structure
The crystal structure of uranium trichloride

An3+: __  /  Cl: __
Space group P63/m
Coordination number An*[9], Cl [3]
Lattice constants a = 762 pm
c = 455 pm
a = 745.2 pm
c = 432.8 pm
a = 739.4 pm
c = 424.3 pm
a = 738.2 pm
c = 421.4 pm
a = 726 pm
c = 414 pm
a = 738.2 pm
c = 412.7 pm
a = 738 pm
c = 409 pm
Actinide fluorides[38][62][94][100][101]
Compound Color Crystal symmetry, type Lattice constants, Å Density, g/cm3
a b c
AcF3 White Hexagonal, LaF3 4.27 - 7.53 7.88
PaF4 Dark brown Monoclinic 12.7 10.7 8.42
PaF5 Black Tetragonal, β-UF5 11.53 - 5.19
ThF4 Colorless Monoclinic 13 10.99 8.58 5.71
UF3 Reddish-purple Hexagonal 7.18 - 7.34 8.54
UF4 Green Monoclinic 11.27 10.75 8.40 6.72
α-UF5 Bluish Tetragonal 6.52 - 4.47 5.81
β-UF5 Bluish Tetragonal 11.47 - 5.20 6.45
UF6 Yellowish Orthorhombic 9.92 8.95 5.19 5.06
NpF3 Black or purple Hexagonal 7.129 - 7.288 9.12
NpF4 Light green Monoclinic 12.67 10.62 8.41 6.8
NpF6 Orange Orthorhombic 9.91 8.97 5.21 5
PuF3 Violet-blue Trigonal 7.09 - 7.25 9.32
PuF4 Pale brown Monoclinic 12.59 10.57 8.28 6.96
PuF6 Red-brown Orthorhombic 9.95 9.02 3.26 4.86
AmF3 Pink or light beige hexagonal, LaF3 7.04[73][102] - 7.255 9.53
AmF4 Orange-red Monoclinic 12.53 10.51 8.20
CmF3 From brown to white Hexagonal 4.041 - 7.179 9.7
CmF4 Yellow Monoclinic, UF4 12.51 10.51 8.20
BkF3 Yellow-green Trigonal, LaF3
Orthorhombic, YF3
6.97
6.7
-
7.09
7.14
4.41
10.15
9.7
BkF4 - Monoclinic, UF4 12.47 10.58 8.17
CfF3 -
-
Trigonal, LaF3
Orthorhombic, YF3
6. 94
6.65
-
7.04
7.10
4.39
CfF4 -
-
Monoclinic, UF4
Monoclinic, UF4
1.242
1.233
1.047
1.040
8.126
8.113
Periodic table forms
Sets of elements
Elements
History
See also

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