Americium

Americium
Atomic number	95
Symbol	Am
Boiling point	2607 °C
Electronegativity	1.13 (Pauling)
Electron configuration	[Rn] 5f7 7s2
Density	13.69 g/cm^3
Main isotopes	241Am, 243Am, 242mAm
Melting point	1176 °C
Block	f
Phase STP	Solid
Oxidation states	+3, +4, +5
Wikidata	Q1872

Americium (Am, atomic number 95) is a synthetic, silvery-white radioactive metal in the actinide series of the periodic table. It is one of the transuranic elements (those beyond uranium, Z>92) and has no stable isotopes. Americium metal is a solid at room temperature and commonly used in tiny quantities (milligrams) in ionization-type smoke detectors, where its alpha-particle emissions help detect fire. Despite its usefulness, americium is highly toxic and poses radiological hazards; its principal long-lived isotopes (^241Am and ^243Am) decay by emitting alpha particles (helium nuclei) over timescales of hundreds to thousands of years.

Atomic Structure and Electron Configuration

Americium atoms contain 95 protons and 95 electrons. The electronic configuration is [Rn] 5f^7 7s^2, meaning americium has a filled radon core plus seven electrons in the 5f orbital and two in the 7s orbital. These seven 5f electrons make americium chemically similar to its lighter actinide and lanthanide neighbors – for example, europium (the lanthanide above americium) also has seven electrons in its outer shell, giving the pair some chemical analogy. Americium’s valence shell (the outermost electrons) can exhibit several oxidation states (charges) in compounds; the +3 state (lis Am^3+, analogous to +3 in lanthanides) is most common, with +4 also fairly common. Other states from +2 up to +6 (and very rarely +7) can occur under special chemical conditions, although these higher and lower states are generally unstable in aqueous solution.

In terms of atomic size, americium is a large atom. Its metallic radius is on the order of 175–180 picometers (pm) and a calculated atomic (covalent) radius about 180 pm, which is slightly smaller than its lighter actinide neighbors due to the progressive actinide contraction (increasing nuclear charge draws electrons slightly closer). In compounds, the Am^3+ ion has an ionic radius of roughly 100–120 pm (depending on coordination number), again similar to the smaller lanthanide ions. Americium’s electronegativity (Pauling scale) is about 1.3, which is relatively low (reflecting its metallic, electropositive character). The first ionization energy – the energy needed to remove one electron – is about 5.99 eV (around 578 kJ/mol), modest compared to many stable metals. These atomic properties place americium among the more reactive actinide metals: it oxidizes in air and reacts with acids and halogens relatively readily (though somewhat less vigorously than the very lightest actinides).

Isotopes and Nuclear Properties

No isotope of americium is stable; all are radioactive. The most important and longest-lived isotopes are americium-241 (^241Am, half-life 432.2 years) and americium-243 (^243Am, half-life 7,370 years). In practice, americium is produced by irradiating plutonium (^239Pu) in nuclear reactors. Plutonium-239 captures neutrons to become ^241Pu, which beta-decays (converts a neutron to a proton and emits an electron) into ^241Am. Some of the ^241Am can further capture a neutron to make ^242Am (half-life 16 hours), which quickly beta-decays to ^242Cm (curium). With enough neutron flux, ^243Am is formed in small quantities as well.

Both ^241Am and ^243Am decay chiefly by emitting alpha particles (helium nuclei). ^241Am’s alpha decay yields neptunium-237 (^237Np). Alpha decay is highly ionizing but short-ranged, so an external alpha source like ^241Am is mainly a hazard if ingested or inhaled. Importantly, ^241Am also emits a weak 59.5 keV gamma ray (and X-rays near 13–18 keV), which allows easy detection and calibration. ^243Am decays by alpha emission to ^239Np. Because these half-lives are long, americium does not "burn up" quickly, and any americium released into the environment persists for centuries.

In addition to these main isotopes, there are about 19 known isotopes and several nuclear isomers (excited states). Notably, ^242mAm (a metastable isomer) has a half-life of about 141 years; it can be created in reactors, but ^242Am decays so quickly that it is rare. All americium isotopes primarily give off alpha particles; some of their short-lived daughter products emit beta and gamma radiation, which can penetrate outside shielding. Because there is no primordial americium on Earth (all would have decayed away early in Earth’s history), the americium found on Earth today comes from human activities (nuclear reactors and weapons testing). Small amounts of ^241Am from past atmospheric weapons tests and reactor releases remain detectable on the soil surface. Americium isotopes have no major role in radiometric dating like uranium or carbon; instead, they serve more as tracers of nuclear contamination and are used as known sources in industrial and research contexts.

Allotropes and Typical Compounds

At ordinary conditions, metallic americium has a crystal structure called double hexagonal close-packed (α–Am). This phase is relatively soft, silvery, and malleable (like many heavy metals). Under varying temperature and pressure, americium metal can adopt other forms: for example, at several gigapascals of pressure the metal shifts to a face-centered cubic (β–Am) structure. In general, americium’s crystal lattice is slowly damaged by its own radioactivity (α-particles knocking atoms out of place), a process called self-irradiation or metamictization, which can darken older samples and broaden X-ray diffraction peaks.

Americium forms a variety of compounds, often in the +3 state. Like the lanthanides, Am^3+ compounds tend to be mostly ionic. For example, americium(III) oxide (Am₂O₃) is a reddish-brown solid, and americium(IV) oxide (AmO₂) is black. Both oxides are important in nuclear technology: AmO₂, in particular, is analogous to PuO₂. Americium hydroxide, Am(OH)₃, precipitates as a light-colored solid when americium(III) is treated with a base. Americium also forms salts with the halogens: AmCl₃, AmBr₃, and AmI₃ are known (typically hydrated forms), all usually crystalline solids (for instance, AmCl₃·6H₂O is yellow). Americium(IV) fluoride (AmF₄) and americium(III) fluoride (AmF₃) can be prepared; AmF₄, like PuF₄, is a green-yellow strong oxidizer. Many of the americium halides are salts that dissolve in water to yield the Am^3+ ion.

Lower oxidation states are rare but possible with strong reducers: an americium(II) compound like AmCl₂ can be made using Na or Mg reductions, but it oxidizes in air back to +3. At the highest levels, Am(V) and Am(VI) appear in solution under very strong oxidizing conditions. For instance, solutions of Am^6+ (as AmO₂^2+) can be prepared with powerful oxidizers like fluorine or sodium bismuthate; these Am(VI) species are typically yellow. A true americium(VII) is speculative and not well established. Overall, americium chemistry is dominated by +3 (and +4) compounds, much as the lanthanides are dominated by +3.

Americium also forms coordination compounds and complexes with organic ligands, similar to other actinides. It can bind chelating agents like α-hydroxyoximes or dithiocarbamates, which have been explored for separation chemistry. A few organometallic americium complexes have been synthesized (for example, Am bound to cyclopentadienyl rings, analogs of ferrocene). However, these are of interest mainly in research; americium’s radioactivity generally limits its practical chemistry to laboratory-scale work.

Physical Properties

Pure americium metal is a dense material. The density of α- americium is about 12 g/cm³ (lighter than plutonium or curium but much heavier than most common metals). It is silvery-white when freshly prepared, but like many actinides it quickly tarnishes in air to a dull surface of oxide or amide. It is a relatively soft metal (it can be cut with a knife) and shows little elasticity. Its crystal structure, as noted, is double-hexagonal close-packed (similar to α- plutonium or certain lanthanides).

In terms of thermal properties, americium melts at about 1173–1176 °C and boils around 2010–2011 °C (at standard pressure). The heat of fusion is roughly 14 kJ/mol. It has a moderately low thermal conductivity (around 10 W·m^–1·K^–1), so it does not conduct heat as well as many common metals (copper is about 400 W/m·K, for comparison). The electrical resistivity of soft, α- americium metal at room temperature is around 690 nΩ·m (about 0.00000069 Ω·m), corresponding to a conductivity on the order of 1.5×10^6 S/m. This makes it a poorer conductor than many transition metals (copper is ~5×10^7 S/m) but typical for heavy f-block metals.

Because americium is radioactive, its spectroscopic properties are somewhat obscured by self-irradiation effects. Metallic americium gradually accumulates crystal defects; at low temperatures this radiation damage broadens its diffraction peaks. The electronic spectra of americium compounds are studied mainly in the ultraviolet and visible range: Am^3+ ions typically give pale pink or peach-colored solutions (reflecting f–f transitions within the 5f electrons). Americium(IV) compounds can be yellow or greenish (AmO₂ is black, but in dilute solution AmO₂^2+ has bright yellow colors). In X-ray and gamma-ray spectroscopy, the familiar signature is the 59.5 keV gamma emission from ^241Am decay, often used to calibrate detectors. Americium itself does not have sharp atomic emission lines easily used in flame or plasma spectroscopy, as its electron transitions are largely in the deep ultraviolet or absorbed by the metal’s conduction band.

Chemical Reactivity and Trends

Americium is a reactive metal, though somewhat less so than the very lightest actinides (protactinium or uranium). In air and moisture, solid americium gradually oxidizes and forms a surface film of americium oxide or americium hydroxide. Finely divided americium can ignite in air. The metal dissolves readily in dilute mineral acids (nitric or hydrochloric), producing Am^3+ solutions (with the evolution of hydrogen gas). Alkali metals or hydriding agents convert americium into americium oxides or hydrides: for example, heating with hydrogen gas produces americium(III) hydride (AmH₃) or mixed AmH₂ species, which are reddish and slowly release hydrogen when exposed to air.

In solution chemistry, americium behaves similarly to the lanthanide rare earths: in aqueous acids it exists mostly as Am^3+, which forms Am(OH)₃ (a gelatinous hydroxide) upon adding base. Americium ions form complexes with various ligands: chloride, nitrate, sulfate, and carbonate bind Am^3+ in familiar ways. For example, americium nitrate (Am(NO₃)₃) and americium sulfate (Am₂(SO₄)₃) are soluble salts that were studied early in lanthanide/actinide chemistry experiments. Am^3+ also strongly complexes with halides (F^–, Cl^–, Br^–) and can form americium halide complexes in solution. In higher oxidation states, Am^4+ can form AmO₂^2+ (the plutonyl analog) and complex with fluoride to give AmF₆^2– (hexafluoride anions). However, +4 compounds (like AmO₂ or AmF₄) are strong oxidizers and tend to get reduced to +3 unless kept in very acidic fluoride media.

Redox-wise, americium acts as a source of reducing power in its lower oxidation states (Am^2+,Am^3+) and as a strong oxidizer in its higher states (Am^5+/Am^6+). But for practical purposes, +3/+4 chemistry dominates. The Am^3+ ion is typically inert to mild reducing or oxidizing agents – it requires strong reagents (like Na amalgam to reduce to Am^2+, or potassium permanganate to reach +6). This pattern is similar to light lanthanides (which are mostly +3 and require strong conditions to vary).

In the context of corrosion and the reactivity series, americium metal would be placed as fairly high (active), yet it is not as easily oxidized as e.g. sodium or even some ferrous metals in common conditions. It is more reactive than uranium and plutonium, meaning that pure americium metal will oxidize or react with many environments where uranium/ plutonium might remain stable. Passivation can occur if a protective oxide layer forms, but since americium oxide is not a robust passive film, corrosion in wet air or acids proceeds steadily.

Occurrence and Production

Americium does not occur naturally in any significant amount, because of its radioactivity and production history. Any primordial americium (present since Earth’s formation) would have long since decayed; its two main isotopes have half-lives much shorter than the age of the Earth. Nevertheless, tiny trace amounts of americium can be produced naturally by neutron capture and beta decay in uranium-bearing minerals, but these are negligible and hard to measure. The main sources of americium on Earth are human-made: nuclear weapons tests in the 1950s-1980s injected americium (mostly ^241Am) into the atmosphere, which then settled onto soils worldwide (especially downwind of test sites). Reactor and fuel reprocessing accidents (such as the Chernobyl disaster) also released americium into the environment. In consequence, small traces of ^241Am can be detected in soils and fallout residue, though its concentration is extremely low (on the order of 0.01 picocurie per gram of soil globally).

Today, almost all americium is produced in nuclear reactors or extracted from spent nuclear fuel. In thermal reactors, uranium-238 captures neutrons to become Pu-239, and Pu-239 eventually decays to Pu-241 and then to ^241Am (as described earlier). About 100 grams of americium (primarily ^241Am and ^243Am) are estimated to be present per metric ton of irradiated fuel. This americium is usually unwanted (as a source of heat and long-term radioactivity in nuclear waste), but it can be chemically separated. Production typically involves the PUREX process or other solvent-extraction methods to remove uranium and plutonium, leaving a mixture of curium, americium, and lanthanides. Further specialized separation (using ion exchange or advanced extractants) isolates americium. Historically, facilities in the United States (Rocky Flats, Savannah River), Europe (Sellafield in the UK, Marcoule in France), and Russia have carried out americium production and purification.

Americium-241 is the most commonly produced isotope, since it arises from the decay of reactor-produced ^241Pu. It is available commercially (at high purity) for research and industrial uses. Americium-243 requires more neutron captures (from ^239Pu to ^243Am) or long decay chains, so it is much rarer and expensive. Some gram-scale isolations of ^243Am have been done by irradiating plutonium in very high flux reactors for long periods. Overall, americium is not mined from ores; it is a secondary product of the nuclear fuel cycle, extracted and stockpiled by nuclear agencies. Worldwide, only a few kilograms of americium are produced each year (often less), mainly as ^241Am.

Applications and Technology

Americium’s main application is in small radiation sources. The single largest use is as the alpha-emitter ^241Am in residential and industrial smoke detectors. Each ionization-type smoke alarm typically contains about 0.9 microcurie (≈3.3×10^4 becquerels) of ^241Am, embedded in a metal foil or thin ceramic. The americium ionizes air in a small chamber; smoke particles disrupt the current and trigger the alarm. The radiation dose from a household smoke detector is tiny and poses no health risk under normal conditions.

Another common use is in neutron sources. Combining americium-beryllium (Am/Be) yields a (α,n) source: ^241Am alpha particles strike ^9Be, producing neutrons. Americium-beryllium sources (with a few curies of ^241Am) are used for well logging (measuring underground formations), for radiography to inspect parts, in neutron counters, and in some medical and research equipment. The neutrons from Am/Be are “fast” (about 4–5 MeV) and can initiate fission in uranium or plutonium, so small Am/Be sources were used as initiators in nuclear bombs historically (though other methods superseded them).

Americium-241 is also used as a γ-ray source for instruments. Its 59.5 keV photon can be used in X-ray fluorescence (XRF) analyzers to determine elemental composition of materials. Some portable XRF analyzers contain ^241Am sources for this purpose. It also serves as a calibration standard for radiation detectors.

A few specialized applications have been proposed or tested. One research measure is to use ^241Am as a fuel in radioisotope thermoelectric generators (RTGs) for space missions, but its heat output is relatively low and it is far less efficient than plutonium-238 (which is the standard RTG fuel). The hypothetical typically-discussed isotope ^242mAm (with its high energy level) was considered as a possible small nuclear battery fuel, but its scarcity and complex handling have prevented practical use.

Other uses are mainly scientific. Americium is used in laboratories to study actinide chemistry, to produce heavier elements (bombarding ^241Am with particles can make curium or even fierbornium), and as a neutron flux monitor or target material. It has been investigated as a catalyst in organic reactions, but this is mostly academic. In industry, americium sources can serve as thickness gauges or density gauges (the radiation absorption indicates material thickness or density), similar to other radioisotopes. However, americium’s roles in most technologies are niche due to safety concerns and cost. Today, outside of smoke detectors, only a small fraction of produced americium finds use in these specialized roles.

Biology, Environment, and Safety

Americium has no biological role; it is entirely foreign to living systems. Its chemical properties are similar enough to calcium and some rare earths that it can bind to bones and tissues if ingested, but its greatest hazard is radioactive. ^241Am and other isotopes emit alpha particles, which cannot penetrate skin but will damage living cells if the source is inside the body (for example, by inhalation or ingestion of americium-containing dust). The primary target organs are bone, liver, and lungs. If americium enters the body (say, via inhalation of airborne particles), it tends to deposit in the skeleton and liver and remains there for long periods, steadily irradiating nearby cells. This greatly increases the risk of bone cancer, liver cancer, or other cancers over time.

In the environment, americium is not highly mobile. Americium compounds are typically insoluble in water (oxides and hydroxides) or bind strongly to soil particles and clays. Thus, americium fallout or discharges tend to stay near the surface or in sediments rather than leaching into groundwater. Organisms do not readily uptake americium from soils or water, so it bioaccumulates to only a small extent (primarily via inhalation of dust by animals or humans). Because of this, most environmental concerns about americium focus on direct gamma exposure from contaminated soils (though ^241Am’s gamma is low energy) and ingestion of contaminated food in extreme cases. Past weapons testing and reactor spills have left traces of americium around test sites, but the levels are generally very low and decrease over time with decay and burial.

Safety guidelines for americium are strict. Its radioactivity demands careful handling: sealed sources and containment (glove boxes or hot cells) are used in laboratories. Protective gear is required to prevent inhalation of americium dust. Regulatory bodies (like the U.S. NRC or IAEA) set annual dose limits and maximum on-site inventories. For example, an occupational exposure limit might be on the order of a few hundred becquerels of ^241Am aerosol per cubic meter (carefully controlled) and annual dose of a few millisieverts; these ensure that any americium taken into the body remains far below levels that significantly increase cancer risk. Even in smoke detectors, the tiny amount of ^241Am is sealed behind metal foil and foil will stop its alpha radiation.

Because americium is an alpha emitter, shielding is relatively simple when it is contained. A few centimeters of air or a sheet of paper will stop the alpha particles. However, ^241Am’s 59-keV gamma rays require denser material for shielding (about 2 mm of lead will cut the radiation by ~50%). In practice, americium sources are stored in steel or lead containers if bulk quantities are present, to protect workers from any gamma. Waste americium (e.g. from spent smoke detectors or used lab sources) is treated as high-level radioactive waste: it must be isolated from the environment for many half-lives (generally centuries). On the positive side, the long half-life of ^241Am means it decays slowly, so groundwater or food-chain transfers remain a longer-term but lower-intensity risk than short-lived fission products.

History and Etymology

Americium was discovered in 1944 during World War II as part of the Manhattan Project effort to study transuranic elements. It was first made and identified by Glenn T. Seaborg, Ralph A. James, Leon O. Morgan, and Albert Ghiorso at the University of California, Berkeley. They bombarded plutonium with neutrons in a cyclotron to create ^239Pu, which by neutron capture became ^241Pu and then decayed to ^241Am. The new element was chemically separated from other actinides at the Metallurgical Laboratory (Chicago). Because of the secrecy of the Manhattan Project, the discovery of americium (and its neighbor curium, element 96) was not announced to the public until 1945.

The element was named by analogy with europium (element 63), since europium was named after Europe. Seaborg proposed “americium” for element 95, referencing the Americas. The name was formally adopted in 1947. In early experiments, the Berkeley group humorously referred to the worked-on samples as “pandemonium” (for element 95) and “delirium” (for element 96) before the names were fixed. “Americium” reflects both the place of discovery (Americas/USA) and the actinide series’s mirroring of the lanthanides.

Initially only microgram quantities of americium could be prepared. The first reported sample of metallic americium, weighing about 1 microgram, was produced in late 1944 or early 1945. Over the years, larger samples were made by reduction of americium salts (for example, AmF₃ reduced with barium metal at high temperature yielded milligram amounts of metal in 1951).

In 1945, Seaborg even mentioned the synthesis of new heavy elements on a children’s radio program, five days before the official conference disclosure. After the war, americium’s radioactive properties and potential uses were studied intensively. The first practical application came in the 1960s with ^241Am neutron sources and gamma calibration sources. Its introduction into smoke detectors happened a bit later (around the 1970s), once the devices became standardized for home safety.

Americium had no use prior to the nuclear era. Its discovery and naming are entirely a product of mid-20th-century nuclear chemistry. Because its most notable isotope (^241Am) arises from plutonium decay, americium became well known in the nuclear industry (and later among consumers, due to smoke alarms). It played a minor yet interesting part in the early history of nuclear research, reflecting humanity’s ability to create new elements beyond those found in nature. Today americium’s legacy is twofold: as a reminder of the atomic age, and as a practical if hidden tool for safety (the humble smoke detector).

Data Table

Property	Value
Symbol	Am
Atomic number (Z)	95
Standard atomic weight	(243) (no stable isotopes)
Element category	Actinide (transuranic)
Period	7
Block	f
Electron configuration	[Rn] 5f⁷ 7s²
Common oxidation states	+3, +4 (also +2, +5, +6 in labs)
Electronegativity	1.3 (Pauling scale)
First ionization energy	~5.99 eV (579 kJ/mol)
Atomic radius (approx)	~175–180 pm (metallic); Am³⁺ ~100 pm
Density (20°C)	~12 g/cm³
Melting point	~1175 °C (1448 K)
Boiling point	~2010 °C (2283 K)
Crystal structure	Double hexagonal close-packed (α-Am)
Isotopes (key)	^241Am (t½ 432 y, α emitter), ^243Am (t½ 7370 y, α); ~17 total
Discovered	1944 (by Seaborg et al.)
Name origin	Named for the Americas (by analogy with europium)
CAS Registry No.	7440-35-9