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120-405: Of the elements with atomic numbers 1 to 92, most can be found in nature, having stable isotopes (such as oxygen ) or very long-lived radioisotopes (such as uranium ), or existing as common decay products of the decay of uranium and thorium (such as radon ). The exceptions are technetium , promethium , astatine , and francium ; all four occur in nature, but only in very minor branches of

240-455: A = m 1 x 1 + m 2 x 2 + . . . + m N x N {\displaystyle {\overline {m}}_{a}=m_{1}x_{1}+m_{2}x_{2}+...+m_{N}x_{N}} where m 1 , m 2 , ..., m N are the atomic masses of each individual isotope, and x 1 , ..., x N are the relative abundances of these isotopes. Several applications exist that capitalize on

360-511: A chemical symbol is used, e.g. "C" for carbon, standard notation (now known as "AZE notation" because A is the mass number , Z the atomic number , and E for element ) is to indicate the mass number (number of nucleons) with a superscript at the upper left of the chemical symbol and to indicate the atomic number with a subscript at the lower left (e.g. 2 He , 2 He , 6 C , 6 C , 92 U , and 92 U ). Because

480-409: A hydrocarbon . The lanthanides and remaining actinides are then separated from the aqueous residue ( raffinate ) by a diamide -based extraction, to give, after stripping, a mixture of trivalent actinides and lanthanides. Americium compounds are then selectively extracted using multi-step chromatographic and centrifugation techniques with an appropriate reagent. A large amount of work has been done on

600-417: A bare Am sphere is about 9–14 kg (the uncertainty results from insufficient knowledge of its material properties). It can be lowered to 3–5 kg with a metal reflector and should become even smaller with a water reflector. Such small critical mass is favorable for portable nuclear weapons , but those based on Am are not known yet, probably because of its scarcity and high price. The critical masses of

720-458: A cubic ( fluorite ) crystal structure. The oxalate of americium(III), vacuum dried at room temperature, has the chemical formula Am 2 (C 2 O 4 ) 3 ·7H 2 O. Upon heating in vacuum, it loses water at 240 °C and starts decomposing into AmO 2 at 300 °C, the decomposition completes at about 470 °C. The initial oxalate dissolves in nitric acid with the maximum solubility of 0.25 g/L. Halides of americium are known for

840-403: A density of 12 g/cm , americium is less dense than both curium (13.52 g/cm ) and plutonium (19.8 g/cm ); but has a higher density than europium (5.264 g/cm )—mostly because of its higher atomic mass. Americium is relatively soft and easily deformable and has a significantly lower bulk modulus than the actinides before it: Th, Pa, U, Np and Pu. Its melting point of 1173 °C

960-505: A double pairing of 2 protons and 2 neutrons prevents any nuclides containing five ( 2 He , 3 Li ) or eight ( 4 Be ) nucleons from existing long enough to serve as platforms for the buildup of heavier elements via nuclear fusion in stars (see triple alpha process ). Only five stable nuclides contain both an odd number of protons and an odd number of neutrons. The first four "odd-odd" nuclides occur in low mass nuclides, for which changing

1080-437: A double- hexagonal close packing with the layer sequence ABAC and so is isotypic with α-lanthanum and several actinides such as α-curium. The crystal structure of americium changes with pressure and temperature. When compressed at room temperature to 5 GPa, α-Am transforms to the β modification, which has a face-centered cubic ( fcc ) symmetry, space group Fm 3 m and lattice constant a  = 489 pm. This fcc structure

1200-458: A drift of some material properties over time, more noticeable in older samples. Although americium was likely produced in previous nuclear experiments, it was first intentionally synthesized , isolated and identified in late autumn 1944, at the University of California, Berkeley , by Glenn T. Seaborg , Leon O. Morgan, Ralph A. James , and Albert Ghiorso . They used a 60-inch cyclotron at

1320-466: A general trend of decreasing as atomic numbers increase. There are exceptions, however, including several isotopes of curium and dubnium . Some heavier elements in this series, around atomic numbers 110–114, are thought to break the trend and demonstrate increased nuclear stability, comprising the theoretical island of stability . Transuranic elements are difficult and expensive to produce, and their prices increase rapidly with atomic number. As of 2008,

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1440-419: A given element all have the same number of electrons and share a similar electronic structure. Because the chemical behaviour of an atom is largely determined by its electronic structure, different isotopes exhibit nearly identical chemical behaviour. The main exception to this is the kinetic isotope effect : due to their larger masses, heavier isotopes tend to react somewhat more slowly than lighter isotopes of

1560-407: A glowing patch on the plate at the point it struck. Thomson observed two separate parabolic patches of light on the photographic plate (see image), which suggested two species of nuclei with different mass-to-charge ratios. He wrote "There can, therefore, I think, be little doubt that what has been called neon is not a simple gas but a mixture of two gases, one of which has an atomic weight about 20 and

1680-617: A half-life of 7,370 years and is the most stable isotope, and Am has a half-life of 432.2 years. The most stable nuclear isomer is Am; it has a long half-life of 141 years. The half-lives of other isotopes and isomers range from 0.64 microseconds for Am to 50.8 hours for Am. As with most other actinides, the isotopes of americium with odd number of neutrons have relatively high rate of nuclear fission and low critical mass. Americium-241 decays to Np emitting alpha particles of 5 different energies, mostly at 5.486 MeV (85.2%) and 5.443 MeV (12.8%). Because many of

1800-468: A nonoptimal number of neutrons or protons decay by beta decay (including positron emission ), electron capture , or other less common decay modes such as spontaneous fission and cluster decay . Most stable nuclides are even-proton-even-neutron, where all numbers Z , N , and A are even. The odd- A stable nuclides are divided (roughly evenly) into odd-proton-even-neutron, and even-proton-odd-neutron nuclides. Stable odd-proton-odd-neutron nuclides are

1920-427: A nucleus. As the number of protons increases, so does the ratio of neutrons to protons necessary to ensure a stable nucleus (see graph at right). For example, although the neutron:proton ratio of 2 He is 1:2, the neutron:proton ratio of 92 U is greater than 3:2. A number of lighter elements have stable nuclides with the ratio 1:1 ( Z = N ). The nuclide 20 Ca (calcium-40)

2040-405: A product of stellar nucleosynthesis or another type of nucleosynthesis such as cosmic ray spallation , and have persisted down to the present because their rate of decay is very slow (e.g. uranium-238 and potassium-40 ). Post-primordial isotopes were created by cosmic ray bombardment as cosmogenic nuclides (e.g., tritium , carbon-14 ), or by the decay of a radioactive primordial isotope to

2160-506: A proton to a neutron or vice versa would lead to a very lopsided proton-neutron ratio ( 1 H , 3 Li , 5 B , and 7 N ; spins 1, 1, 3, 1). The only other entirely "stable" odd-odd nuclide, 73 Ta (spin 9), is thought to be the rarest of the 251 stable nuclides, and is the only primordial nuclear isomer , which has not yet been observed to decay despite experimental attempts. Many odd-odd radionuclides (such as

2280-478: A radioactive radiogenic nuclide daughter (e.g. uranium to radium ). A few isotopes are naturally synthesized as nucleogenic nuclides, by some other natural nuclear reaction , such as when neutrons from natural nuclear fission are absorbed by another atom. As discussed above, only 80 elements have any stable isotopes, and 26 of these have only one stable isotope. Thus, about two-thirds of stable elements occur naturally on Earth in multiple stable isotopes, with

2400-424: A single stable isotope (of these, 19 are so-called mononuclidic elements , having a single primordial stable isotope that dominates and fixes the atomic weight of the natural element to high precision; 3 radioactive mononuclidic elements occur as well). In total, there are 251 nuclides that have not been observed to decay. For the 80 elements that have one or more stable isotopes, the average number of stable isotopes

2520-497: A stable (non-radioactive) element was found by J. J. Thomson in 1912 as part of his exploration into the composition of canal rays (positive ions). Thomson channelled streams of neon ions through parallel magnetic and electric fields, measured their deflection by placing a photographic plate in their path, and computed their mass to charge ratio using a method that became known as the Thomson's parabola method. Each stream created

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2640-622: A total 30 + 2(9) = 48 stable odd-even isotopes. There are also five primordial long-lived radioactive odd-even isotopes, 37 Rb , 49 In , 75 Re , 63 Eu , and 83 Bi . The last two were only recently found to decay, with half-lives greater than 10 years. Actinides with odd neutron number are generally fissile (with thermal neutrons ), whereas those with even neutron number are generally not, though they are fissionable with fast neutrons . All observationally stable odd-odd nuclides have nonzero integer spin. This

2760-681: Is aluminium-26 , which is not naturally found on Earth but is found in abundance on an astronomical scale. The tabulated atomic masses of elements are averages that account for the presence of multiple isotopes with different masses. Before the discovery of isotopes, empirically determined noninteger values of atomic mass confounded scientists. For example, a sample of chlorine contains 75.8% chlorine-35 and 24.2% chlorine-37 , giving an average atomic mass of 35.5 atomic mass units . According to generally accepted cosmology theory , only isotopes of hydrogen and helium, traces of some isotopes of lithium and beryllium, and perhaps some boron, were created at

2880-483: Is −2.08 ± 0.01 V . Americium metal readily reacts with oxygen and dissolves in aqueous acids . The most stable oxidation state for americium is +3. The chemistry of americium(III) has many similarities to the chemistry of lanthanide (III) compounds. For example, trivalent americium forms insoluble fluoride , oxalate , iodate , hydroxide , phosphate and other salts. Compounds of americium in oxidation states +2, +4, +5, +6 and +7 have also been studied. This

3000-547: Is 251/80 ≈ 3.14 isotopes per element. The proton:neutron ratio is not the only factor affecting nuclear stability. It depends also on evenness or oddness of its atomic number Z , neutron number N and, consequently, of their sum, the mass number A . Oddness of both Z and N tends to lower the nuclear binding energy , making odd nuclei, generally, less stable. This remarkable difference of nuclear binding energy between neighbouring nuclei, especially of odd- A isobars , has important consequences: unstable isotopes with

3120-453: Is a black solid isomorphic with LaSi, it has an orthorhombic crystal symmetry. AmSi x has a bright silvery lustre and a tetragonal crystal lattice (space group I 4 1 /amd), it is isomorphic with PuSi 2 and ThSi 2 . Borides of americium include AmB 4 and AmB 6 . The tetraboride can be obtained by heating an oxide or halide of americium with magnesium diboride in vacuum or inert atmosphere. Analogous to uranocene , americium

3240-641: Is a radioactive form of carbon, whereas C and C are stable isotopes. There are about 339 naturally occurring nuclides on Earth, of which 286 are primordial nuclides , meaning that they have existed since the Solar System 's formation. Primordial nuclides include 35 nuclides with very long half-lives (over 100 million years) and 251 that are formally considered as " stable nuclides ", because they have not been observed to decay. In most cases, for obvious reasons, if an element has stable isotopes, those isotopes predominate in

3360-443: Is an artificial element of recent origin, and thus does not have a biological requirement . It is harmful to life . It has been proposed to use bacteria for removal of americium and other heavy metals from rivers and streams. Thus, Enterobacteriaceae of the genus Citrobacter precipitate americium ions from aqueous solutions, binding them into a metal-phosphate complex at their cell walls. Several studies have been reported on

3480-663: Is because the single unpaired neutron and unpaired proton have a larger nuclear force attraction to each other if their spins are aligned (producing a total spin of at least 1 unit), instead of anti-aligned. See deuterium for the simplest case of this nuclear behavior. Only 78 Pt , 4 Be , and 7 N have odd neutron number and are the most naturally abundant isotope of their element. Elements are composed either of one nuclide ( mononuclidic elements ), or of more than one naturally occurring isotopes. The unstable (radioactive) isotopes are either primordial or postprimordial. Primordial isotopes were

3600-502: Is concentrated in the areas used for the atmospheric nuclear weapons tests conducted between 1945 and 1980, as well as at the sites of nuclear incidents, such as the Chernobyl disaster . For example, the analysis of the debris at the testing site of the first U.S. hydrogen bomb , Ivy Mike , (1 November 1952, Enewetak Atoll ), revealed high concentrations of various actinides including americium; but due to military secrecy, this result

3720-435: Is denoted with symbols "u" (for unified atomic mass unit) or "Da" (for dalton ). The atomic masses of naturally occurring isotopes of an element determine the standard atomic weight of the element. When the element contains N isotopes, the expression below is applied for the average atomic mass m ¯ a {\displaystyle {\overline {m}}_{a}} : m ¯

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3840-551: Is different from plutonium and curium which show a rapid rise up to 60 K followed by saturation. The room temperature value for americium is lower than that of neptunium, plutonium and curium, but higher than for uranium, thorium and protactinium. Americium is paramagnetic in a wide temperature range, from that of liquid helium , to room temperature and above. This behavior is markedly different from that of its neighbor curium which exhibits antiferromagnetic transition at 52 K. The thermal expansion coefficient of americium

3960-409: Is different from β-Am, and at 1075 °C it converts to a body-centered cubic structure. The pressure-temperature phase diagram of americium is thus rather similar to those of lanthanum, praseodymium and neodymium . As with many other actinides, self-damage of the crystal structure due to alpha-particle irradiation is intrinsic to americium. It is especially noticeable at low temperatures, where

4080-422: Is equivalent to the closest packing with the sequence ABC. Upon further compression to 23 GPa, americium transforms to an orthorhombic γ-Am structure similar to that of α-uranium. There are no further transitions observed up to 52 GPa, except for an appearance of a monoclinic phase at pressures between 10 and 15 GPa. There is no consistency on the status of this phase in the literature, which also sometimes lists

4200-701: Is first separated and then converted by neutron bombardment in special reactors to short-lived nuclides. This procedure is well known as nuclear transmutation , but it is still being developed for americium. The transuranic elements from americium to fermium occurred naturally in the natural nuclear fission reactor at Oklo , but no longer do so. Americium is also one of the elements that have theoretically been detected in Przybylski's Star . Americium has been produced in small quantities in nuclear reactors for decades, and kilograms of its Am and Am isotopes have been accumulated by now. Nevertheless, since it

4320-435: Is less pronounced at room temperature, due to annihilation of radiation defects; also heating to room temperature the sample which was kept for hours at low temperatures restores its resistivity. In fresh samples, the resistivity gradually increases with temperature from about 2 μOhm·cm at liquid helium to 69 μOhm·cm at room temperature; this behavior is similar to that of neptunium, uranium, thorium and protactinium , but

4440-628: Is observationally the heaviest stable nuclide with the same number of neutrons and protons. All stable nuclides heavier than calcium-40 contain more neutrons than protons. Of the 80 elements with a stable isotope, the largest number of stable isotopes observed for any element is ten (for the element tin ). No element has nine or eight stable isotopes. Five elements have seven stable isotopes, eight have six stable isotopes, ten have five stable isotopes, nine have four stable isotopes, five have three stable isotopes, 16 have two stable isotopes (counting 73 Ta as stable), and 26 elements have only

4560-413: Is poorly soluble and precipitates upon reaction of Am and fluoride ions in weak acidic solutions: The tetravalent americium(IV) fluoride (AmF 4 ) is obtained by reacting solid americium(III) fluoride with molecular fluorine : Another known form of solid tetravalent americium fluoride is KAmF 5 . Tetravalent americium has also been observed in the aqueous phase. For this purpose, black Am(OH) 4

4680-634: Is predicted to form the organometallic compound amerocene with two cyclooctatetraene ligands, with the chemical formula (η -C 8 H 8 ) 2 Am. A cyclopentadienyl complex is also known that is likely to be stoichiometrically AmCp 3 . Formation of the complexes of the type Am(n-C 3 H 7 -BTP) 3 , where BTP stands for 2,6-di(1,2,4-triazin-3-yl)pyridine, in solutions containing n-C 3 H 7 -BTP and Am ions has been confirmed by EXAFS . Some of these BTP-type complexes selectively interact with americium and therefore are useful in its selective separation from lanthanides and another actinides. Americium

4800-475: Is produced instead in a process where Pu captures four neutrons under high neutron flux : Most synthesis routines yield a mixture of different actinide isotopes in oxide forms, from which isotopes of americium can be separated. In a typical procedure, the spent reactor fuel (e.g. MOX fuel ) is dissolved in nitric acid , and the bulk of uranium and plutonium is removed using a PUREX -type extraction ( P lutonium– UR anium EX traction) with tributyl phosphate in

4920-549: Is rather slow: half of the original amount of Pu decays to Am after about 15 years, and the Am amount reaches a maximum after 70 years. The obtained Am can be used for generating heavier americium isotopes by further neutron capture inside a nuclear reactor. In a light water reactor (LWR), 79% of Am converts to Am and 10% to its nuclear isomer Am: Americium-242 has a half-life of only 16 hours, which makes its further conversion to Am extremely inefficient. The latter isotope

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5040-411: Is significantly higher than that of plutonium (639 °C) and europium (826 °C), but lower than for curium (1340 °C). At ambient conditions, americium is present in its most stable α form which has a hexagonal crystal symmetry , and a space group P6 3 /mmc with cell parameters a  = 346.8  pm and c  = 1124 pm, and four atoms per unit cell . The crystal consists of

5160-439: Is slightly anisotropic and amounts to (7.5 ± 0.2) × 10  /°C along the shorter a axis and (6.2 ± 0.4) × 10  /°C for the longer c hexagonal axis. The enthalpy of dissolution of americium metal in hydrochloric acid at standard conditions is −620.6 ± 1.3 kJ/mol , from which the standard enthalpy change of formation (Δ f H °) of aqueous Am ion is −621.2 ± 2.0 kJ/mol . The standard potential Am /Am

5280-419: Is the reduction of americium dioxide by metallic lanthanum or thorium : In the periodic table , americium is located to the right of plutonium, to the left of curium, and below the lanthanide europium , with which it shares many physical and chemical properties. Americium is a highly radioactive element. When freshly prepared, it has a silvery-white metallic lustre, but then slowly tarnishes in air. With

5400-647: Is the widest range that has been observed with actinide elements. The color of americium compounds in aqueous solution is as follows: Am (yellow-reddish), Am (yellow-reddish), Am O + 2 ; (yellow), Am O 2+ 2 (brown) and Am O 5− 6 (dark green). The absorption spectra have sharp peaks, due to f - f transitions' in the visible and near-infrared regions. Typically, Am(III) has absorption maxima at ca. 504 and 811 nm, Am(V) at ca. 514 and 715 nm, and Am(VI) at ca. 666 and 992 nm. Americium compounds with oxidation state +4 and higher are strong oxidizing agents, comparable in strength to

5520-558: Is typical. The chemistry of Am(V) and Am(VI) is comparable to the chemistry of uranium in those oxidation states. In particular, compounds like Li 3 AmO 4 and Li 6 AmO 6 are comparable to uranates and the ion AmO 2+ 2 is comparable to the uranyl ion, UO 2+ 2 . Such compounds can be prepared by oxidation of Am(III) in dilute nitric acid with ammonium persulfate . Other oxidising agents that have been used include silver(I) oxide , ozone and sodium persulfate . Three americium oxides are known, with

5640-474: The Big Bang , while all other nuclides were synthesized later, in stars and supernovae, and in interactions between energetic particles such as cosmic rays, and previously produced nuclides. (See nucleosynthesis for details of the various processes thought responsible for isotope production.) The respective abundances of isotopes on Earth result from the quantities formed by these processes, their spread through

5760-447: The CNO cycle . The nuclides 3 Li and 5 B are minority isotopes of elements that are themselves rare compared to other light elements, whereas the other six isotopes make up only a tiny percentage of the natural abundance of their elements. 53 stable nuclides have an even number of protons and an odd number of neutrons. They are a minority in comparison to

5880-532: The Girdler sulfide process . Uranium isotopes have been separated in bulk by gas diffusion, gas centrifugation, laser ionization separation, and (in the Manhattan Project ) by a type of production mass spectrometry . Americium Americium is a synthetic chemical element ; it has symbol Am and atomic number 95. It is radioactive and a transuranic member of the actinide series in

6000-410: The binding energy of the nucleus (see mass defect ), the slight difference in mass between proton and neutron, and the mass of the electrons associated with the atom, the latter because the electron:nucleon ratio differs among isotopes. The mass number is a dimensionless quantity . The atomic mass, on the other hand, is measured using the atomic mass unit based on the mass of the carbon-12 atom. It

6120-405: The biosorption and bioaccumulation of americium by bacteria and fungi. In the laboratory, both americium and curium were found to support the growth of methylotrophs . The isotope Am (half-life 141 years) has the largest cross sections for absorption of thermal neutrons (5,700 barns ), that results in a small critical mass for a sustained nuclear chain reaction . The critical mass for

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6240-465: The fissile 92 U . Because of their odd neutron numbers, the even-odd nuclides tend to have large neutron capture cross-sections, due to the energy that results from neutron-pairing effects. These stable even-proton odd-neutron nuclides tend to be uncommon by abundance in nature, generally because, to form and enter into primordial abundance, they must have escaped capturing neutrons to form yet other stable even-even isotopes, during both

6360-692: The isotope Am, but they are as yet hindered by the scarcity and high price of this nuclear isomer . Americium is a relatively soft radioactive metal with a silvery appearance. Its most common isotopes are Am and Am. In chemical compounds, americium usually assumes the oxidation state +3, especially in solutions. Several other oxidation states are known, ranging from +2 to +7, and can be identified by their characteristic optical absorption spectra. The crystal lattices of solid americium and its compounds contain small intrinsic radiogenic defects, due to metamictization induced by self-irradiation with alpha particles, which accumulates with time; this can cause

6480-425: The isotope concept (grouping all atoms of each element) emphasizes chemical over nuclear. The neutron number greatly affects nuclear properties, but its effect on chemical properties is negligible for most elements. Even for the lightest elements, whose ratio of neutron number to atomic number varies the most between isotopes, it usually has only a small effect although it matters in some circumstances (for hydrogen,

6600-554: The periodic table , located under the lanthanide element europium and was thus named after the Americas by analogy. Americium was first produced in 1944 by the group of Glenn T. Seaborg from Berkeley, California , at the Metallurgical Laboratory of the University of Chicago , as part of the Manhattan Project . Although it is the third element in the transuranic series, it was discovered fourth, after

6720-550: The permanganate ion ( MnO − 4 ) in acidic solutions. Whereas the Am ions are unstable in solutions and readily convert to Am , compounds such as americium dioxide (AmO 2 ) and americium(IV) fluoride (AmF 4 ) are stable in the solid state. The pentavalent oxidation state of americium was first observed in 1951. In acidic aqueous solution the AmO + 2 ion is unstable with respect to disproportionation . The reaction

6840-436: The residual strong force . Because protons are positively charged, they repel each other. Neutrons, which are electrically neutral, stabilize the nucleus in two ways. Their copresence pushes protons slightly apart, reducing the electrostatic repulsion between the protons, and they exert an attractive nuclear force on each other and on protons. For this reason, one or more neutrons are necessary for two or more protons to bind into

6960-421: The s-process and r-process of neutron capture, during nucleosynthesis in stars . For this reason, only 78 Pt and 4 Be are the most naturally abundant isotopes of their element. 48 stable odd-proton-even-neutron nuclides, stabilized by their paired neutrons, form most of the stable isotopes of the odd-numbered elements; the very few odd-proton-odd-neutron nuclides comprise

7080-536: The solvent extraction of americium. For example, a 2003 EU -funded project codenamed "EUROPART" studied triazines and other compounds as potential extraction agents. A bis -triazinyl bipyridine complex was proposed in 2009 as such a reagent is highly selective to americium (and curium). Separation of americium from the highly similar curium can be achieved by treating a slurry of their hydroxides in aqueous sodium bicarbonate with ozone , at elevated temperatures. Both Am and Cm are mostly present in solutions in

7200-594: The sulfide AmS 2 , selenides AmSe 2 and Am 3 Se 4 , and tellurides Am 2 Te 3 and AmTe 2 . The pnictides of americium ( Am) of the AmX type are known for the elements phosphorus , arsenic , antimony and bismuth . They crystallize in the rock-salt lattice. Americium monosilicide (AmSi) and "disilicide" (nominally AmSi x with: 1.87 < x < 2.0) were obtained by reduction of americium(III) fluoride with elementary silicon in vacuum at 1050 °C (AmSi) and 1150−1200 °C (AmSi x ). AmSi

7320-441: The +3 valence state; whereas curium remains unchanged, americium oxidizes to soluble Am(IV) complexes which can be washed away. Metallic americium is obtained by reduction from its compounds. Americium(III) fluoride was first used for this purpose. The reaction was conducted using elemental barium as reducing agent in a water- and oxygen-free environment inside an apparatus made of tantalum and tungsten . An alternative

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7440-577: The Earth's formation) atoms of these elements, have long since decayed. Trace amounts of neptunium and plutonium form in some uranium-rich rock, and small amounts are produced during atmospheric tests of nuclear weapons . These two elements are generated by neutron capture in uranium ore with subsequent beta decays (e.g. U + n → U → Np → Pu ). All elements beyond plutonium are entirely synthetic ; they are created in nuclear reactors or particle accelerators . The half-lives of these elements show

7560-788: The United States (elements 93–101, 106, and joint credit for 103–105), the Joint Institute for Nuclear Research (JINR) in Russia (elements 102 and 114–118, and joint credit for 103–105), the GSI Helmholtz Centre for Heavy Ion Research in Germany (elements 107–112), and RIKEN in Japan (element 113). Superheavy elements , (also known as superheavies , or superheavy atoms , commonly abbreviated SHE ) usually refer to

7680-551: The University of California, Berkeley. The element was chemically identified at the Metallurgical Laboratory (now Argonne National Laboratory ) of the University of Chicago . Following the lighter neptunium , plutonium , and heavier curium , americium was the fourth transuranium element to be discovered. At the time, the periodic table had been restructured by Seaborg to its present layout, containing

7800-485: The actinide row below the lanthanide one. This led to americium being located right below its twin lanthanide element europium; it was thus by analogy named after the Americas : "The name americium (after the Americas) and the symbol Am are suggested for the element on the basis of its position as the sixth member of the actinide rare-earth series, analogous to europium, Eu, of the lanthanide series." The new element

7920-429: The almost integral masses for the two isotopes Cl and Cl. After the discovery of the neutron by James Chadwick in 1932, the ultimate root cause for the existence of isotopes was clarified, that is, the nuclei of different isotopes for a given element have different numbers of neutrons, albeit having the same number of protons. A neutral atom has the same number of electrons as protons. Thus different isotopes of

8040-463: The atomic number is given by the element symbol, it is common to state only the mass number in the superscript and leave out the atomic number subscript (e.g. He , He , C , C , U , and U ). The letter m (for metastable) is sometimes appended after the mass number to indicate a nuclear isomer , a metastable or energetically excited nuclear state (as opposed to

8160-410: The average radioactivity of surface soil due to residual americium is only about 0.01  picocuries per gram (0.37  mBq /g). Atmospheric americium compounds are poorly soluble in common solvents and mostly adhere to soil particles. Soil analysis revealed about 1,900 times higher concentration of americium inside sandy soil particles than in the water present in the soil pores; an even higher ratio

8280-640: The beta decay of actinium-230 forms thorium-230. The term "isotope", Greek for "at the same place", was suggested to Soddy by Margaret Todd , a Scottish physician and family friend, during a conversation in which he explained his ideas to her. He received the 1921 Nobel Prize in Chemistry in part for his work on isotopes. In 1914 T. W. Richards found variations between the atomic weight of lead from different mineral sources, attributable to variations in isotopic composition due to different radioactive origins. The first evidence for multiple isotopes of

8400-568: The color and exact structure between the halogens. So, chloride (AmCl 3 ) is reddish and has a structure isotypic to uranium(III) chloride (space group P6 3 /m) and the melting point of 715 °C. The fluoride is isotypic to LaF 3 (space group P6 3 /mmc) and the iodide to BiI 3 (space group R 3 ). The bromide is an exception with the orthorhombic PuBr 3 -type structure and space group Cmcm. Crystals of americium(III) chloride hexahydrate (AmCl 3 ·6H 2 O) can be prepared by dissolving americium dioxide in hydrochloric acid and evaporating

8520-556: The cost of weapons-grade plutonium was around $ 4,000/gram, and californium exceeded $ 60,000,000/gram. Einsteinium is the heaviest element that has been produced in macroscopic quantities. Transuranic elements that have not been discovered, or have been discovered but are not yet officially named, use IUPAC 's systematic element names . The naming of transuranic elements may be a source of controversy . So far, essentially all transuranium elements have been discovered at four laboratories: Lawrence Berkeley National Laboratory (LBNL) in

8640-410: The development of compact nuclear weapons. The potential everyday applications are vast; americium is used in devices such as smoke detectors and spectrometers . Isotope Isotopes are distinct nuclear species (or nuclides ) of the same chemical element . They have the same atomic number (number of protons in their nuclei ) and position in the periodic table (and hence belong to

8760-510: The element carbon with mass numbers 12, 13, and 14, respectively. The atomic number of carbon is 6, which means that every carbon atom has 6 protons so that the neutron numbers of these isotopes are 6, 7, and 8 respectively. A nuclide is a species of an atom with a specific number of protons and neutrons in the nucleus, for example, carbon-13 with 6 protons and 7 neutrons. The nuclide concept (referring to individual nuclear species) emphasizes nuclear properties over chemical properties, whereas

8880-500: The elemental abundance found on Earth and in the Solar System. However, in the cases of three elements ( tellurium , indium , and rhenium ) the most abundant isotope found in nature is actually one (or two) extremely long-lived radioisotope(s) of the element, despite these elements having one or more stable isotopes. Theory predicts that many apparently "stable" nuclides are radioactive, with extremely long half-lives (discounting

9000-410: The even-even isotopes, which are about 3 times as numerous. Among the 41 even- Z elements that have a stable nuclide, only two elements (argon and cerium) have no even-odd stable nuclides. One element (tin) has three. There are 24 elements that have one even-odd nuclide and 13 that have two odd-even nuclides. Of 35 primordial radionuclides there exist four even-odd nuclides (see table at right), including

9120-486: The galaxy, and the rates of decay for isotopes that are unstable. After the initial coalescence of the Solar System , isotopes were redistributed according to mass, and the isotopic composition of elements varies slightly from planet to planet. This sometimes makes it possible to trace the origin of meteorites . The atomic mass ( m r ) of an isotope (nuclide) is determined mainly by its mass number (i.e. number of nucleons in its nucleus). Small corrections are due to

9240-401: The ground state of tantalum-180) with comparatively short half-lives are known. Usually, they beta-decay to their nearby even-even isobars that have paired protons and paired neutrons. Of the nine primordial odd-odd nuclides (five stable and four radioactive with long half-lives), only 7 N is the most common isotope of a common element. This is the case because it is a part of

9360-602: The heavier curium . The discovery was kept secret and only released to the public in November 1945. Most americium is produced by uranium or plutonium being bombarded with neutrons in nuclear reactors – one tonne of spent nuclear fuel contains about 100 grams of americium. It is widely used in commercial ionization chamber smoke detectors , as well as in neutron sources and industrial gauges. Several unusual applications, such as nuclear batteries or fuel for space ships with nuclear propulsion , have been proposed for

9480-409: The integers 20 and 22 and that neither is equal to the known molar mass (20.2) of neon gas. This is an example of Aston's whole number rule for isotopic masses, which states that large deviations of elemental molar masses from integers are primarily due to the fact that the element is a mixture of isotopes. Aston similarly showed in 1920 that the molar mass of chlorine (35.45) is a weighted average of

9600-464: The largest number of stable isotopes for an element being ten, for tin ( 50 Sn ). There are about 94 elements found naturally on Earth (up to plutonium inclusive), though some are detected only in very tiny amounts, such as plutonium-244 . Scientists estimate that the elements that occur naturally on Earth (some only as radioisotopes) occur as 339 isotopes ( nuclides ) in total. Only 251 of these naturally occurring nuclides are stable, in

9720-445: The latter half of the 20th century and are continually being created during the 21st century as technology advances. They are created through the bombardment of elements in a particle accelerator, in quantities on the atomic scale, and no method of mass creation has been found. Transuranic elements may be used to synthesize superheavy elements. Elements of the island of stability have potentially important military applications, including

9840-482: The least common. The 146 even-proton, even-neutron (EE) nuclides comprise ~58% of all stable nuclides and all have spin 0 because of pairing. There are also 24 primordial long-lived even-even nuclides. As a result, each of the 41 even-numbered elements from 2 to 82 has at least one stable isotope , and most of these elements have several primordial isotopes. Half of these even-numbered elements have six or more stable isotopes. The extreme stability of helium-4 due to

9960-517: The lightest element, the isotope effect is large enough to affect biology strongly). The term isotopes (originally also isotopic elements , now sometimes isotopic nuclides ) is intended to imply comparison (like synonyms or isomers ). For example, the nuclides 6 C , 6 C , 6 C are isotopes (nuclides with the same atomic number but different mass numbers ), but 18 Ar , 19 K , 20 Ca are isobars (nuclides with

10080-402: The liquid. Those crystals are hygroscopic and have yellow-reddish color and a monoclinic crystal structure. Oxyhalides of americium in the form Am O 2 X 2 , Am O 2 X, Am OX 2 and Am OX can be obtained by reacting the corresponding americium halide with oxygen or Sb 2 O 3 , and AmOCl can also be produced by vapor phase hydrolysis : The known chalcogenides of americium include

10200-774: The listeners asked whether any new transuranium element besides plutonium and neptunium had been discovered during the war. After the discovery of americium isotopes Am and Am, their production and compounds were patented listing only Seaborg as the inventor. The initial americium samples weighed a few micrograms; they were barely visible and were identified by their radioactivity. The first substantial amounts of metallic americium weighing 40–200 micrograms were not prepared until 1951 by reduction of americium(III) fluoride with barium metal in high vacuum at 1100 °C. The longest-lived and most common isotopes of americium, Am and Am, have half-lives of 432.2 and 7,370 years, respectively. Therefore, any primordial americium (americium that

10320-402: The longest-lived isotope), and thorium X ( Ra) are impossible to separate. Attempts to place the radioelements in the periodic table led Soddy and Kazimierz Fajans independently to propose their radioactive displacement law in 1913, to the effect that alpha decay produced an element two places to the left in the periodic table, whereas beta decay emission produced an element one place to

10440-699: The lowest-energy ground state ), for example 73 Ta ( tantalum-180m ). The common pronunciation of the AZE notation is different from how it is written: 2 He is commonly pronounced as helium-four instead of four-two-helium, and 92 U as uranium two-thirty-five (American English) or uranium-two-three-five (British) instead of 235-92-uranium. Some isotopes/nuclides are radioactive , and are therefore referred to as radioisotopes or radionuclides , whereas others have never been observed to decay radioactively and are referred to as stable isotopes or stable nuclides . For example, C

10560-457: The meaning behind the name is that different isotopes of a single element occupy the same position on the periodic table . It was coined by Scottish doctor and writer Margaret Todd in a 1913 suggestion to the British chemist Frederick Soddy , who popularized the term. The number of protons within the atom's nucleus is called its atomic number and is equal to the number of electrons in

10680-436: The mobility of the produced structure defects is relatively low, by broadening of X-ray diffraction peaks. This effect makes somewhat uncertain the temperature of americium and some of its properties, such as electrical resistivity . So for americium-241, the resistivity at 4.2 K increases with time from about 2 μOhm·cm to 10 μOhm·cm after 40 hours, and saturates at about 16 μOhm·cm after 140 hours. This effect

10800-504: The most common reactor material – but from the plutonium isotope Pu. The latter needs to be produced first, according to the following nuclear process: The capture of two neutrons by Pu (a so-called (n,γ) reaction), followed by a β-decay, results in Am: The plutonium present in spent nuclear fuel contains about 12% of Pu. Because it beta-decays to Am, Pu can be extracted and may be used to generate further Am. However, this process

10920-421: The neutral (non-ionized) atom. Each atomic number identifies a specific element, but not the isotope; an atom of a given element may have a wide range in its number of neutrons . The number of nucleons (both protons and neutrons) in the nucleus is the atom's mass number , and each isotope of a given element has a different mass number. For example, carbon-12 , carbon-13 , and carbon-14 are three isotopes of

11040-417: The other about 22. The parabola due to the heavier gas is always much fainter than that due to the lighter, so that probably the heavier gas forms only a small percentage of the mixture." F. W. Aston subsequently discovered multiple stable isotopes for numerous elements using a mass spectrograph . In 1919 Aston studied neon with sufficient resolution to show that the two isotopic masses are very close to

11160-689: The other naturally occurring nuclides are radioactive but occur on Earth due to their relatively long half-lives, or else due to other means of ongoing natural production. These include the afore-mentioned cosmogenic nuclides , the nucleogenic nuclides, and any radiogenic nuclides formed by ongoing decay of a primordial radioactive nuclide, such as radon and radium from uranium. An additional ~3000 radioactive nuclides not found in nature have been created in nuclear reactors and in particle accelerators. Many short-lived nuclides not found naturally on Earth have also been observed by spectroscopic analysis, being naturally created in stars or supernovae . An example

11280-726: The others. There are 41 odd-numbered elements with Z = 1 through 81, of which 39 have stable isotopes ( technetium ( 43 Tc ) and promethium ( 61 Pm ) have no stable isotopes). Of these 39 odd Z elements, 30 elements (including hydrogen-1 where 0 neutrons is even ) have one stable odd-even isotope, and nine elements: chlorine ( 17 Cl ), potassium ( 19 K ), copper ( 29 Cu ), gallium ( 31 Ga ), bromine ( 35 Br ), silver ( 47 Ag ), antimony ( 51 Sb ), iridium ( 77 Ir ), and thallium ( 81 Tl ), have two odd-even stable isotopes each. This makes

11400-409: The oxidation states +2 (AmO), +3 (Am 2 O 3 ) and +4 (AmO 2 ). Americium(II) oxide was prepared in minute amounts and has not been characterized in detail. Americium(III) oxide is a red-brown solid with a melting point of 2205 °C. Americium(IV) oxide is the main form of solid americium which is used in nearly all its applications. As most other actinide dioxides, it is a black solid with

11520-460: The oxidation states +2, +3 and +4, where the +3 is most stable, especially in solutions. Reduction of Am(III) compounds with sodium amalgam yields Am(II) salts – the black halides AmCl 2 , AmBr 2 and AmI 2 . They are very sensitive to oxygen and oxidize in water, releasing hydrogen and converting back to the Am(III) state. Specific lattice constants are: Americium(III) fluoride (AmF 3 )

11640-415: The possibility of proton decay , which would make all nuclides ultimately unstable). Some stable nuclides are in theory energetically susceptible to other known forms of decay, such as alpha decay or double beta decay, but no decay products have yet been observed, and so these isotopes are said to be "observationally stable". The predicted half-lives for these nuclides often greatly exceed the estimated age of

11760-432: The primary exceptions). The vibrational modes of a molecule are determined by its shape and by the masses of its constituent atoms; so different isotopologues have different sets of vibrational modes. Because vibrational modes allow a molecule to absorb photons of corresponding energies, isotopologues have different optical properties in the infrared range. Atomic nuclei consist of protons and neutrons bound together by

11880-457: The properties of the various isotopes of a given element. Isotope separation is a significant technological challenge, particularly with heavy elements such as uranium or plutonium. Lighter elements such as lithium, carbon, nitrogen, and oxygen are commonly separated by gas diffusion of their compounds such as CO and NO. The separation of hydrogen and deuterium is unusual because it is based on chemical rather than physical properties, for example in

12000-586: The relative mass difference between isotopes is much less so that the mass-difference effects on chemistry are usually negligible. (Heavy elements also have relatively more neutrons than lighter elements, so the ratio of the nuclear mass to the collective electronic mass is slightly greater.) There is also an equilibrium isotope effect . Similarly, two molecules that differ only in the isotopes of their atoms ( isotopologues ) have identical electronic structures, and therefore almost indistinguishable physical and chemical properties (again with deuterium and tritium being

12120-451: The right. Soddy recognized that emission of an alpha particle followed by two beta particles led to the formation of an element chemically identical to the initial element but with a mass four units lighter and with different radioactive properties. Soddy proposed that several types of atoms (differing in radioactive properties) could occupy the same place in the table. For example, the alpha-decay of uranium-235 forms thorium-231, whereas

12240-452: The same chemical element), but different nucleon numbers ( mass numbers ) due to different numbers of neutrons in their nuclei. While all isotopes of a given element have similar chemical properties, they have different atomic masses and physical properties. The term isotope is derived from the Greek roots isos ( ἴσος "equal") and topos ( τόπος "place"), meaning "the same place"; thus,

12360-441: The same element. This is most pronounced by far for protium ( H ), deuterium ( H ), and tritium ( H ), because deuterium has twice the mass of protium and tritium has three times the mass of protium. These mass differences also affect the behavior of their respective chemical bonds, by changing the center of gravity ( reduced mass ) of the atomic systems. However, for heavier elements,

12480-489: The same mass number ). However, isotope is the older term and so is better known than nuclide and is still sometimes used in contexts in which nuclide might be more appropriate, such as nuclear technology and nuclear medicine . An isotope and/or nuclide is specified by the name of the particular element (this indicates the atomic number) followed by a hyphen and the mass number (e.g. helium-3 , helium-4 , carbon-12 , carbon-14 , uranium-235 and uranium-239 ). When

12600-406: The sense of never having been observed to decay as of the present time. An additional 35 primordial nuclides (to a total of 286 primordial nuclides), are radioactive with known half-lives, but have half-lives longer than 100 million years, allowing them to exist from the beginning of the Solar System. See list of nuclides for details. All the known stable nuclides occur naturally on Earth;

12720-452: The transactinide elements beginning with rutherfordium (atomic number 104). (Lawrencium, the first 6d element, is sometimes but not always included as well.) They have only been made artificially and currently serve no practical purpose because their short half-lives cause them to decay after a very short time, ranging from a few hours to just milliseconds, which also makes them extremely hard to study. Superheavies have all been created since

12840-622: The two readily available isotopes, Am and Am, are relatively high – 57.6 to 75.6 kg for Am and 209 kg for Am. Scarcity and high price yet hinder application of americium as a nuclear fuel in nuclear reactors . There are proposals of very compact 10-kW high-flux reactors using as little as 20 grams of Am. Such low-power reactors would be relatively safe to use as neutron sources for radiation therapy in hospitals. About 18 isotopes and 11 nuclear isomers are known for americium, having mass numbers 229, 230, and 232 through 247. There are two long-lived alpha-emitters; Am has

12960-420: The universe, and in fact, there are also 31 known radionuclides (see primordial nuclide ) with half-lives longer than the age of the universe. Adding in the radioactive nuclides that have been created artificially, there are 3,339 currently known nuclides . These include 905 nuclides that are either stable or have half-lives longer than 60 minutes. See list of nuclides for details. The existence of isotopes

13080-413: The uranium and thorium decay chains, and thus all save francium were first discovered by synthesis in the laboratory rather than in nature. All elements with higher atomic numbers have been first discovered in the laboratory, with neptunium and plutonium later discovered in nature. They are all radioactive , with a half-life much shorter than the age of the Earth , so any primordial (i.e. present at

13200-408: The α, β and γ phases as I, II and III. The β-γ transition is accompanied by a 6% decrease in the crystal volume; although theory also predicts a significant volume change for the α-β transition, it is not observed experimentally. The pressure of the α-β transition decreases with increasing temperature, and when α-americium is heated at ambient pressure, at 770 °C it changes into an fcc phase which

13320-449: Was directly obtained from plutonium upon absorption of two neutrons. It decays by emission of a α-particle to Np; the half-life of this decay was first determined as 510 ± 20 years but then corrected to 432.2 years. The second isotope Am was produced upon neutron bombardment of the already-created Am. Upon rapid β-decay , Am converts into the isotope of curium Cm (which had been discovered previously). The half-life of this decay

13440-505: Was dissolved in perchloric acid . Further separation was carried out by ion exchange , yielding a certain isotope of curium. The separation of curium and americium was so painstaking that those elements were initially called by the Berkeley group as pandemonium (from Greek for all demons or hell ) and delirium (from Latin for madness ). Initial experiments yielded four americium isotopes: Am, Am, Am and Am. Americium-241

13560-592: Was dissolved in 15- M NH 4 F with the americium concentration of 0.01 M. The resulting reddish solution had a characteristic optical absorption spectrum which is similar to that of AmF 4 but differed from other oxidation states of americium. Heating the Am(IV) solution to 90 °C did not result in its disproportionation or reduction, however a slow reduction was observed to Am(III) and assigned to self-irradiation of americium by alpha particles. Most americium(III) halides form hexagonal crystals with slight variation of

13680-427: Was first offered for sale in 1962, its price, about US$ 1,500 per gram (US$ 43,000/oz) of Am, remains almost unchanged owing to the very complex separation procedure. The heavier isotope Am is produced in much smaller amounts; it is thus more difficult to separate, resulting in a higher cost of the order US$ 100,000–US$ 160,000 per gram (US$ 2,800,000–US$ 4,500,000/oz). Americium is not synthesized directly from uranium –

13800-513: Was first suggested in 1913 by the radiochemist Frederick Soddy , based on studies of radioactive decay chains that indicated about 40 different species referred to as radioelements (i.e. radioactive elements) between uranium and lead, although the periodic table only allowed for 11 elements between lead and uranium inclusive. Several attempts to separate these new radioelements chemically had failed. For example, Soddy had shown in 1910 that mesothorium (later shown to be Ra), radium ( Ra,

13920-478: Was initially determined at 17 hours, which was close to the presently accepted value of 16.02 h. The discovery of americium and curium in 1944 was closely related to the Manhattan Project ; the results were confidential and declassified only in 1945. Seaborg leaked the synthesis of the elements 95 and 96 on the U.S. radio show for children Quiz Kids five days before the official presentation at an American Chemical Society meeting on 11 November 1945, when one of

14040-457: Was isolated from its oxides in a complex, multi-step process. First plutonium -239 nitrate ( PuNO 3 ) solution was coated on a platinum foil of about 0.5 cm area, the solution was evaporated and the residue was converted into plutonium dioxide (PuO 2 ) by calcining . After cyclotron irradiation, the coating was dissolved with nitric acid , and then precipitated as the hydroxide using concentrated aqueous ammonia solution . The residue

14160-449: Was measured in loam soils. Americium is produced mostly artificially in small quantities, for research purposes. A tonne of spent nuclear fuel contains about 100 grams of various americium isotopes, mostly Am and Am. Their prolonged radioactivity is undesirable for the disposal, and therefore americium, together with other long-lived actinides, must be neutralized. The associated procedure may involve several steps, where americium

14280-480: Was not published until later, in 1956. Trinitite , the glassy residue left on the desert floor near Alamogordo, New Mexico , after the plutonium -based Trinity nuclear bomb test on 16 July 1945, contains traces of americium-241. Elevated levels of americium were also detected at the crash site of a US Boeing B-52 bomber aircraft, which carried four hydrogen bombs, in 1968 in Greenland . In other regions,

14400-468: Was present on Earth during its formation) should have decayed by now. Trace amounts of americium probably occur naturally in uranium minerals as a result of neutron capture and beta decay ( U → Pu → Pu → Am), though the quantities would be tiny and this has not been confirmed. Extraterrestrial long-lived Cm is probably also deposited on Earth and has Am as one of its intermediate decay products, but again this has not been confirmed. Existing americium

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