The actinoid (, also called actinide ) series encompasses the 15 metallic s with s from 89 to 103, through . The actinoid series derives its name from the first element in the series, actinium. The informal chemical symbol An is used in general discussions of actinoid chemistry to refer to any actinoid. Since "actinoid" means "actinium-like" (cf. humanoid or android), it has been argued for semantic reasons that actinium cannot logically be an actinoid, but acknowledges its inclusion based on common usage. All the actinides are elements, except the final one (lawrencium) which is a element. Actinium has sometimes been considered d-block instead of lawrencium, but the classification with lawrencium in the d-block is more often adopted by those who study the matter. The series mostly corresponds to the filling of the 5f , although in the ground state many have anomalous configurations involving the filling of the 6d shell due to interelectronic repulsion. In comparison with the s, also mostly elements, the actinides show much more variable . They all have very large and 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 s in their chemistry, with neptunium and plutonium occupying an intermediate position. All actinides are and release energy upon radioactive decay; naturally occurring and , and synthetically produced are the most abundant actinides on Earth. These are used in s and s. Uranium and thorium also have diverse current or historical uses, and is used in the s of most modern s. Of the actinides, and occur naturally in substantial quantities. The radioactive decay of uranium produces transient amounts of and , and atoms of and plutonium are occasionally produced from reactions in s. The other actinides are purely s.Greenwood, p. 1250 Nuclear weapons tests have released at least six actinides heavier than into the ; analysis of debris from a 1952 explosion showed the presence of , , , , and . In presentations of the , the f-block elements are customarily shown as two additional rows below the main body of the table. This convention is entirely a matter of and formatting practicality; a rarely used wide-formatted periodic table inserts the 4f and 5f series in their proper places, as parts of the table's sixth and seventh rows (periods).

Discovery, isolation and synthesis

Like the s, the actinides form a family of elements with similar properties. Within the actinides, there are two overlapping groups: s, which follow uranium in the ; and , which follow plutonium. Compared to the lanthanides, which (except for ) 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. The existence of transuranium elements was suggested in 1934 by , based on his experiments. 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 , and , 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, , can reach oxidation state of 6) prompted to formulate an "". 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. At present, there are two major methods of producing s of transplutonium elements: (1) irradiation of the lighter elements with s; (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. In 1962–1966, there were attempts in the United States to produce transplutonium isotopes using a series of six . Small samples of rock were extracted from the blast area immediately after the test to study the explosion products, but no isotopes with greater than 257 could be detected, despite predictions that such isotopes would have relatively long of . This non-observation was attributed to owing to the large speed of the products and to other decay channels, such as neutron emission and .

From actinium to uranium

and were the first actinides . Uranium was identified in 1789 by the German chemist in ore. He named it after the planet , which had been discovered eight years earlier. Klaproth was able to precipitate a yellow compound (likely ) by dissolving in and neutralizing the solution with . He then reduced the obtained yellow powder with charcoal, and extracted a black substance that he mistook for metal. Sixty years later, the French scientist identified it as uranium oxide. He also isolated the first sample of uranium metal by heating with metallic . The of uranium was then calculated as 120, but in 1872 corrected it to 240 using his periodicity laws. This value was confirmed experimentally in 1882 by K. Zimmerman. was discovered by in the mineral , which was found in Norway (1827). 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 of thunder and lightning . The same isolation method was later used by Péligot for uranium. was discovered in 1899 by , an assistant of , in the pitchblende waste left after removal of radium and polonium. He described the substance (in 1899) as similar to and (in 1900) as similar to thorium. The discovery of actinium by Debierne was however questioned in 1971 and 2000, arguing that Debierne's publications in 1904 contradicted his earlier work of 1899–1900. This view instead credits the 1902 work of , 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. 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 in 1937. was possibly isolated in 1900 by . It was first identified in 1913, when 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); the name was changed to ''protoactinium'' (from πρῶτος + ἀκτίς meaning "first beam element") in 1918 when two groups of scientists, led by the Austrian and of Germany and 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.Greenwood, p. 1251

Neptunium and above

Neptunium (named for the planet , the next out from Uranus, after which uranium was named) was discovered by and in 1940 in . They produced the 239Np isotope (half-life = 2.4 days) by bombarding uranium with slow s. It was the first produced synthetically. Transuranium elements do not occur in sizeable quantities in nature and are commonly synthesized via s conducted with nuclear reactors. For example, under irradiation with reactor neutrons, partially converts to : : \ce \left( \ce \right) \ce This synthesis reaction was used by Fermi and his collaborators in their design of the reactors located at the , which produced significant amounts of plutonium-239 for the nuclear weapons of the and the United States' post-war nuclear arsenal. Actinides with the highest mass numbers are synthesized by bombarding uranium, plutonium, curium and californium with s of nitrogen, oxygen, carbon, neon or boron in a . Thus was produced by bombarding uranium-238 with as : _^U + _^Ne -> _^No + 4_0^1n. The first isotopes of transplutonium elements, and , were synthesized in 1944 by Glenn T. Seaborg, Ralph A. James and . Curium-242 was obtained by bombarding plutonium-239 with 32-MeV α-particles : _^Pu + _2^4He -> _^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 and her husband who are noted for discovering and for their work in . Bombarding curium-242 with α-particles resulted in an isotope of californium (1950), and a similar procedure yielded in 1949 from americium-241. The new elements were named after , by analogy with its , which was named after the village of in Sweden. In 1945, B. B. Cunningham obtained the first bulk chemical compound of a transplutonium element, namely . Over the few years, milligram quantities of americium and microgram amounts of curium were accumulated that allowed production of isotopes of berkelium (Thomson, 1949) and californium (Thomson, 1950). Sizeable amounts of these elements were produced in 1958 (Burris B. Cunningham and Stanley G. Thomson), and the first californium compound (0.3 µg of CfOCl) was obtained in 1960 by B. B. Cunningham and J. C. Wallmann. Einsteinium and fermium were identified in 1952–1953 in the fallout from the "" 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 yielded and . 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 tensions. 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. The "Ivy Mike" studies were declassified and published in 1955. The first significant (submicrograms) amounts of einsteinium were produced in 1961 by Cunningham and colleagues, but this has not been done for fermium yet. The first isotope of mendelevium, (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 s in the 60-inch of ; this was the first isotope of any element to be synthesized one atom at a time. 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 by the Russian group ( ''et al.'') in 1965, as acknowledged by the in 1992. In their experiments, Flyorov ''et al.'' bombarded uranium-238 with neon-22. In 1961, Ghiorso ''et al.'' obtained the first isotope of lawrencium by irradiating californium (mostly ) with and ions. The of this isotope was not clearly established (possibly 258 or 259) at the time. In 1965, was synthesized by Flyorov ''et al.'' from and . Thus IUPAC recognized the nuclear physics teams at Dubna and Berkeley as the co-discoverers of lawrencium.


32 and eight excited of some of its s were identified by 2016. Three isotopes, , and , 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 ;Greenwood, p. 1254 it was first discovered in 1947 as a decay product of , 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. 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 gram of 227Ac. Actinium-228 is a member of the formed by the decay of ; it is a β emitter with a half-life of 6.15 hours. In one tonne of thorium there is 5 gram of 228Ac. It was discovered by in 1906. There are 31 known ranging in mass number from 208 to 238. Of these, the longest-lived is 232Th, whose half-life of means that it still exists in nature as a . 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 are known with mass numbers 212–239 as well as three excited . Only and 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 , 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 , and thus was used in most studies of protactinium chemistry. Protactinium-233 is a with a half-life of 26.97 days. There are 26 known , having mass numbers 215–242 (except 220 and 241). Three of them, , 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 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 years. There are 24 with mass numbers of 219, 220, and 223–244; they are all highly radioactive. The most popular among scientists are long-lived 237Np (t1/2 = 2.20 years) and short-lived 239Np, 238Np (t1/2 ~ 2 days). Eighteen are known with mass numbers from 229 to 247 (with the exception of 231). 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.Myasoedov, p. 18 Among 19 , ranging in mass number from 233 to 251, 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 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 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 years), but is not formed in large quantities because of the strong fission induced by thermal neutrons. Seventeen were identified with mass numbers 233–234, 236, 238, and 240–252. Only 249Bk is available in large quantities; it has a relatively short half-life of 330 days and emits mostly soft , which are inconvenient for detection. Its is rather weak (1.45% 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. The 20 with mass numbers 237–256 are formed in nuclear reactors; 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.Myasoedov, p. 22 Among the 18 known with mass numbers from 240 to 257, 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). Twenty are known with mass numbers of 241–260. 254Fm, 255Fm and 256Fm are 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. Among the 17 known (mass numbers from 244 to 260), 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. Long-lived and (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. 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.

Formation in nuclear reactors

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-time. 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 (242Am). The formation of actinide nuclides is primarily characterised by: * 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 , marked with an arrow pointing up-left, plays a major role for the balance of the particle densities of the nuclides. Nuclides decaying by (beta-plus decay) or (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, 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 (T1/2 = 163 d) and 236Pu (T1/2 = 2.9 y). Only for these two cases, the α decay is marked on the nuclide map by a long arrow pointing down-left.

Distribution in nature

Thorium and uranium are the most abundant actinides in nature with the respective mass concentrations of 16 ppm and 4 ppm. Uranium mostly occurs in the Earth's crust as a mixture of its oxides in the mineral , which is also called pitchblende because of its black color. There are several dozens of other such as (KUO2VO4·3H2O) and (Ca(UO2)2(PO4)2·nH2O). The isotopic composition of natural uranium is (relative abundance 99.2742%), (0.7204%) and (0.0054%); of these 238U has the largest half-life of 4.51 years. The worldwide production of uranium in 2009 amounted to 50,572 s, of which 27.3% was mined in . Other important uranium mining countries are Canada (20.1%), Australia (15.7%), (9.1%), (7.0%), and (6.4%). The most abundant are (ThO2), (ThSiO4) and , ((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). The abundance of actinium in the Earth's crust is only about 5%. 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. 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. As actinium, the distribution of protactinium follows that of 235U. The half-life of the longest-lived isotope of neptunium, , 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. 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%. Plutonium could not be detected in samples of lunar soil. Owing to its scarcity in nature, most plutonium is produced synthetically.


Owing to the low abundance of actinides, their extraction is a complex, multistep process. s of actinides are usually used because they are insoluble in water and can be easily separated with reactions. Fluorides are reduced with , or :Golub, pp. 215–217 : \begin\\ \ce\\ \ce\\ \ce\\ \end Among the actinides, thorium and uranium are the easiest to isolate. Thorium is extracted mostly from : thorium (ThP2O7) is reacted with , and the produced thorium nitrate treated with . impurities are separated by increasing the in sulfate solution. In another extraction method, monazite is decomposed with a 45% aqueous solution of at 140 °C. Mixed metal hydroxides are extracted first, filtered at 80 °C, washed with water and dissolved with concentrated . 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 s. An efficient method is the dissolution of thorium hydroxide in nitric acid, because the resulting solution can be purified by with organic solvents: :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: :ThO2 + 2 Ca → 2 CaO + Th Sometimes thorium is extracted by of a fluoride in a mixture of sodium and potassium chloride at 700–800 °C in a crucible. Highly pure thorium can be extracted from its iodide with the . 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 as UO4·2H2O. When the uranium ore is rich in such minerals as , , 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 , which converts uranium into a complex O2(CO3)3sup>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 (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 → 2 O2(CO3)3sup>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 in the presence of nickel yielding an insoluble uranium tetracarbonate. Another separation method uses polymeric resins as a . Ion exchange processes in the resins result in separation of uranium. Uranium from resins is washed with a solution of or nitric acid that yields 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 changes it to , which yields uranium metal upon reaction with magnesium metal: : 4 HF + UO2 → UF4 + 2 H2O To extract plutonium, neutron-irradiated uranium is dissolved in nitric acid, and a reducing agent (, or ) 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 to pH = 8 that results in precipitation of Pu4+ compounds. In another method, Pu4+ and are first extracted with tributyl phosphate, then reacted with washing out the recovered plutonium. 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.


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

Physical properties

Actinides are typical metals. All of them are soft and have a silvery color (but tarnish in air),Greenwood, p. 1264 relatively high and plasticity. Some of them can be cut with a knife. Their varies between 15 and 150 µOhm·cm. 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, , and, with the exception of actinium, have several crystalline phases: plutonium has seven, and uranium, neptunium and californium three. The s of protactinium, uranium, neptunium and plutonium do not have clear analogs among the lanthanides and are more similar to those of the 3''d''-s. All actinides are , especially when finely divided, that is, they spontaneously ignite upon reaction with air at room temperature. The 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 of 5''f'' and 6''d'' orbitals and the formation of directional bonds in these metals.

Chemical properties

Like the lanthanides, all actinides are highly reactive with s and s; however, the actinides react more easily. Actinides, especially those with a small number of 5''f''-electrons, are prone to . This is explained by the similarity of the electron energies at the 5''f'', 7''s'' and 6''d'' 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.Golub, pp. 222–227 Actinium is chemically similar to lanthanum, which is explained by their similar ionic radii and electronic structures. Like lanthanum, actinium almost always has an oxidation state of +3 in compounds, but it is less reactive and has more pronounced properties. Among other trivalent actinides Ac3+ is least acidic, i.e. has the weakest tendency to hydrolyze in aqueous solutions. Thorium is rather active chemically. Owing to lack of s on 6''d'' and 5''f'' orbitals, the tetravalent thorium compounds are colorless. At pH < 3, the solutions of thorium salts are dominated by the cations h(H2O)8sup>4+. The Th4+ ion is relatively large, and depending on the 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 both in water and polar organic solvents. 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 (V) and (V). The complex chemical behavior of protactinium is a consequence of the start of the filling of the 5''f'' shell in this element. Uranium has a valence from 3 to 6, the last being most stable. In the hexavalent state, uranium is very similar to the s. Many compounds of uranium(IV) and uranium(VI) are , 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 . Most of them contain the linear "" group, . Between 4 and 6 ligands can be accommodated in an equatorial plane perpendicular to the uranyl group. The uranyl group acts as a and forms stronger complexes with oxygen-donor ligands than with nitrogen-donor ligands. and 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 s, such as UIII(C5H5)3 and UIV(C5H5)4.Greenwood, p. 1278 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 s. 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 even at temperatures as low as 25–50 °C; it also easily forms s and s. Hydrolysis reactions of plutonium ions of different oxidation states are quite diverse. Plutonium(V) can enter reactions. 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 (). 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.Myasoedov, pp. 25–29 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 4+. 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. The redox potential \mathit E_\frac 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 s with the general formula of AnC or AnC2 (U2C3 for uranium) as well as sulfides An2S3 and AnS2. File:Uranyl Nitrate.jpg, Uranyl nitrate (UO2(NO3)2) File:U Oxstufen.jpg, Aqueous solutions of uranium III, IV, V, VI salts File:Np ox st .jpg, Aqueous solutions of neptunium III, IV, V, VI, VII salts File:Plutonium in solution.jpg, Aqueous solutions of plutonium III, IV, V, VI, VII salts File:UCl4.jpg, File:Uranium hexafluoride crystals sealed in an ampoule.jpg, File:Yellowcake.jpg, U3O8 (yellowcake)


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 and An2O3, AnO2 and An2O5 are basic, they easily react with water, forming bases: : An2O3 + 3 H2O → 2 An(OH)3. These bases are poorly soluble in water and by their activity are close to the s of rare-earth metals. Np(OH)3 has not yet been synthesized, Pu(OH)3 has a blue color while Am(OH)3 is pink and Cm(OH)3 is colorless. 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. The strongest base is of actinium. All compounds of actinium are colorless, except for black (Ac2S3). Dioxides of tetravalent actinides crystallize in the , same as in . Thorium reacting with oxygen exclusively forms the dioxide: : Th + O2 ->[\ce] \overbrace^ Thorium dioxide is a refractory material with the highest melting point among any known oxide (3390 °C). 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 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. Decomposition of certain salts of uranium, for example UO2(NO3)·6H2O in air at 400 °C, yields orange or yellow UO3. This oxide is amphoteric and forms several hydroxides, the most stable being 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). 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.


: *An – actinide
**Depending on the isotopes
Actinides easily react with halogens forming salts with the formulas MX3 and MX4 (X = ). So the first berkelium compound, BkCl3, was synthesized in 1962 with an amount of 3 nanograms. Like the halogens of rare earth elements, actinide s, s, and s are water-soluble, and s are insoluble. Uranium easily yields a colorless hexafluoride, which at a temperature of 56.5 °C; because of its volatility, it is used in the separation of uranium isotopes with or . Actinide hexafluorides have properties close to s. They are very sensitive to moisture and hydrolyze forming AnO2F2.Greenwood, p.1269 The pentachloride and black hexachloride of uranium were synthesized, but they are both unstable. 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. 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 s, s, s, s and s of actinides are weakly soluble in water; they precipitate as hydrates, such as ThF4·3H2O and Th(CrO4)2·3H2O. 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 s, 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 s (for example Na2SO4·Th(SO4)2·5H2O), s and thiocyanates. Salts with the general formula An2Th(NO3)6·''n''H2O are of coordination nature, with the 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. .


While actinides have some established daily-life applications, such as in smoke detectors (americium)Greenwood, p. 1262 and s (thorium),Greenwood, p. 1255 they are mostly used in s and as in nuclear reactors. The last two areas exploit the property of actinides to release enormous energy in nuclear reactions, which under certain conditions may become self-sustaining . The most important isotope for applications is . It is used in the , and its concentration in natural uranium does not exceed 0.72%. This isotope strongly absorbs s releasing much energy. One fission act of 1 gram of 235U converts into about 1 MW·day. Of importance, is that emits more neutrons than it absorbs;Golub, pp. 220–221 upon reaching the , enters into a self-sustaining chain reaction. Typically, uranium nucleus is divided into two fragments with the release of 2–3 neutrons, for example: : + ⟶ + + 3 Other promising actinide isotopes for nuclear power are and its product from the , . 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. converts via 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. Thorium is also added into multicomponent s of and . 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 s. Thorium also has good properties, with long lifetime and low potential barrier for the emission. The relative content of thorium and uranium isotopes is widely used to estimate the age of various objects, including stars (see ). The major application of plutonium has been in s, where the isotope plutonium-239 was a key component due to its ease of fission and availability. Plutonium-based designs allow reducing the to about a third of that for uranium-235. The "Fat Man"-type plutonium bombs produced during the 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 equivalent to 20 kilotons of . (See also .) 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. 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) 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 s and water systems of some space satellites and stations. So and spacecraft (e.g. ) 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. 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 in chemical research, as it emits high-energy electrons (2.18 MeV) that can be easily detected. - mixtures are widely used as an intense gamma-source in industry and medicine. 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.


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 and . Together with radium and transuranium elements, actinium is one of the most dangerous radioactive poisons with high specific . The most important feature of actinium is its ability to accumulate and remain in the surface layer of s. At the initial stage of poisoning, actinium accumulates in the . Another danger of actinium is that it undergoes radioactive decay faster than being excreted. from the digestive tract is much smaller (~0.05%) for actinium than radium. 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 that corresponds to 0.5 micrograms of 231Pa. This isotope, which might be present in the air as , is 2.5 times more toxic than . 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 (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 , 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 , 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. 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. 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.

Nuclear properties

See also

* * * *

References and notes


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External links

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''
{{Authority control Actinides