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The actinide () or actinoid () series encompasses at least the 14 metallic in the 5f series, with from 89 to 102, actinium through nobelium. Number 103, lawrencium, is also generally included despite being part of the 6d transition series. 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.
The 1985 IUPAC Red Book recommends that actinoid be used rather than actinide, since the suffix -ide normally indicates a negative ion. However, owing to widespread current use, actinide is still allowed.
Actinium through nobelium are f-block elements, while lawrencium is a d-block element and a transition metal. The series mostly corresponds to the filling of the 5f electron shell, although as isolated atoms in the ground state many have anomalous configurations involving the filling of the 6d shell due to interelectronic repulsion. In comparison with the , also mostly f-block elements, the actinides show much more variable valence. They all have very large atomic radius and ionic radius and exhibit an unusually large range of physical properties. While actinium and the late actinides (from curium onwards) behave similarly to the lanthanides, the elements thorium, protactinium, and uranium are much more similar to in their chemistry, with neptunium, plutonium, and americium 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 have been used in , and uranium and plutonium are critical elements of . Uranium and thorium also have diverse current or historical uses, and americium is used in the ionization chambers of most modern .
Due to their long half-lives, only thorium and uranium are found on Earth and astrophysically 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 . The other actinides are purely synthetic elements.Greenwood, p. 1250 Nuclear weapons tests have released at least six actinides heavier than plutonium into the environment; analysis of debris from the Ivy Mike of a hydrogen bomb showed the presence of americium, curium, berkelium, californium, and the discovery of einsteinium and fermium.
In presentations of the periodic table, 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 aesthetics 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).
| +Synthesis of transuranium elementsGreenwood, p. 1252Nobelium and lawrencium were almost simultaneously discovered by Soviet and American scientists ! Element !Year !Method | ||
| Neptunium | 1940 | Bombarding 238U with |
| Plutonium | 1941 | Bombarding 238U with |
| Americium | 1944 | Bombarding 239Pu with neutrons |
| Curium | 1944 | Bombarding 239Pu with Alpha particle |
| Berkelium | 1949 | Bombarding 241Am with α-particles |
| Californium | 1950 | Bombarding 242Cm with α-particles |
| Einsteinium | 1952 | As a product of nuclear explosion |
| Fermium | 1952 | As a product of nuclear explosion |
| Mendelevium | 1955 | Bombarding 253Es with α-particles |
| Nobelium | 1965 | Bombarding 243Am with 15N or 238U with 22Ne |
| Lawrencium | 1961 –1971 | Bombarding 252Cf with 10B or 11B and of 243Am with 18O |
Like the , 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.Myasoedov, p. 7
The existence of transuranium elements was suggested in 1934 by Enrico Fermi, 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 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 concept". 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 of transplutonium elements: (1) irradiation of the lighter elements with ; (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.Myasoedov, p. 9
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-life of Alpha 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.Myasoedov, p. 14
Thorium oxide was discovered by Friedrich Wöhler in the mineral thorianite, which was found in Norway (1827).Golub, p. 214 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 mythology of thunder and lightning Thor. (modern citation: Annalen der Physik, vol. 92, no. 7, pp. 385–415) The same isolation method was later used by Péligot for uranium.
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 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 Friedrich Oskar Giesel, who discovered a radioactive element named emanium that behaved similarly to lanthanum. The name actinium comes from the , meaning beam or ray. This metal was discovered not by its own radiation but by the radiation of the daughter products.Golub, p. 213 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.
Protactinium was possibly isolated in 1900 by William Crookes. 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 chain. They named the new element brevium (from Latin brevis meaning brief); the name was changed to protoactinium (from Greek language 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 Arnold 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 Alfred 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
Transuranium elements do not occur in sizeable quantities in nature and are commonly synthesized via conducted with nuclear reactors. For example, under irradiation with reactor neutrons, uranium-238 partially converts to plutonium-239:
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