Actinide

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. Most do not 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 in 1934 by Enrico Fermi, based on his experiments.[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 an "actinide hypothesis". Studies of known actinides and discoveries of further transuranic elements provided more data in support of this position, 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 neutrons; (2) irradiation with accelerated charged particles. The first method is more 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 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,[8] which had been discovered 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] Sixty 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 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.[8]

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 Ancient Greek: ακτίς, ακτίνος (aktis, aktinos), 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 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:

${\displaystyle {\ce {{^{238}_{92}U}+{}_{0}^{1}n->{}_{92}^{239}U->[\beta ^{-}][23.5\ {\ce {min}}]{}_{93}^{239}Np->[\beta ^{-}][2.3\ {\ce {days}}]{}_{94}^{239}Pu}}\left({\ce {->[\alpha ][2.4\cdot 10^{4}\ {\ce {years}}]}}\right){\ce {^{235}_{92}U}}}$

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. Thus nobelium was produced by bombarding uranium-238 with neon-22 as

${\displaystyle {\ce {_{92}^{238}U + _{10}^{22}Ne -> _{102}^{256}No + 4_0^1n}}}$.

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

${\displaystyle {\ce {_{94}^{239}Pu + _2^4He -> _{96}^{242}Cm + _0^1n}}}$.

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 few 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 in 1958 (Burris B. Cunningham and Stanley G. Thomson),[50] and the first californium compound (0.3 µg of CfOCl) was obtained 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 US military until 1955 due to Cold War tensions.[9][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 those 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.[10]

In 1961, Ghiorso et al. obtained the first isotope of lawrencium by irradiating californium (mostly californium-252) with boron-10 and boron-11 ions.[10] 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.

Actinides have 89–103 protons and usually 117–159 neutrons.

32 isotopes of actinium and eight excited isomeric states of some of its nuclides were identified by 2016.[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;[60] 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 2×10⁻¹⁰ gram of ²²⁷Ac.[30][57] Actinium-228 is a member of the radioactive thorium series formed by the decay of ²²⁸Ra;[60] it is a β⁻ emitter with a half-life of 6.15 hours. In one tonne of thorium there is 5×10⁻⁸ gram of ²²⁸Ac. It was discovered by Otto Hahn in 1906.[30]

There are 31 known isotopes of thorium ranging in mass number from 208 to 238.[57] Of these, the longest-lived is ²³²Th, whose half-life of 1.4×10¹⁰ years means that it still exists in nature as a primordial nuclide. The next longest-lived is ²³⁰Th, an intermediate decay product of ²³⁸U with a half-life of 75,400 years. Several other thorium isotopes have half-lives over a day; all of these are also transient in the decay chains of ²³²Th, ²³⁵U, and ²³⁸U.

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 lifetimes, 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][61]

There are 26 known isotopes of uranium, having mass numbers 215–242 (except 220 and 241).[58] 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 transformation 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.[62]

There are 24 isotopes of neptunium with mass numbers of 219, 220, and 223–244;[58] they are all highly radioactive. The most popular among scientists are long-lived ²³⁷Np (t_1/2 = 2.20×10⁶ years) and short-lived ²³⁹Np, ²³⁸Np (t_1/2 ~ 2 days).[38]

There are 20 known isotopes of plutonium, having mass numbers 228–247.[58] The most stable isotope of plutonium is ²⁴⁴Pu with half-life of 8.13×10⁷ years.[63]

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, ranging in mass number from 233 to 251,[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.

Seventeen isotopes of berkelium were identified with mass numbers 233–234, 236, 238, and 240–252.[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 (1.45×10⁻³% 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]

The 20 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]

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 (t_1/2 = 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 (t_1/2 = 100 days) can accumulate upon prolonged and strong irradiation. All these isotopes are characterized by high rates of spontaneous fission.[65][67]

Among the 17 known isotopes of mendelevium (mass numbers from 244 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 (t_1/2 = 3 min) and ²⁵⁶Lr (t_1/2 = 35 s). The longest-lived nobelium isotope, ²⁵⁹No, has a half-life of approximately 1 hour.[57] Lawrencium has 13 known isotopes with mass numbers 251–262 and 266. The most stable of them all is ²⁶⁶Lr with a half life of 11 hours.

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]

Table of nuclides: Buildup of actinides in a nuclear reactor, including radioative decay

The figure buildup of actinides is a table of nuclides with the number of neutrons on the horizontal axis (isotopes) and the number of protons on the vertical axis (elements). The red dot divides the nuclides in two groups, so the figure is more compact. Each nuclide is represented by a square with the mass number of the element and its half-life.[69] Naturally existing actinide isotopes (Th, U) are marked with a bold border, alpha emitters have a yellow colour, and beta emitters have a blue colour. Pink indicates electron capture (²³⁶Np), whereas white stands for a long-lasting metastable state (²⁴²Am).

The formation of actinide nuclides is primarily characterised by:[70]

• Neutron capture reactions (n,γ), which are represented in the figure by a short right arrow.
• The (n,2n) reactions and the less frequently occurring (γ,n) reactions are also taken into account, both of which are marked by a short left arrow.
• Even more rarely and only triggered by fast neutrons, the (n,3n) reaction occurs, which is represented in the figure with one example, marked by a long left arrow.

In addition to these neutron- or gamma-induced nuclear reactions, the radioactive conversion of actinide nuclides also affects the nuclide inventory in a reactor. These decay types are marked in the figure by diagonal arrows. The beta-minus decay, marked with an arrow pointing up-left, plays a major role for the balance of the particle densities of the nuclides. Nuclides decaying by positron emission (beta-plus decay) or electron capture (ϵ) do not occur in a nuclear reactor except as products of knockout reactions; their decays are marked with arrows pointing down-right. Due to the long half-lives of the given nuclides, alpha decay plays almost no role in the formation and decay of the actinides in a power reactor, as the residence time of the nuclear fuel in the reactor core is rather short (a few years). Exceptions are the two relatively short-lived nuclides ²⁴²Cm (T_1/2 = 163 d) and ²³⁶Pu (T_1/2 = 2.9 y). Only for these two cases, the α decay is marked on the nuclide map by a long arrow pointing down-left. A few long-lived actinide isotopes, such as ²⁴⁴Pu and ²⁵⁰Cm, cannot be produced in reactors because neutron capture does not happen quickly enough to bypass the short-lived beta-decaying nuclides ²⁴³Pu and ²⁴⁹Cm; they can however be generated in nuclear explosions, which have much higher neutron fluxes.

Unprocessed uranium ore

Thorium and uranium are the most abundant actinides in nature with the respective mass concentrations of 16 ppm and 4 ppm.[71] Uranium mostly occurs in the Earth's crust as a mixture of its oxides in the mineral 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.[72][73] 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%).[74]

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).[76]

The abundance of actinium in the Earth's crust is only about 5×10⁻¹⁵%.[61] 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.[61]

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 3×10⁻²⁰%. Plutonium could not be detected in samples of lunar soil. Owing to its scarcity in nature, most plutonium is produced synthetically.[75]

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:[77]

${\displaystyle {\begin{array}{l}{}\\{\ce {2AmF3{}+3Ba->[{\ce {1150-1350^{\circ }C}}]3BaF2{}+2Am}}\\{\ce {PuF4{}+2Ba->[{\ce {1200^{\circ }C}}]2BaF2{}+Pu}}\\{\ce {UF4{}+2Mg->[{\ce {>500^{\circ }C}}]U{}+2MgF2}}\\{}\end{array}}}$

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.[77]

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:[77]

Separation of uranium and plutonium from spent nuclear fuel using the PUREX process[78]

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:[79]

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.[80]

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.[77]

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.[77]

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:[79]

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.[77]

In another method, Pu⁴⁺ and UO²⁺
₂ are first extracted with tributyl phosphate, then reacted with hydrazine washing out the recovered plutonium.[77]

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]

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.[81] As in lanthanides (see lanthanide contraction), the ionic radius of actinides monotonically decreases with atomic number (see also Aufbau principle).[82]

Physical properties

Major crystal structures of some actinides vs. temperature	Metallic and ionic radii of actinides[83]

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.

Californium

Actinides are typical metals. All of them are soft and have a silvery color (but tarnish in air),[88] relatively high density and plasticity. Some of them can be cut with a knife. Their electrical resistivity varies between 15 and 150 µΩ·cm.[83] 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.[73]

All actinides are pyrophoric, especially when finely divided, that is, they spontaneously ignite upon reaction with air at room temperature.[88][89] 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.[73]

Comparison of ionic radii of lanthanides and actinides[90]

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	–	–

Interior of a smoke detector containing americium-241.

While actinides have some established daily-life applications, such as in smoke detectors (americium)[108][109] and gas mantles (thorium),[79] they are mostly used in nuclear weapons and as fuel in nuclear reactors.[79] 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 reactions.

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
₉₂U
emits more neutrons than it absorbs;[110] upon reaching the critical mass, ²³⁵
₉₂U
enters into a self-sustaining chain reaction.[73] Typically, uranium nucleus is divided into two fragments with the release of 2–3 neutrons, for example:

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

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

Nuclear reactor[73][111][112]

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.[79] 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.[110] The relative content of thorium and uranium isotopes is widely used to estimate the age of various objects, including stars (see radiometric dating).[113]

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.[114] 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.[115] (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.[116]

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)[109][117] by decay even when the fission chain reaction is stopped by control rods. Its application is limited by its 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[118]) 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.[109]

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.[119]

Schematic illustration of penetration of radiation through sheets of paper, aluminium and lead brick

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 known 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.[61]

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 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.[93]

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.[120] 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.[119]

• Actinides in the environment
• Lanthanides
• Major actinides
• Minor actinides
• Transuranics

• Golub, A. M. (1971). Общая и неорганическая химия (General and Inorganic Chemistry). Vol. 2.
• Greenwood, Norman N.; Earnshaw, Alan (1997). Chemistry of the Elements (2nd ed.). Butterworth-Heinemann. ISBN 978-0-08-037941-8.
• Myasoedov, B. (1972). Analytical chemistry of transplutonium elements. Moscow: Nauka. ISBN 978-0-470-62715-0.

Wikimedia Commons has media related to Actinides.

• Lawrence Berkeley Laboratory image of historic periodic table by Seaborg showing actinide series for the first time
• Lawrence Livermore National Laboratory, Uncovering the Secrets of the Actinides
• Los Alamos National Laboratory, Actinide Research Quarterly