Americium
Americium is a radioactive transuranic chemical element with symbol Am and atomic number 95. This member of the actinide series is located in the periodic table under the lanthanide element europium, and thus by analogy was named after the Americas. Americium was first produced in 1944 by the group of Glenn T.Seaborg from Berkeley, California, at the metallurgical laboratory of University of Chicago.
Identity
- Atomic mass
- [243] u
- Category
- Actinides
- Group
- 3
- Period
- 7
- Block
- f
- Oxidation States
- +2, +3, +4, +5, +6
- Phase (25 °C)
- Solid
- Appearance
- silvery white
Electronic Structure
- Electron configuration
- 1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p6 5s2 4d10 5p6 6s2 4f14 5d10 6p6 7s2 5f7
- Short configuration
- [Rn] 5f7 7s2
- Electrons per shell
- 2, 8, 18, 32, 25, 8, 2
- Electronegativity (Pauling)
- 1.13
- Electron affinity
- 9.93 kJ/mol
- 1st ionization energy
- 578 kJ/mol
Physical properties
- Melting point
- 1175.8 °C (1449 K)
- Boiling point
- 2606.8 °C (2880 K)
- Density
- 12 g/cm³
- Molar heat capacity
- 62.7 J/(mol·K)
Chemical behavior of americium
Americium has seven outer electrons ([Rn] 5f⁷ 7s²) and shows states from +2 to +6, with +3 the most stable in solution. Its electronegativity is very low (1.13) and it is dense, malleable, and radioactive: it heats its compounds and oxidizes in air. Its half-filled 5f shell makes it the actinide most similar to europium; Am-241 emits steady alphas for centuries.
Reactivity
4 Am + 3 O₂ → 2 Am₂O₃
In air it tarnishes rapidly to the dark dioxide, the ceramic form used in detectors and sources. Fine powder can burn spontaneously.
2 Am + 6 HCl → 2 AmCl₃ + 3 H₂
It dissolves in acids to give pink americium(III) salts. The chloride is the route for purifying Am-241 from aged plutonium.
AmO₂ + 4 HF → AmF₄ + 2 H₂O
The dioxide converts to the tetrafluoride, the precursor of the metal by reduction with barium and of targets for berkelium.
²⁴¹Am → ²³⁷Np + α
Am-241 decays emitting 5.5 MeV alphas that ionize the air inside detectors and, with beryllium, produce neutrons.
Notable compounds
| Compound | Formula | Use or importance |
|---|---|---|
| Americium-241 dioxide | ²⁴¹AmO₂ | Smoke detectors and sources |
| Am-Be source | Am-Be | Neutrons for prospecting |
| Americium tetrafluoride | AmF₄ | Metal production |
| Americium oxalate | Am₂(C₂O₄)₃ | Radiochemical purification |
| Am-243 target | ²⁴³Am | Superheavy element synthesis |
Electron shell distribution
View in the atom builderIsotopes
No americium isotope is stable. Americium-241 (²⁴¹Am, 432 years) dominates detectors and sources and is bred from Pu-241. Americium-243 (²⁴³Am, 7,370 years) is a target for curium and berkelium. Americium-242m is one of the most potent fissile isotopes known.
Occurrence and production
It does not occur in nature in appreciable amounts: only traces from neutron capture in uranium. All americium is bred in reactors, when Pu-239 captures neutrons up to Pu-241, which decays into Am-241. It is extracted from spent fuel or aged plutonium by advanced PUREX.
History and origin of the name
Glenn Seaborg and his team synthesized it in 1944 by bombarding plutonium with neutrons in Chicago and named it americium, after America: europium's twin on the other side of the Atlantic. It was announced in 1945 on a children's radio program. Smoke detectors carried it into every home.
Applications and relevance of americium
Uses of the element
As Am-241 in ceramic pellets it ionizes the smoke detectors in every house and, with beryllium, generates neutrons for oil exploration and soils. As Am-243 it serves as a target to create berkelium and superheavy elements.
Uses of its compounds
The sealed dioxide measures thickness and density in factories; Am-Be mixtures calibrate reactors and start-up sources. Oxalates purify the isotope in hot cells.
Health, safety and environment
Biological role
It has no biological function and is highly radiotoxic: inhaled it accumulates in bone and liver for decades. The dose in an intact detector is lower than annual natural radiation.
Safety
It is handled only in nuclear facilities, sealed in steel and ceramic. If a detector breaks, do not touch it: ventilate and call radiation protection. It must never be opened or drilled.
Environment
Retired detectors are collected as low-level radioactive waste with full traceability. Am-241 decays into extremely long-lived Np-237, which shapes geological repositories.
Frequently asked questions about Americium
Why do ionization smoke detectors contain americium-241?
Americium-241 emits alpha particles that ionize the air inside a sealed chamber. That ionization allows a very small electric current to flow between two electrodes. When smoke enters, its particles attach to the ions and reduce the current flow; the circuit detects that drop and triggers the alarm. The amount of americium is tiny and is encapsulated in layers of metal and ceramic.
What is americium-241 used for in industry?
Besides smoke detectors, americium-241 is used in thickness, density, and liquid or solid level gauges. It is also combined with beryllium to produce neutron sources used in oil prospecting, soil moisture measurement, and materials analysis. These applications take advantage of its stable alpha emission and long half-life to obtain reliable measurements for years.
What role does americium play in superheavy element research?
Americium is used as starting material to synthesize heavier elements in particle accelerators. By bombarding americium atoms with heavy ions, scientists can create nuclei of superheavy elements that do not occur naturally. These syntheses require tiny amounts of material and high-energy facilities. Americium is therefore a stepping stone in the search for new elements of the periodic table.
What happens if a smoke detector containing americium breaks?
If the sealed source is damaged, there is a risk of contamination from americium-241 particles, especially if they are inhaled. For this reason the detector must not be opened, drilled, or dismantled to extract the material. In case of accidental breakage, it is recommended not to touch the debris, ventilate the area, and contact the radiation protection authorities or the manufacturer for safe disposal.
What is the difference between an ionization smoke detector and a photoelectric one?
The ionization detector uses americium-241 to create an electric current and responds quickly to the very small particles produced by fast-flaming fires. The photoelectric detector uses a light beam and a sensor: when smoke scatters the light, it triggers the alarm. Ionization detectors are therefore usually more sensitive to fast-burning fires, while photoelectric ones respond better to the dense smoke of slow-burning fires.
Why does americium-241 decay into neptunium-237?
Americium-241 decays by alpha emission and transforms into neptunium-237, an element with a half-life of about 2.14 million years. This process is very slow: americium's half-life is about 432 years, so the amount of neptunium produced in a household detector is insignificant. The transformation is part of a broader radioactive chain within the actinides.
Why is americium-241 suitable for neutron sources?
When the alpha particles from americium-241 hit light atoms such as beryllium, they can trigger nuclear reactions that release neutrons. This combination makes it possible to build compact neutron sources for prospecting, industrial control, and materials analysis. Americium's advantage is its long half-life and steady emission, which guarantee a constant neutron flux for many years without a reactor.
Main sources consulted: PubChem, Royal Society of Chemistry, NIST, IUPAC. Last revision: November 2026.