Americium is a
synthetic elementIn chemistry, a synthetic element is a chemical element that is too unstable to occur naturally on Earth, and therefore has to be created artificially. So far 30 synthetic elements have been discovered—that is, synthesized...
that has the symbol
Am and
atomic numberIn chemistry and physics, the atomic number is the number of protons found in the nucleus of an atom and therefore identical to the charge number of the nucleus. It is conventionally represented by the symbol Z. The atomic number uniquely identifies a chemical element...
95. This transuranic element of the
actinideThe actinide or actinoid series encompasses the 15 metallic chemical elements with atomic numbers from 89 to 103, actinium through lawrencium.The actinide series derives its name from the group 3 element actinium...
series is located in the
periodic tableThe periodic table of the chemical elements is a tabular display of the 118 known chemical elements organized by selected properties of their atomic structures. Elements are presented by increasing atomic number, the number of protons in an atom's atomic nucleus...
below the
lanthanideThe lanthanide or lanthanoid series comprises the fifteen metallic chemical elements with atomic numbers 57 through 71, from lanthanum through lutetium...
element
europiumEuropium is a chemical element with the symbol Eu and atomic number 63. It is named after the continent of Europe. It is a moderately hard silvery metal which readily oxidizes in air and water...
, and thus by analogy was named after another continent,
AmericaThe Americas, or America , are lands in the Western hemisphere, also known as the New World. In English, the plural form the Americas is often used to refer to the landmasses of North America and South America with their associated islands and regions, while the singular form America is primarily...
.
Americium was first produced in 1944 by the group of
Glenn T. SeaborgGlenn Theodore Seaborg was an American scientist who won the 1951 Nobel Prize in Chemistry for "discoveries in the chemistry of the transuranium elements", contributed to the discovery and isolation of ten elements, and developed the actinide concept, which led to the current arrangement of the...
at the
University of California, BerkeleyThe University of California, Berkeley , is a teaching and research university established in 1868 and located in Berkeley, California, USA...
. Although it is the third element in the transuranic series, it was discovered fourth, after the heavier
curiumCurium is a synthetic chemical element with the symbol Cm and atomic number 96. This radioactive transuranic element of the actinide series was named after Marie Skłodowska-Curie and her husband Pierre Curie. Curium was first intentionally produced and identified in summer 1944 by the group of...
. The discovery was kept secret and only released to the public in November 1945. Most americium is produced by bombarding
uraniumUranium is a silvery-white metallic chemical element in the actinide series of the periodic table, with atomic number 92. It is assigned the chemical symbol U. A uranium atom has 92 protons and 92 electrons, of which 6 are valence electrons...
or
plutoniumPlutonium is a transuranic radioactive chemical element with the chemical symbol Pu and atomic number 94. It is an actinide metal of silvery-gray appearance that tarnishes when exposed to air, forming a dull coating when oxidized. The element normally exhibits six allotropes and four oxidation...
with
alpha particleAlpha particles consist of two protons and two neutrons bound together into a particle identical to a helium nucleus, which is classically produced in the process of alpha decay, but may be produced also in other ways and given the same name...
s in
nuclear reactorA nuclear reactor is a device to initiate and control a sustained nuclear chain reaction. Most commonly they are used for generating electricity and for the propulsion of ships. Usually heat from nuclear fission is passed to a working fluid , which runs through turbines that power either ship's...
s – one
tonneThe tonne, known as the metric ton in the US , often put pleonastically as "metric tonne" to avoid confusion with ton, is a metric system unit of mass equal to 1000 kilograms. The tonne is not an International System of Units unit, but is accepted for use with the SI...
of spent
nuclear fuelNuclear fuel is a material that can be 'consumed' by fission or fusion to derive nuclear energy. Nuclear fuels are the most dense sources of energy available...
contains about 100 grams of americium. It is widely used in commercial
ionization chamberThe ionization chamber is the simplest of all gas-filled radiation detectors, and is used for the detection or measurement of ionizing radiation...
smoke detectors, as well as in
neutron sourceA Neutron source is a device that emits neutrons. There is a wide variety of different sources, ranging from hand-held radioactive sources to neutron research facilities operating research reactors and spallation sources...
s and industrial gauges. Several unusual applications, such as a nuclear battery or fuel for space ships with nuclear propulsion, have been proposed for the
isotopeIsotopes are variants of atoms of a particular chemical element, which have differing numbers of neutrons. Atoms of a particular element by definition must contain the same number of protons but may have a distinct number of neutrons which differs from atom to atom, without changing the designation...
242mAm, but they are as yet hindered by the scarcity and high price of this
nuclear isomerA nuclear isomer is a metastable state of an atomic nucleus caused by the excitation of one or more of its nucleons . "Metastable" refers to the fact that these excited states have half-lives more than 100 to 1000 times the half-lives of the other possible excited nuclear states...
.
Americium is a relatively soft
radioactiveRadioactive decay is the process by which an atomic nucleus of an unstable atom loses energy by emitting ionizing particles . The emission is spontaneous, in that the atom decays without any physical interaction with another particle from outside the atom...
metal with silvery-white appearance. Its most common isotopes are
241Am and
243Am. In chemical compounds, they usually assume the
oxidation stateIn chemistry, the oxidation state is an indicator of the degree of oxidation of an atom in a chemical compound. The formal oxidation state is the hypothetical charge that an atom would have if all bonds to atoms of different elements were 100% ionic. Oxidation states are typically represented by...
+3, especially in solutions. Several other oxidation states are known, which range from +2 to +7 and can be identified by their characteristic optical absorption spectra. The crystal lattice of solid americium and its compounds contains intrinsic defects, which are induced by self-irradiation with alpha particles and accumulate with time; this results in a drift of some material properties.
History
Although americium was likely produced in previous nuclear experiments, it was
first intentionally synthesizedThe discovery of the elements known to exist today is presented here in chronological order. The elements are listed generally in the order in which each was first defined as the pure element, as the exact date of discovery of most elements cannot be accurately defined.Given is each element's name,...
, isolated and identified in late autumn 1944, at the
University of California, BerkeleyThe University of California, Berkeley , is a teaching and research university established in 1868 and located in Berkeley, California, USA...
by
Glenn T. SeaborgGlenn Theodore Seaborg was an American scientist who won the 1951 Nobel Prize in Chemistry for "discoveries in the chemistry of the transuranium elements", contributed to the discovery and isolation of ten elements, and developed the actinide concept, which led to the current arrangement of the...
, Leon O. Morgan, Ralph A. James, and
Albert GhiorsoAlbert Ghiorso was an American nuclear scientist and co-discoverer of a record 12 chemical elements on the periodic table. His research career spanned five decades, from the early 1940s to the late 1990s.-Early life:...
. They used a 60-inch
cyclotronIn technology, a cyclotron is a type of particle accelerator. In physics, the cyclotron frequency or gyrofrequency is the frequency of a charged particle moving perpendicularly to the direction of a uniform magnetic field, i.e. a magnetic field of constant magnitude and direction...
at the University of California, Berkeley. The element was chemically identified at the Metallurgical Laboratory (now
Argonne National LaboratoryArgonne National Laboratory is the first science and engineering research national laboratory in the United States, receiving this designation on July 1, 1946. It is the largest national laboratory by size and scope in the Midwest...
) of the
University of ChicagoThe University of Chicago is a private research university in Chicago, Illinois, USA. It was founded by the American Baptist Education Society with a donation from oil magnate and philanthropist John D. Rockefeller and incorporated in 1890...
. Following the lighter
neptuniumNeptunium is a chemical element with the symbol Np and atomic number 93. A radioactive metal, neptunium is the first transuranic element and belongs to the actinide series. Its most stable isotope, 237Np, is a by-product of nuclear reactors and plutonium production and it can be used as a...
,
plutoniumPlutonium is a transuranic radioactive chemical element with the chemical symbol Pu and atomic number 94. It is an actinide metal of silvery-gray appearance that tarnishes when exposed to air, forming a dull coating when oxidized. The element normally exhibits six allotropes and four oxidation...
, and heavier
curiumCurium is a synthetic chemical element with the symbol Cm and atomic number 96. This radioactive transuranic element of the actinide series was named after Marie Skłodowska-Curie and her husband Pierre Curie. Curium was first intentionally produced and identified in summer 1944 by the group of...
, americium was the fourth
transuranium elementIn chemistry, transuranium elements are the chemical elements with atomic numbers greater than 92...
to be discovered. At the time, the
periodic tableThe periodic table of the chemical elements is a tabular display of the 118 known chemical elements organized by selected properties of their atomic structures. Elements are presented by increasing atomic number, the number of protons in an atom's atomic nucleus...
had been restructured by Seaborg to its present layout, containing the actinide row below the
lanthanideThe lanthanide or lanthanoid series comprises the fifteen metallic chemical elements with atomic numbers 57 through 71, from lanthanum through lutetium...
one. This led to americium being located right below its twin lanthanide element europium; it was thus by analogy named after another continent,
AmericaThe Americas, or America , are lands in the Western hemisphere, also known as the New World. In English, the plural form the Americas is often used to refer to the landmasses of North America and South America with their associated islands and regions, while the singular form America is primarily...
: "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 was isolated from its
oxideAn oxide is a chemical compound that contains at least one oxygen atom in its chemical formula. Metal oxides typically contain an anion of oxygen in the oxidation state of −2....
s in a complex, multi-step process. First
plutoniumPlutonium is a transuranic radioactive chemical element with the chemical symbol Pu and atomic number 94. It is an actinide metal of silvery-gray appearance that tarnishes when exposed to air, forming a dull coating when oxidized. The element normally exhibits six allotropes and four oxidation...
-239 nitrate (
239PuNO
3) solution was coated on a
platinumPlatinum is a chemical element with the chemical symbol Pt and an atomic number of 78. Its name is derived from the Spanish term platina del Pinto, which is literally translated into "little silver of the Pinto River." It is a dense, malleable, ductile, precious, gray-white transition metal...
foil of about 0.5 cm
2 area, the solution was evaporated and the residue was converted into plutonium dioxide (PuO
2) by annealing. After cyclotron irradiation, the coating was dissolved with
nitric acidNitric acid , also known as aqua fortis and spirit of nitre, is a highly corrosive and toxic strong acid.Colorless when pure, older samples tend to acquire a yellow cast due to the accumulation of oxides of nitrogen. If the solution contains more than 86% nitric acid, it is referred to as fuming...
, and then precipitated as the hydroxide using concentrated aqueous
ammonia solutionAmmonia solution, also known as ammonium hydroxide, ammonia water, ammonical liquor, ammonia liquor, aqua ammonia, aqueous ammonia, or simply ammonia, is a solution of ammonia in water. It can be denoted by the symbols NH3...
. The residue was dissolved in
perchloric acidPerchloric acid is the inorganic compound with the formula HClO4. Usually encountered as an aqueous solution, this colourless compound is a strong acid comparable in strength to sulfuric and nitric acids. It is a powerful oxidizer, but its aqueous solutions up to appr. 70% are remarkably inert,...
. Further separation was carried out by
ion exchangeIon exchange is an exchange of ions between two electrolytes or between an electrolyte solution and a complex. In most cases the term is used to denote the processes of purification, separation, and decontamination of aqueous and other ion-containing solutions with solid polymeric or mineralic 'ion...
, 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:
241Am,
242Am,
239Am and
238Am. Americium-241 was directly obtained from plutonium upon absorption of one neutron. It decays by emission of a
α-particleAlpha particles consist of two protons and two neutrons bound together into a particle identical to a helium nucleus, which is classically produced in the process of alpha decay, but may be produced also in other ways and given the same name...
to
237Np; the
half-lifeHalf-life, abbreviated t½, is the period of time it takes for the amount of a substance undergoing decay to decrease by half. The name was originally used to describe a characteristic of unstable atoms , but it may apply to any quantity which follows a set-rate decay.The original term, dating to...
of this decay was first determined as 510 ± 20 years but then corrected to 432.2 years.
- The times are half-lives
Half-life, abbreviated t½, is the period of time it takes for the amount of a substance undergoing decay to decrease by half. The name was originally used to describe a characteristic of unstable atoms , but it may apply to any quantity which follows a set-rate decay.The original term, dating to...
The second isotope
242Am was produced upon neutron bombardment of the already-created
241Am. Upon rapid β-decay,
242Am converts into the isotope of curium
242Cm (which had been discovered previously). The half-life of this decay 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 ProjectThe Manhattan Project was a research and development program, led by the United States with participation from the United Kingdom and Canada, that produced the first atomic bomb during World War II. From 1942 to 1946, the project was under the direction of Major General Leslie Groves of the US Army...
; 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, the
Quiz KidsQuiz Kids, a popular radio-TV series of the 1940s and 1950s, was created by Chicago public relations and advertising man Louis G. Cowan . Originally sponsored by Alka-Seltzer, the series was first broadcast on NBC from Chicago, June 28, 1940, airing as a summer replacement show for Alec Templeton...
, five days before the official presentation at an
American Chemical SocietyThe American Chemical Society is a scientific society based in the United States that supports scientific inquiry in the field of chemistry. Founded in 1876 at New York University, the ACS currently has more than 161,000 members at all degree-levels and in all fields of chemistry, chemical...
meeting on November 11, 1945, when one of the listeners asked whether any new transuranium element beside plutonium and neptunium had been discovered during the war. After the discovery of americium isotopes
241Am and
242Am, 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
bariumBarium is a chemical element with the symbol Ba and atomic number 56. It is the fifth element in Group 2, a soft silvery metallic alkaline earth metal. Barium is never found in nature in its pure form due to its reactivity with air. Its oxide is historically known as baryta but it reacts with...
metal in high vacuum at 1100 °C.
Occurrence
The longest-lived and most common isotopes of americium,
241Am and
243Am, have half-lives of 432.2 and 7,370 years, respectively. Therefore, all
primordialIn geochemistry and geonuclear physics, primordial nuclides or primordial isotopes are nuclides found on the earth that have existed in their current form since before Earth was formed. Only 288 such nuclides are known...
americium (americium that was present on Earth during its formation) should have decayed by now. Existing americium 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 disasterThe Chernobyl disaster was a nuclear accident that occurred on 26 April 1986 at the Chernobyl Nuclear Power Plant in Ukraine , which was under the direct jurisdiction of the central authorities in Moscow...
. For example, the analysis of the debris at the testing site of the first U.S. hydrogen bomb,
Ivy MikeIvy Mike was the codename given to the first United States test of a thermonuclear weapon, in which a major part of the explosive yield came from nuclear fusion. It was detonated on November 1, 1952 by the United States at on Enewetak, an atoll in the Pacific Ocean, as part of Operation Ivy...
, (November 1, 1952, Enewetak Atoll), revealed high concentrations of various actinides including americium. Due to military secrecy, this result was published only in 1956. Elevated levels of americium were also detected at the crash site of a US B-52 bomber, which carried four hydrogen bombs, in 1968 in
GreenlandGreenland is an autonomous country within the Kingdom of Denmark, located between the Arctic and Atlantic Oceans, east of the Canadian Arctic Archipelago. Though physiographically a part of the continent of North America, Greenland has been politically and culturally associated with Europe for...
.
In other regions, the average radioactivity due to residual americium is only about 0.01
picocuriesThe curie is a unit of radioactivity, defined asThis is roughly the activity of 1 gram of the radium isotope 226Ra, a substance studied by the pioneers of radiology, Marie and Pierre Curie, for whom the unit was named. In addition to the curie, activity can be measured using an SI derived unit,...
(0.37
mBqThe becquerel is the SI-derived unit of radioactivity. One Bq is defined as the activity of a quantity of radioactive material in which one nucleus decays per second. The Bq unit is therefore equivalent to an inverse second, s−1...
). Atmospheric americium compounds are poorly soluble in common solvents and mostly adhere to soil particles. Soil analysis revealed about 1,900 higher concentration of americium inside sandy soil particles than in the water present in the soil pores; an even higher ratio was measured in
loamLoam is soil composed of sand, silt, and clay in relatively even concentration . Loam soils generally contain more nutrients and humus than sandy soils, have better infiltration and drainage than silty soils, and are easier to till than clay soils...
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
241Am and
243Am. Their prolonged radioactivity is undesirable for the disposal, and therefore americium, together with other long-lived actinides, have to be neutralized. The associated procedure may involve several steps, where americium is first separated and then converted by neutron bombardment in special reactors to short-lived nuclides. This procedure is well known as
nuclear transmutationNuclear transmutation is the conversion of one chemical element or isotope into another. In other words, atoms of one element can be changed into atoms of other element by 'transmutation'...
, but it is still being developed for americium.
Isotope nucleosyntheses
Americium has been produced in small quantities in
nuclear reactorA nuclear reactor is a device to initiate and control a sustained nuclear chain reaction. Most commonly they are used for generating electricity and for the propulsion of ships. Usually heat from nuclear fission is passed to a working fluid , which runs through turbines that power either ship's...
s for decades, and kilograms of its
241Am and
243Am isotopes have been accumulated by now. Nevertheless, since it was first offered for sale in 1962, its price, about 1,500
USDThe United States dollar , also referred to as the American dollar, is the official currency of the United States of America. It is divided into 100 smaller units called cents or pennies....
per gram of
241Am, remains almost unchanged owing to the very complex separation procedure. The heavier isotope
243Am is produced in much smaller amounts; it is thus more difficult to separate, resulting in a higher cost of the order 100–160 USD/mg.
Americium is not synthesized directly from uranium – the most common reactor material – but from the plutonium isotope
239Pu. The latter needs to be produced first, according to the following nuclear process:
-
The capture of two neutrons by
239Pu (a so-called (n,γ) reaction), followed by a β-decay, results in
241Am:
-
The plutonium present in spent nuclear fuel contains about 12% of
241Pu. Because it spontaneously converts to
241Am,
241Pu can be extracted and may be used to generate further
241Am. However, this process is rather slow: half of the original amount of
241Pu decays to
241Am after about 15 years, and the
241Am amount reaches a maximum after 70 years.
The obtained
241Am can be used for generating heavier americium isotopes by further neutron capture inside a nuclear reactor. In a
light water reactorThe light water reactor is a type of thermal reactor that uses normal water as its coolant and neutron moderator. Thermal reactors are the most common type of nuclear reactor, and light water reactors are the most common type of thermal reactor...
(LWR), 79% of
241Am converts to
242Am and 10% to its
nuclear isomerA nuclear isomer is a metastable state of an atomic nucleus caused by the excitation of one or more of its nucleons . "Metastable" refers to the fact that these excited states have half-lives more than 100 to 1000 times the half-lives of the other possible excited nuclear states...
242mAm:
[The "metastable" state is marked by letter m.]
- 79%:
- 10%:
Americium-242 has a half-life of only 16 hours, which makes its further up-conversion to
243Am, extremely inefficient. The latter isotope is produced instead in a process where
239Pu captures four neutrons under high
neutron fluxThe neutron flux is a quantity used in reactor physics corresponding to the total length travelled by all neutrons per unit time and volume . The neutron fluence is defined as the neutron flux integrated over a certain time period....
:
-
Metal generation
Most synthesis routines yield a mixture of different actinide isotopes in oxide forms, from which isotopes of americium need to be separated. In a typical procedure, the spent reactor fuel (e.g.
MOX fuelMixed oxide fuel, commonly referred to as MOX fuel, is nuclear fuel that contains more than one oxide of fissile material. MOX fuel contains plutonium blended with natural uranium, reprocessed uranium, or depleted uranium. MOX fuel is an alternative to the low-enriched uranium fuel used in the...
) is dissolved in
nitric acidNitric acid , also known as aqua fortis and spirit of nitre, is a highly corrosive and toxic strong acid.Colorless when pure, older samples tend to acquire a yellow cast due to the accumulation of oxides of nitrogen. If the solution contains more than 86% nitric acid, it is referred to as fuming...
, and the bulk of uranium and plutonium is removed using a
PUREXPUREX is an acronym standing for Plutonium - URanium EXtraction — de facto standard aqueous nuclear reprocessing method for the recovery of uranium and plutonium from used nuclear fuel. It is based on liquid-liquid extraction ion-exchange.The PUREX process was invented by Herbert H. Anderson and...
-type extraction (
Plutonium –
URanium
EXtraction) with
tributyl phosphateTributyl phosphate, known commonly as TBP, is an organophosphorus compound with the formula 3PO. This colourless, odorless liquid finds some applications as an extractant and a plasticizer. It is an ester of orthophosphoric acid with n-butanol.- Production :Tributyl phosphate is manufactured by...
in a
hydrocarbonIn organic chemistry, a hydrocarbon is an organic compound consisting entirely of hydrogen and carbon. Hydrocarbons from which one hydrogen atom has been removed are functional groups, called hydrocarbyls....
. The lanthanides and remaining actinides are then separated from the aqueous residue (
raffinateRaffinating something is a technique used in metallurgy to remove impurities from liquid material. There are many different kinds of raffination, for example you can use vacuum to extract hydrogen from metals....
) 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 the solvent extraction of americium. For example, a recent EU funded project codenamed "EUROPART" studied
triazineA triazine is one of three organic chemicals, isomeric with each other, whose molecular formula is 333 and whose empirical formula is CHN.- Structure :...
s and other compounds as potential extraction agents.
Bis-triazinyl bipyridineThe bis-triazinyl bipyridines are a class of chemical compounds which are tetradentate ligands similar in shape to quaterpyridine. The BTBPs are made by the reaction of hydrazine and a 1,2-diketone with 6,6'-dicyano-2,2'-bipyridine...
complex has been recently proposed as such reagent as 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 bicarbonateSodium bicarbonate or sodium hydrogen carbonate is the chemical compound with the formula Na HCO3. Sodium bicarbonate is a white solid that is crystalline but often appears as a fine powder. It has a slightly salty, alkaline taste resembling that of washing soda . The natural mineral form is...
with
ozoneOzone , or trioxygen, is a triatomic molecule, consisting of three oxygen atoms. It is an allotrope of oxygen that is much less stable than the diatomic allotrope...
, at elevated temperatures. Both Am and Cm are mostly present in solutions in 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
reductionRedox reactions describe all chemical reactions in which atoms have their oxidation state changed....
from its compounds. Americium(III) fluoride was first used for this purpose. The reaction was conducted using elemental
bariumBarium is a chemical element with the symbol Ba and atomic number 56. It is the fifth element in Group 2, a soft silvery metallic alkaline earth metal. Barium is never found in nature in its pure form due to its reactivity with air. Its oxide is historically known as baryta but it reacts with...
as reducing agent in a water- and oxygen-free environment inside an apparatus made of
tantalumTantalum is a chemical element with the symbol Ta and atomic number 73. Previously known as tantalium, the name comes from Tantalus, a character in Greek mythology. Tantalum is a rare, hard, blue-gray, lustrous transition metal that is highly corrosion resistant. It is part of the refractory...
and
tungstenTungsten , also known as wolfram , is a chemical element with the chemical symbol W and atomic number 74.A hard, rare metal under standard conditions when uncombined, tungsten is found naturally on Earth only in chemical compounds. It was identified as a new element in 1781, and first isolated as...
.
-
An alternative is the reduction of americium dioxide by metallic
lanthanumLanthanum is a chemical element with the symbol La and atomic number 57.Lanthanum is a silvery white metallic element that belongs to group 3 of the periodic table and is the first element of the lanthanide series. It is found in some rare-earth minerals, usually in combination with cerium and...
or
thoriumThorium is a natural radioactive chemical element with the symbol Th and atomic number 90. It was discovered in 1828 and named after Thor, the Norse god of thunder....
:
-
Physical properties
In the
periodic tableThe periodic table of the chemical elements is a tabular display of the 118 known chemical elements organized by selected properties of their atomic structures. Elements are presented by increasing atomic number, the number of protons in an atom's atomic nucleus...
, americium is located right to plutonium, left to curium, and below the lanthanide
europiumEuropium is a chemical element with the symbol Eu and atomic number 63. It is named after the continent of Europe. It is a moderately hard silvery metal which readily oxidizes in air and water...
, with which it shares many similarities in 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 a density of 12 g/cm
3, americium is lighter than both curium (13.52 g/cm
3) and plutonium (19.8 g/cm
3); but is heavier than europium (5.264 g/cm
3)—mostly because of its higher atomic mass. Americium is relatively soft and easily deformable and has a significantly lower
bulk modulusThe bulk modulus of a substance measures the substance's resistance to uniform compression. It is defined as the pressure increase needed to decrease the volume by a factor of 1/e...
than the actinides before it: Th, Pa, U, Np and Pu. Its melting point of 1173 °C 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 symmetryIn crystallography, the hexagonal crystal system is one of the 7 crystal systems, the hexagonal lattice system is one of the 7 lattice systems, and the hexagonal crystal family is one of the 6 crystal families...
, and a
space groupIn mathematics and geometry, a space group is a symmetry group, usually for three dimensions, that divides space into discrete repeatable domains.In three dimensions, there are 219 unique types, or counted as 230 if chiral copies are considered distinct...
P6
3/mmc with lattice parameters
a = 346.8 pm and
c = 1124 pm, and four atoms per unit cell. The crystal consists of 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 cubicIn crystallography, the cubic crystal system is a crystal system where the unit cell is in the shape of a cube. This is one of the most common and simplest shapes found in crystals and minerals....
(
fcc) symmetry, space group Fmm and lattice constant
a = 489 pm. This
fcc structure is equivalent to the closest packing with the sequence ABC. Upon further compression to 23 GPa, americium transforms to an
orthorhombicIn crystallography, the orthorhombic crystal system is one of the seven lattice point groups. Orthorhombic lattices result from stretching a cubic lattice along two of its orthogonal pairs by two different factors, resulting in a rectangular prism with a rectangular base and height , such that a,...
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 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 is different from β-Am, and at 1075 °C it converts to a
body-centered cubicIn crystallography, the cubic crystal system is a crystal system where the unit cell is in the shape of a cube. This is one of the most common and simplest shapes found in crystals and minerals....
structure. The pressure-temperature phase diagram of americium is thus rather similar to those of lanthanum,
praseodymiumPraseodymium is a chemical element that has the symbol Pr and atomic number 59. Praseodymium is a soft, silvery, malleable and ductile metal in the lanthanide group. It is too reactive to be found in native form, and when artificially prepared, it slowly develops a green oxide coating.The element...
and
neodymiumNeodymium is a chemical element with the symbol Nd and atomic number 60. It is a soft silvery metal that tarnishes in air. Neodymium was discovered in 1885 by the Austrian chemist Carl Auer von Welsbach. It is present in significant quantities in the ore minerals monazite and bastnäsite...
.
As with many other actinides, self-damage of the crystal lattice due to alpha-particle irradiation is intrinsic to americium. It is especially noticeable at low temperatures, where the mobility of the produced
lattice defectsInterstitials are a variety of crystallographic defects, i.e. atoms which occupy a site in the crystal structure at which there is usually not an atom, or two or more atoms sharing one or more lattice sites such that the number of atoms is larger than the number of lattice sites.They are generally...
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
resistivityElectrical resistivity is a measure of how strongly a material opposes the flow of electric current. A low resistivity indicates a material that readily allows the movement of electric charge. The SI unit of electrical resistivity is the ohm metre...
. 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 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 heliumHelium exists in liquid form only at extremely low temperatures. The boiling point and critical point depend on the isotope of the helium; see the table below for values. The density of liquid helium-4 at its boiling point and 1 atmosphere is approximately 0.125 g/mL Helium-4 was first liquefied...
to 69 µOhm·cm at room temperature; this behavior is similar to that of neptunium, uranium, thorium and
protactiniumProtactinium is a chemical element with the symbol Pa and atomic number 91. It is a dense, silvery-gray metal which readily reacts with oxygen, water vapor and inorganic acids. It forms various chemical compounds where protactinium is usually present in the oxidation state +5, but can also assume...
, but 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
paramagneticParamagnetism is a form of magnetism whereby the paramagnetic material is only attracted when in the presence of an externally applied magnetic field. In contrast with this, diamagnetic materials are repulsive when placed in a magnetic field...
in a wide temperature range, from that of
liquid heliumHelium exists in liquid form only at extremely low temperatures. The boiling point and critical point depend on the isotope of the helium; see the table below for values. The density of liquid helium-4 at its boiling point and 1 atmosphere is approximately 0.125 g/mL Helium-4 was first liquefied...
, to room temperature, and above. This behavior is markedly different from that of its neighbor curium which exhibit antiferromagnetic transition at 52 K. The
thermal expansionThermal expansion is the tendency of matter to change in volume in response to a change in temperature.When a substance is heated, its particles begin moving more and thus usually maintain a greater average separation. Materials which contract with increasing temperature are rare; this effect is...
coefficient of americium is slightly anisotropic and amounts to (7.5 ± 0.2)/°C along the shorter
a axis and (6.2 ± 0.4)/°C for the longer
c hexagonal axis. The
enthalpy of dissolutionThe enthalpy of solution, enthalpy of dissolution, or heat of solution is the enthalpy change associated with the dissolution of a substance in a solvent at constant pressure resulting in infinite dilution....
of americium metal in
hydrochloric acidHydrochloric acid is a solution of hydrogen chloride in water, that is a highly corrosive, strong mineral acid with many industrial uses. It is found naturally in gastric acid....
at standard conditions is −620.6 ± 1.3 kJ/mol, from which the
standard enthalpy change of formationThese tables include heat of formation data gathered from a variety of sources, including the primary and secondary literature, as well as the NIST Chemistry WebBook...
(Δ
fH°) of aqueous Am
3+ ion is −621.2 ± 2.0 kJ/mol
−1. The standard potential Am
3+/Am
0 is 2.08 ± 0.01 V.
Chemical properties
Americium readily reacts with oxygen and dissolves well in
acidAn acid is a substance which reacts with a base. Commonly, acids can be identified as tasting sour, reacting with metals such as calcium, and bases like sodium carbonate. Aqueous acids have a pH of less than 7, where an acid of lower pH is typically stronger, and turn blue litmus paper red...
s. The most common
oxidation stateIn chemistry, the oxidation state is an indicator of the degree of oxidation of an atom in a chemical compound. The formal oxidation state is the hypothetical charge that an atom would have if all bonds to atoms of different elements were 100% ionic. Oxidation states are typically represented by...
for americium is +3, in which americium compounds are rather stable against oxidation and reduction. In this sense, americium is chemically similar to most lanthanides. The trivalent americium forms insoluble
fluorideFluoride is the anion F−, the reduced form of fluorine when as an ion and when bonded to another element. Both organofluorine compounds and inorganic fluorine containing compounds are called fluorides. Fluoride, like other halides, is a monovalent ion . Its compounds often have properties that are...
,
oxalateOxalate , is the dianion with formula C2O42− also written 22−. Either name is often used for derivatives, such as disodium oxalate, 2C2O42−, or an ester of oxalic acid Oxalate (IUPAC: ethanedioate), is the dianion with formula C2O42− also written (COO)22−. Either...
,
iodateAn iodate is a conjugate base of iodic acid. In the iodate anion, iodine is bonded to three oxygen atoms and the molecular formula is IO3−. The molecular geometry of iodate is trigonal pyramidal....
,
hydroxideHydroxide is a diatomic anion with chemical formula OH−. It consists of an oxygen and a hydrogen atom held together by a covalent bond, and carrying a negative electric charge. It is an important but usually minor constituent of water. It functions as a base, as a ligand, a nucleophile, and a...
,
phosphateA phosphate, an inorganic chemical, is a salt of phosphoric acid. In organic chemistry, a phosphate, or organophosphate, is an ester of phosphoric acid. Organic phosphates are important in biochemistry and biogeochemistry or ecology. Inorganic phosphates are mined to obtain phosphorus for use in...
and other salts. Other oxidation states have been observed between +2 and +7, which is the widest range among the actinide elements. Their color in aqueous solutions varies as follows: Am
3+ (colorless to yellow-reddish), Am
4+ (yellow-reddish), Am
VO
2+; (yellow), Am
VIO
22+ (brown) and Am
VIIO
65- (dark green). All oxidation states have their characteristic optical absorption spectra, with a few sharp peaks in the visible and mid-infrared regions, and the position and intensity of these peaks can be converted into the concentrations of the corresponding oxidation states. For example, Am(III) has two sharp peaks at 504 and 811 nm, Am(V) at 514 and 715 nm, and Am(VI) at 666 and 992 nm.
Americium compounds with oxidation state +4 and higher are strong oxidizing agents, comparable in strength to the
permanganateA permanganate is the general name for a chemical compound containing the manganate ion, . Because manganese is in the +7 oxidation state, the permanganate ion is a strong oxidizing agent. The ion has tetrahedral geometry...
ion (MnO
4-) in acidic solutions. Whereas the Am
4+ ions are unstable in solutions and readily convert to Am
3+, the +4 oxidation state occurs well in solids, such as americium dioxide (AmO
2) and americium(IV) fluoride (AmF
4).
All pentavalent and hexavalent americium compounds are complex salts such as KAmO
2F
2, Li
3AmO
4 and Li
6AmO
6, Ba
3AmO
6, AmO
2F
2. These high oxidation states Am(IV), Am(V) and Am(VI) can be prepared from Am(III) by oxidation with
ammonium persulfateAmmonium persulfate 2S2O8 is a strong oxidizing agent. It is very soluble in water; the dissolution of the salt in water is endothermic. It is a radical initiator. It is used to etch copper on printed circuit boards as an alternative to ferric chloride solution...
in dilute nitric acid, with silver(I) oxide in
perchloric acidPerchloric acid is the inorganic compound with the formula HClO4. Usually encountered as an aqueous solution, this colourless compound is a strong acid comparable in strength to sulfuric and nitric acids. It is a powerful oxidizer, but its aqueous solutions up to appr. 70% are remarkably inert,...
, or with
ozoneOzone , or trioxygen, is a triatomic molecule, consisting of three oxygen atoms. It is an allotrope of oxygen that is much less stable than the diatomic allotrope...
or
sodium persulfateSodium persulfate is a chemical compound. It is a strong oxidizer. It is a severe irritant of skin, eyes, and respiratory system. It is almost non-hygroscopic and has particularly good ability to be stored for long time. It is easy and safe to handle...
in
sodium carbonateSodium carbonate , Na2CO3 is a sodium salt of carbonic acid. It most commonly occurs as a crystalline heptahydrate, which readily effloresces to form a white powder, the monohydrate. Sodium carbonate is domestically well-known for its everyday use as a water softener. It can be extracted from the...
solutions. The pentavalent oxidation state of americium was first observed in 1951. It is present in aqueous solution in the form of AmO
2+ ions (acidic) or AmO
3- ions (alkaline) which are however unstable and subject to several rapid
disproportionationDisproportionation, also known as dismutation is used to describe a specific type of redox reaction in which a species is simultaneously reduced and oxidized so as to form two different products....
reactions:
-
-
Oxygen compounds
Two americium oxides are known, with the oxidation states +3 (Am
2O
3) and +4 (AmO
2). 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 a cubic (
fluoriteFluorite is a halide mineral composed of calcium fluoride, CaF2. It is an isometric mineral with a cubic habit, though octahedral and more complex isometric forms are not uncommon...
) crystal structure.
The oxalate of americium(III), vacuum dried at room temperature, has the chemical formula Am
2(C
2O
4)
3·7H
2O. 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
HalideA halide is a binary compound, of which one part is a halogen atom and the other part is an element or radical that is less electronegative than the halogen, to make a fluoride, chloride, bromide, iodide, or astatide compound. Many salts are halides...
s of americium are known for the oxidation states +2, +3 and +4, where the +3 is most stable, especially in solutions.
| Oxidation state |
F |
Cl |
Br |
I |
| +4 |
Americium(IV) fluoride
AmF4
pale pink |
|
|
|
| +3 |
Americium(III) fluoride
AmF3
pink |
Americium(III) chlorideAmericium chloride or americium trichloride is the chemical compound composed of americium and chlorine with the formula AmCl3. It forms pink hexagonal crystals....
AmCl3
pink |
Americium(III) bromide
AmBr3
light yellow |
Americium(III) iodide
AmI3
light yellow |
| +2 |
|
Americium(II) chloride
AmCl2
black |
Americium(II) bromide
AmBr2
black |
Americium(II) iodide
AmI2
black |
Reduction of Am(III) compounds with sodium
amalgamAn amalgam is a substance formed by the reaction of mercury with another metal. Almost all metals can form amalgams with mercury, notable exceptions being iron and platinum. Silver-mercury amalgams are important in dentistry, and gold-mercury amalgam is used in the extraction of gold from ore.The...
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:
- Orthorhombic
In crystallography, the orthorhombic crystal system is one of the seven lattice point groups. Orthorhombic lattices result from stretching a cubic lattice along two of its orthogonal pairs by two different factors, resulting in a rectangular prism with a rectangular base and height , such that a,...
AmCl2: a = 896.3 ± 0.8 pm, b = 757.3 ± 0.8 pm and c = 453.2 ± 0.6 pm
- Tetragonal
In crystallography, the tetragonal crystal system is one of the 7 lattice point groups. Tetragonal crystal lattices result from stretching a cubic lattice along one of its lattice vectors, so that the cube becomes a rectangular prism with a square base and height .There are two tetragonal Bravais...
AmBr2: a = 1159.2 ± 0.4 and c = 712.1 ± 0.3 pm.
They can also be prepared by reacting metallic americium with an appropriate mercury halide HgX
2, where X = Cl, Br or I:
-
Americium(III) fluoride (AmF
3) is poorly soluble and precipitates upon reaction of Am
3+ and fluoride ions in weak acidic solutions:
-
The tetravalent americium(IV) fluoride (AmF
4) is obtained by reacting solid americium(III) fluoride with molecular
fluorineFluorine is the chemical element with atomic number 9, represented by the symbol F. It is the lightest element of the halogen column of the periodic table and has a single stable isotope, fluorine-19. At standard pressure and temperature, fluorine is a pale yellow gas composed of diatomic...
:
-
Another known form of solid tetravalent americium chloride is KAmF
5. Tetravalent americium has also been observed in the aqueous phase. For this purpose, black Am(OH)
4 was dissolved in 15-
MThe mole is a unit of measurement used in chemistry to express amounts of a chemical substance, defined as an amount of a substance that contains as many elementary entities as there are atoms in 12 grams of pure carbon-12 , the isotope of carbon with atomic weight 12. This corresponds to a value...
NH
3F 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 the color and exact structure between the halogens. So, chloride (AmCl
3) is reddish and has a structure isotypic to
uranium(III) chlorideUranium chloride , UCl3, is a chemical compound that contains the earth metal uranium and chlorine. UCl3 is used mostly to reprocess spent nuclear fuel. Uranium chloride is synthesized various ways from uranium chloride; however, UCl3 is less stable than UCl4.-Preparation:There are two ways to...
(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). The bromide is an exception with the orthorhombic PuBr
3-type structure and space group Cmcm. Crystals of americium hexahydrate (AmCl
3·6H
2O) can be prepared by dissolving americium dioxide in hydrochloric acid and evaporating the liquid. Those crystals are hygroscopic and have yellow-reddish color and a
monoclinicIn crystallography, the monoclinic crystal system is one of the 7 lattice point groups. A crystal system is described by three vectors. In the monoclinic system, the crystal is described by vectors of unequal length, as in the orthorhombic system. They form a rectangular prism with a...
crystal structure.
Oxyhalides of americium in the form Am
VIO
2X
2, Am
VO
2X, Am
IVOX
2 and Am
IIIOX can be obtained by reacting the corresponding americium halide with oxygen or Sb
2O
3, and AmOCl can also be produced by vapor phase
hydrolysisHydrolysis is a chemical reaction during which molecules of water are split into hydrogen cations and hydroxide anions in the process of a chemical mechanism. It is the type of reaction that is used to break down certain polymers, especially those made by condensation polymerization...
:
-
Chalcogenides and pnictides
The known
chalcogenideA chalcogenide is a chemical compound consisting of at least one chalcogen ion and at least one more electropositive element. Although all group 16 elements of the periodic table are defined as chalcogens, the term is more commonly reserved for sulfides, selenides, and tellurides, rather than...
s of americium include the
sulfideA sulfide is an anion of sulfur in its lowest oxidation state of 2-. Sulfide is also a slightly archaic term for thioethers, a common type of organosulfur compound that are well known for their bad odors.- Properties :...
AmS
2, selenides AmSe
2 and Am
3Se
4, and
telluridesThe telluride ion is Te2−. It is the final stable member of the series of dianions O2−, S2−, and Se2− ....
Am
2Te
3 and AmTe
2. The
pnictideThe nitrogen group is a periodic table group consisting of nitrogen , phosphorus , arsenic , antimony , bismuth and ununpentium ....
s of americium (
243Am) of the AmX type are known for the elements
phosphorusPhosphorus is the chemical element that has the symbol P and atomic number 15. A multivalent nonmetal of the nitrogen group, phosphorus as a mineral is almost always present in its maximally oxidized state, as inorganic phosphate rocks...
,
arsenicArsenic is a chemical element with the symbol As, atomic number 33 and relative atomic mass 74.92. Arsenic occurs in many minerals, usually in conjunction with sulfur and metals, and also as a pure elemental crystal. It was first documented by Albertus Magnus in 1250.Arsenic is a metalloid...
,
antimonyAntimony is a toxic chemical element with the symbol Sb and an atomic number of 51. A lustrous grey metalloid, it is found in nature mainly as the sulfide mineral stibnite...
and
bismuthBismuth is a chemical element with symbol Bi and atomic number 83. Bismuth, a trivalent poor metal, chemically resembles arsenic and antimony. Elemental bismuth may occur naturally uncombined, although its sulfide and oxide form important commercial ores. The free element is 86% as dense as lead...
. They crystallize in the
rock-saltIn crystallography, the cubic crystal system is a crystal system where the unit cell is in the shape of a cube. This is one of the most common and simplest shapes found in crystals and minerals....
lattice.
Silicides and borides
Americium
monosilicideA silicide is a compound that has silicon with more electropositive elements.Silicon is more electropositive than carbon. Silicides are structurally closer to borides than to carbides....
(AmSi) and "disilicide" (nominally AmSi
x with: 1.87 < x < 2.0) were obtained by reduction of americium(III) fluoride with elementary
siliconSilicon is a chemical element with the symbol Si and atomic number 14. A tetravalent metalloid, it is less reactive than its chemical analog carbon, the nonmetal directly above it in the periodic table, but more reactive than germanium, the metalloid directly below it in the table...
in vacuum at 1050 °C (AmSi) and 1150-1200 °C (AmSi
x). AmSi 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
I4
1/amd), it is isomorphic with PuSi
2 and ThSi
2.
BorideIn chemistry a boride is a chemical compound between boron and a less electronegative element, for example silicon boride . The borides are a very large group of compounds that are generally high melting and are not ionic in nature. Some borides exhibit very useful physical properties. The term...
s of americium include AmB
4 and AmB
6. The tetraboride can be obtained by heating an oxide or halide of americium with
magnesium diborideMagnesium diboride is a simple ionic binary compound that has proven to be an inexpensive and useful superconducting material.Its superconductivity was announced in the journal Nature in March 2001. Its critical temperature of is the highest amongst conventional superconductors...
in vacuum or inert atmosphere.
Organometallic compounds
Analogous to
uranoceneUranocene U2 is the most notable cyclooctatetraenide of the f elements and one of the first organouranium compounds to be synthesized. Uranocene is a member of the actinocenes, a group of metallocenes incorporating elements from the actinide series...
, americium forms an organometallic compound with two
cyclooctatetraene1,3,5,7-Cyclooctatetraene is an unsaturated derivative of cyclooctane, with the formula C8H8. It is also known as [8]annulene. This polyunsaturated hydrocarbon is a colorless to light yellow flammable liquid at room temperature...
ligands, that is (η
8-C
8H
8)
2Am. It also makes trigonal (η
5-C
5H
5)
3Am complexes with three
cyclopentadienylA cyclopentadienyl complex is a metal complex with one or more cyclopentadienyl groups . Based on the type of bonding between the metals and the cyclopentadienyl]] moieties, cyclopentadienyl complexes are classified into the following three categories: a) π-complexes, b) σ-complexes, and c) ionic...
rings.
Formation of the complexes of the type Am(n-C
3H
7-BTP)
3, where BTP stands for 2,6-di(1,2,4-triazin-3-yl)pyridine, in solutions containing n-C
3H
7-BTP and Am
3+ ions has been confirmed by
EXAFSX-ray Absorption Spectroscopy includes both Extended X-Ray Absorption Fine Structure and X-ray Absorption Near Edge Structure . XAS is the measurement of the x-ray absorption coefficient of a material as a function of energy...
. Some of these BTP-type complexes selectively interact with americium and therefore are useful in its selective separation from lanthanides and another actinides.
Biological aspects
Americium is an artificial element, and thus a biological function involving the element, like all elements heavier than
tungstenTungsten , also known as wolfram , is a chemical element with the chemical symbol W and atomic number 74.A hard, rare metal under standard conditions when uncombined, tungsten is found naturally on Earth only in chemical compounds. It was identified as a new element in 1781, and first isolated as...
, and thus all artificial elements, would be impossible. It has been proposed to use bacteria for removal of americium and other heavy metals from rivers and streams. Thus,
EnterobacteriaceaeThe Enterobacteriaceae is a large family of bacteria that includes many of the more familiar pathogens, such as Salmonella, Escherichia coli, Yersinia pestis, Klebsiella and Shigella. This family is the only representative in the order Enterobacteriales of the class Gammaproteobacteria in the...
of the genus
CitrobacterCitrobacter is a genus of Gram-negative coliform bacteria in the Enterobacteriaceae family.The species C. amalonaticus, C. koseri, and C. freundii use solely citrate as a carbon source...
precipitate americium ions from aqueous solutions, binding them into a metal-phosphate complex at their cell walls. Several studies have been reported on the
biosorptionBiosorption is a physiochemical process that occurs naturally in certain biomass which allows it to passively concentrate and bind contaminants onto its cellular structure...
and
bioaccumulationBioaccumulation refers to the accumulation of substances, such as pesticides, or other organic chemicals in an organism. Bioaccumulation occurs when an organism absorbs a toxic substance at a rate greater than that at which the substance is lost...
of americium by bacteria and fungi.
Fission
The isotope
242m1Am (half-life 141 years) has the largest cross sections for absorption of thermal neutrons (5,700
barnsA barn is a unit of area. Originally used in nuclear physics for expressing the cross sectional area of nuclei and nuclear reactions, today it is used in all fields of high energy physics to express the cross sections of any scattering process, and is best understood as a measure of the...
), that results in a small
critical massA critical mass is the smallest amount of fissile material needed for a sustained nuclear chain reaction. The critical mass of a fissionable material depends upon its nuclear properties A critical mass is the smallest amount of fissile material needed for a sustained nuclear chain reaction. The...
for a sustained
nuclear chain reactionA nuclear chain reaction occurs when one nuclear reaction causes an average of one or more nuclear reactions, thus leading to a self-propagating number of these reactions. The specific nuclear reaction may be the fission of heavy isotopes or the fusion of light isotopes...
. The critical mass for a bare
242m1Am 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 weaponA nuclear weapon is an explosive device that derives its destructive force from nuclear reactions, either fission or a combination of fission and fusion. Both reactions release vast quantities of energy from relatively small amounts of matter. The first fission bomb test released the same amount...
s, but those based on
242m1Am are not known yet, probably because of its scarcity and high price. The critical masses of two other readily available isotopes,
241Am and
243Am, are relatively high – 57.6 to 75.6 kg for
241Am and 209 kg for
243Am. Scarcity and high price yet hinder application of americium as a
nuclear fuelNuclear fuel is a material that can be 'consumed' by fission or fusion to derive nuclear energy. Nuclear fuels are the most dense sources of energy available...
in
nuclear reactorA nuclear reactor is a device to initiate and control a sustained nuclear chain reaction. Most commonly they are used for generating electricity and for the propulsion of ships. Usually heat from nuclear fission is passed to a working fluid , which runs through turbines that power either ship's...
s.
There are proposals of very compact 10-kW high-flux reactors using as little as 20 grams of
242m1Am. Such low-power reactors would be relatively safe to use as
neutron sourceA Neutron source is a device that emits neutrons. There is a wide variety of different sources, ranging from hand-held radioactive sources to neutron research facilities operating research reactors and spallation sources...
s for
radiation therapyIn nuclear medicine procedures, elemental radionuclides are combined with other elements to form chemical compounds, or else combined with existing pharmaceutical compounds, to form radiopharmaceuticals. These radiopharmaceuticals, once administered to the patient, can localize to specific organs...
in hospitals.
Isotopes
About 19
isotopeIsotopes are variants of atoms of a particular chemical element, which have differing numbers of neutrons. Atoms of a particular element by definition must contain the same number of protons but may have a distinct number of neutrons which differs from atom to atom, without changing the designation...
s and 8
nuclear isomerA nuclear isomer is a metastable state of an atomic nucleus caused by the excitation of one or more of its nucleons . "Metastable" refers to the fact that these excited states have half-lives more than 100 to 1000 times the half-lives of the other possible excited nuclear states...
s are known for americium. There are two long-lived alpha-emitters,
241Am and
243Am with half-lives of 432.2 and 7,370 years, respectively, and the nuclear isomer
242m1Am has a long half-life of 141 years. The half-lives of other isotopes and isomers range from 0.64 microseconds for
245m1Am to 50.8 hours for
240Am. 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
237Np emitting alpha particles of 5 different energies, mostly at 5.486 MeV (85.2%) and 5.443 MeV (12.8%). Because many of the resulting states are metastable, they also emit gamma-rays with the discrete energies between 26.3 and 158.5 keV.
Americium-242 is a short-lived isotope with a half-life of 16.02 h. It mostly (82.7%) converts by β-decay to
242Cm, but also by
electron captureElectron capture is a process in which a proton-rich nuclide absorbs an inner atomic electron and simultaneously emits a neutrino...
to
242Pu (17.3%). Both
242Cm and
242Pu transform via nearly the same decay chain through
238Pu down to
234U.
Nearly all (99.541%) of
242m1Am decays by
internal conversionInternal conversion is a radioactive decay process where an excited nucleus interacts with an electron in one of the lower atomic orbitals, causing the electron to be emitted from the atom. Thus, in an internal conversion process, a high-energy electron is emitted from the radioactive atom, but...
to
242Am and the remaining 0.459% by α-decay to
238Np. The latter breaks down to
238Pu and then to
234U.
Americium-243 transforms by α-emission into
239Np, which converts by β-decay to
239Pu, and the
239Pu changes into
235U by emitting an α-particle.
Applications
Ionization detectors
Americium is the only synthetic element to have found its way into the household, where one common type of
smoke detectorA smoke detector is a device that detects smoke, typically as an indicator of fire. Commercial, industrial, and mass residential devices issue a signal to a fire alarm system, while household detectors, known as smoke alarms, generally issue a local audible and/or visual alarm from the detector...
uses
241Am in the form of americium dioxide as its source of
ionizing radiationIonizing radiation is radiation composed of particles that individually have sufficient energy to remove an electron from an atom or molecule. This ionization produces free radicals, which are atoms or molecules containing unpaired electrons...
. This isotope is preferred against
226RaRadium is a chemical element with atomic number 88, represented by the symbol Ra. Radium is an almost pure-white alkaline earth metal, but it readily oxidizes on exposure to air, becoming black in color. All isotopes of radium are highly radioactive, with the most stable isotope being radium-226,...
because it emits 5 times more alpha particles and relatively little of harmful γ-radiation. The amount of americium in a typical new smoke detector is 1
microcurieThe curie is a unit of radioactivity, defined asThis is roughly the activity of 1 gram of the radium isotope 226Ra, a substance studied by the pioneers of radiology, Marie and Pierre Curie, for whom the unit was named. In addition to the curie, activity can be measured using an SI derived unit,...
(37
kBqThe becquerel is the SI-derived unit of radioactivity. One Bq is defined as the activity of a quantity of radioactive material in which one nucleus decays per second. The Bq unit is therefore equivalent to an inverse second, s−1...
) or 0.28
microgramIn the metric system, a microgram is a unit of mass equal to one millionth of a gram , or 1/1000 of a milligram. It is one of the smallest units of mass commonly used...
. This amount declines slowly as the americium decays into
neptuniumNeptunium is a chemical element with the symbol Np and atomic number 93. A radioactive metal, neptunium is the first transuranic element and belongs to the actinide series. Its most stable isotope, 237Np, is a by-product of nuclear reactors and plutonium production and it can be used as a...
-237, a different transuranic element with a much longer half-life (about 2.14 million years). With its half-life of 432.2 years, the americium in a smoke detector includes about 3%
neptuniumNeptunium is a chemical element with the symbol Np and atomic number 93. A radioactive metal, neptunium is the first transuranic element and belongs to the actinide series. Its most stable isotope, 237Np, is a by-product of nuclear reactors and plutonium production and it can be used as a...
after 19 years, and about 5% after 32 years. The radiation passes through an
ionization chamberThe ionization chamber is the simplest of all gas-filled radiation detectors, and is used for the detection or measurement of ionizing radiation...
, an air-filled space between two
electrodeAn electrode is an electrical conductor used to make contact with a nonmetallic part of a circuit...
s, and permits a small, constant
currentElectric current is a flow of electric charge through a medium.This charge is typically carried by moving electrons in a conductor such as wire...
between the electrodes. Any smoke that enters the chamber absorbs the alpha particles, which reduces the ionization and affects this current, triggering the alarm. Compared to the alternative optical smoke detector, the ionization smoke detector is cheaper and can detect particles which are too small to produce significant light scattering; however, it is more prone to
false alarmsIn statistical test theory the notion of statistical error is an integral part of hypothesis testing. The test requires an unambiguous statement of a null hypothesis, which usually corresponds to a default "state of nature", for example "this person is healthy", "this accused is not guilty" or...
.
Radionuclide
As
241Am has a significantly longer half-life than
238Pu (432.2 years vs. 87 years), it has been proposed as an active element of
radioisotope thermoelectric generatorA radioisotope thermoelectric generator is an electrical generator that obtains its power from radioactive decay. In such a device, the heat released by the decay of a suitable radioactive material is converted into electricity by the Seebeck effect using an array of thermocouples.RTGs can be...
s, for example in spacecraft. Although americium produces less heat and electricity – the power yield is 114.7 mW/g for
241Am and 6.31 mW/g for
243Am (cf. 390 mW/g for
238Pu) – and its radiation poses more threat to humans owing to neutron emission, the
European Space AgencyThe European Space Agency , established in 1975, is an intergovernmental organisation dedicated to the exploration of space, currently with 18 member states...
is planning to use americium for its space probes.
Another proposed space-related application of americium is a fuel for space ships with nuclear propulsion. It relies on the very high rate of nuclear fission of
242mAm, which can be maintained even in a micrometer-thick foil. Small thickness avoids the problem of self-absorption of emitted radiation. This problem is pertinent to uranium or plutonium rods, in which only surface layers provide alpha-particles. The fission products of
242mAm can either directly propel the spaceship or they can heat up a thrusting gas; they can also transfer their energy to a fluid and generate electricity through a
magnetohydrodynamic generatorThe MHD generator or dynamo transforms thermal energy or kinetic energy directly into electricity. MHD generators are different from traditional electric generators in that they can operate at high temperatures without moving parts...
.
One more proposal which utilizes the high nuclear fission rate of
242mAm is a nuclear battery. Its design relies not on the energy of the emitted by americium alpha particles, but on their charge, that is the americium acts as the self-sustaining "cathode". A single 3.2 kg
242mAm charge of such battery could provide about 140 kW of power over a period of 80 days. With all the potential benefits, the current applications of
242mAm are as yet hindered by the scarcity and high price of this
nuclear isomerA nuclear isomer is a metastable state of an atomic nucleus caused by the excitation of one or more of its nucleons . "Metastable" refers to the fact that these excited states have half-lives more than 100 to 1000 times the half-lives of the other possible excited nuclear states...
.
Neutron source
The oxide of
241Am pressed with
berylliumBeryllium is the chemical element with the symbol Be and atomic number 4. It is a divalent element which occurs naturally only in combination with other elements in minerals. Notable gemstones which contain beryllium include beryl and chrysoberyl...
is an efficient
neutron sourceA Neutron source is a device that emits neutrons. There is a wide variety of different sources, ranging from hand-held radioactive sources to neutron research facilities operating research reactors and spallation sources...
. Here americium acts as the alpha source, and beryllium produces neutrons owing to its large cross-section for the (α,n) nuclear reaction:
-
-
The most widespread use of
241AmBe neutron sources a
neutron probeA neutron probe is a device used to measure the quantity of water present in soil.A typical neutron probe contains a pellet of americium-241 and beryllium. The alpha particles emitted by the decay of the americium collide with the light beryllium nuclei, producing fast neutrons...
– a device used to measure the quantity of water present in soil, as well as moisture/density for quality control in highway construction.
241Am neutron sources are also used in well logging applications, as well as in
neutron radiographyNeutron Radiography is the process by which film is exposed by first passing neutrons through an object to produce a visible image of the materials that make up the object. Primarily used in scientific investigations.- Brief History of Neutron Imaging :...
, tomography and other radiochemical investigations.
Production of other elements
Americium is a starting material for the production of other transuranic elements and transactinides – for example, 82.7% of
242Am decays to
242Cm and 17.3% to
242Pu. In the nuclear reactor,
242Am is also up-converted by neutron capture to
243Am and
244Am, which transforms by β-decay to
244Cm:
-
Irradiation of
241Am by
12C or
22Ne ions yields the isotopes
247Es (
einsteiniumEinsteinium is a synthetic element with the symbol Es and atomic number 99. It is the seventh transuranic element, and an actinide.Einsteinium was discovered in the debris of the first hydrogen bomb explosion in 1952, and named after Albert Einstein...
) or
260Db (
dubniumThe Soviet team proposed the name nielsbohrium in honor of the Danish nuclear physicist Niels Bohr. The American team proposed that the new element should be named hahnium , in honor of the late German chemist Otto Hahn...
), respectively. Furthermore, the element
berkeliumBerkelium , is a synthetic element with the symbol Bk and atomic number 97, a member of the actinide and transuranium element series. It is named after the city of Berkeley, California, the location of the University of California Radiation Laboratory where it was discovered in December 1949...
(
243Bk isotope) had been first intentionally produced and identified by bombarding
241Am with alpha particles, in 1949, by the same Berkeley group, using the same 60-inch cyclotron. Similarly,
nobeliumNobelium is a synthetic element with the symbol No and atomic number 102. It was first correctly identified in 1966 by scientists at the Flerov Laboratory of Nuclear Reactions in Dubna, Soviet Union...
was produced at the
Joint Institute for Nuclear ResearchThe Joint Institute for Nuclear Research, JINR , in Dubna, Moscow Oblast , Russia, is an international research centre for nuclear sciences, with 5500 staff members, 1200 researchers including 1000 Ph.D.s from eighteen member states The Joint Institute for Nuclear Research, JINR , in Dubna, Moscow...
,
DubnaDubna is a town in Moscow Oblast, Russia. It has a status of naukograd , being home to the Joint Institute for Nuclear Research, an international nuclear physics research centre and one of the largest scientific foundations in the country. It is also home to MKB Raduga, a defence aerospace company...
, Russia, in 1965 in several reactions, one of which included irradiation of
243Am with
15N ions. Besides, one of the synthesis reactions for
lawrenciumLawrencium is a radioactive synthetic chemical element with the symbol Lr and atomic number 103. In the periodic table of the elements, it is a period 7 d-block element and the last element of actinide series...
, discovered by scientists at Berkeley and Dubna, included bombardment of
243Am with
18O.
Spectrometer
Americium-241 has been used as a portable source of both gamma rays and alpha particles for a number of medical and industrial uses. The 60-keV gamma ray emissions from
241Am in such sources can be used for indirect analysis of materials in
radiographyRadiography is the use of X-rays to view a non-uniformly composed material such as the human body. By using the physical properties of the ray an image can be developed which displays areas of different density and composition....
and
X-ray fluorescenceX-ray fluorescence is the emission of characteristic "secondary" X-rays from a material that has been excited by bombarding with high-energy X-rays or gamma rays...
spectroscopy, as well as for quality control in fixed
nuclear density gaugeA nuclear density gauge is a tool used in civil construction and the petroleum industry, as well as for mining and archaeology purposes. It consists of a radiation source that emits a directed beam of particles and a sensor that counts the received particles that are either reflected by the test...
s and
nuclear densometerA nuclear densometer is a field instrument used in geotechnical engineering to determine the density of a compacted material. Also known as a soil density gauge, the device uses the interaction of gamma radiation with matter to measure density, either through direct transmission or the...
s. For example, the element has been employed to gauge
glassGlass is an amorphous solid material. Glasses are typically brittle and optically transparent.The most familiar type of glass, used for centuries in windows and drinking vessels, is soda-lime glass, composed of about 75% silica plus Na2O, CaO, and several minor additives...
thickness to help create flat glass. Americium-241 is also suitable for calibration of gamma-ray spectrometers in the low-energy range, since its spectrum consists of nearly a single peak and negligible Compton continuum (at least three orders of magnitude lower intensity). Americium-241 gamma rays were also used to provide passive diagnosis of thyroid function. This medical application is however obsolete.
Health issues
As a highly radioactive element, americium and its compounds must be handled only in an appropriate laboratory under special arrangements. Although most americium isotopes predominantly emit alpha particles which can be blocked by thin layers of common materials, many of the daughter products emit gamma-rays and neutrons which have a long penetration depth.
If consumed, americium is excreted within a few days and only 0.05% is absorbed in the blood. From there, roughly 45% of it goes to the
liverThe liver is a vital organ present in vertebrates and some other animals. It has a wide range of functions, including detoxification, protein synthesis, and production of biochemicals necessary for digestion...
and 45% to the bones, and the remaining 10% is excreted. The uptake to the liver depends on the individual and increases with age. In the bones, americium is first deposited over
corticalIn anatomy and zoology the cortex is the outermost layer of an organ. Organs with well-defined cortical layers include kidneys, adrenal glands, ovaries, the thymus, and portions of the brain, including the cerebral cortex, the most well-known of all cortices.The cerebellar cortex is the thin gray...
and
trabeculaA trabecula is a small, often microscopic, tissue element in the form of a small beam, strut or rod, generally having a mechanical function, and usually composed of dense collagenous tissue They can be composed of other materials; in the heart, for example, muscles such as trabeculae carneae...
r surfaces and slowly redistributes over the bone with time. The biological half-life of
241Am is 50 years in the bones and 20 years in the liver, whereas in the
gonadThe gonad is the organ that makes gametes. The gonads in males are the testes and the gonads in females are the ovaries. The product, gametes, are haploid germ cells. For example, spermatozoon and egg cells are gametes...
s (testicles and ovaries) it remains permanently; in all these organs, americium promotes formation of cancer cells as a result of its radioactivity.
Americium often enters landfills from discarded
smoke detectorA smoke detector is a device that detects smoke, typically as an indicator of fire. Commercial, industrial, and mass residential devices issue a signal to a fire alarm system, while household detectors, known as smoke alarms, generally issue a local audible and/or visual alarm from the detector...
s. The rules associated with the disposal of smoke detectors are relaxed in most jurisdictions. In the U.S., the "Radioactive Boy Scout"
David HahnDavid Charles Hahn , also called the "Radioactive Boy Scout" or the "Nuclear Boy Scout", is an American who attempted to build a homemade breeder nuclear reactor in 1994, at age 17. A scout in the Boy Scouts of America, Hahn conducted his experiments in secret in a backyard shed at his mother's...
was able to concentrate americium from smoke detectors after managing to buy a hundred of them at remainder prices and also stealing a few. There have been cases of humans being contaminated with americium, the worst case being that of
Harold McCluskeyHarold R. McCluskey was a chemical operations technician at the Hanford Plutonium Finishing Plant located in Washington state who is known for having survived, on April 24, 1976, exposure to the highest dose of americium radiation ever recorded...
, who at the age of 64 was exposed to 500 times the occupational standard for americium-241 as a result of an explosion in his lab. McCluskey died at the age of 75, not as a result of exposure, but of a
heart diseaseHeart disease, cardiac disease or cardiopathy is an umbrella term for a variety of diseases affecting the heart. , it is the leading cause of death in the United States, England, Canada and Wales, accounting for 25.4% of the total deaths in the United States.-Types:-Coronary heart disease:Coronary...
which he had before the accident.
See also
- Actinides in the environment
Actinides in the environment refer to the sources, environmental behaviour and effects of actinides in Earth's environment. Environmental radioactivity is not limited solely to actinides; non-actinides such as radon and radium are of note....
- :Category:Americium compounds
Further reading
- Nuclides and Isotopes – 14th Edition, GE Nuclear Energy, 1989.
- Guide to the Elements – Revised Edition, Albert Stwertka, (Oxford University Press; 1998) ISBN 0-19-508083-1
External links