Atomic number 95

Americium

Am
+ phase
− phase

Shells Orbitals 5f 7s

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95 electrons in 7 shells: 2, 8, 18, 32, 25, 8, 2.

Shells are a teaching model: an electron does not travel on a ring at a fixed distance.

Category: Actinide
Period: 7
Block: f
Atomic mass: 243.061380 Da (longest-lived isotope)
Electron configuration: [Rn] 5f⁷ 7s²
Phase at 25 °C: solid
Melting point: 1176 °C
Boiling point: 2607 °C
Density: 13.69 g/cm³
Electronegativity (Pauling): 1.3
Discovered: 1944, Glenn Seaborg, Albert Ghiorso, Ralph James and Leon Morgan

Neighbours in the table

Pu Cm

Americium takes its name from America. The choice mirrored europium, its lighter lanthanide counterpart, which had been named after Europe, the continent of its discovery, Los Alamos explains. A team of Glenn Seaborg, Albert Ghiorso, Ralph James and Leon Morgan made element 95 in late 1944, Los Alamos records. The first synthesis took place at a University of California laboratory in Berkeley, in IUPAC’s account. Americium was the fourth synthetic transuranic element, one of the elements heavier than uranium, and wartime secrecy under the Manhattan Project hid it until declassification.

Earlier transuranic elements had been placed in the main block, yet americium behaved like the lanthanides, and so did curium, element 96. That behaviour led Seaborg to revise the table and create the actinide series, Los Alamos records. The configuration [Rn] 5f⁷ 7s² puts seven electrons in the 5f subshell.

In solution americium takes the oxidation states +3, +4, +5 and +6, an oxidation state being the charge an atom would carry if every bond were ionic. In carbonate solutions it was the second element after plutonium shown to hold all four at once, according to Los Alamos. The +3 state is the most stable in water. Americium(IV) is unstable in acid but persists in strongly basic carbonate solutions.

Where it is found

Americium does not occur naturally in Earth’s crust, IUPAC notes, and it has no natural or stable isotope, according to ATSDR, the US Agency for Toxic Substances and Disease Registry. It forms when plutonium absorbs neutrons in a nuclear reactor or a weapons test, the US Environmental Protection Agency (EPA) explains. In IUPAC’s scheme, plutonium-239 captures one neutron to give plutonium-240 and a second to give plutonium-241, which beta-decays to americium-241.

Weapons testing has left small amounts in soil, plants and water, mostly as microscopic dust, EPA reports. On soil it clings to surface particles rather than sinking deep, and plants on contaminated ground can take a little up.

PubChem gives the mass of its longest-lived isotope, americium-243, 243.061380 Da. That isotope halves in 7,364 years in the International Atomic Energy Agency’s (IAEA) evaluation, 7,350 in the Atomic Mass Data Center’s figures and 7,370 by Los Alamos. It decays by alpha emission to neptunium-239, and spontaneous fission, a nucleus breaking apart on its own, takes 3.7 × 10⁻⁹ per cent of decays in the IAEA’s evaluation. High-purity americium-241 and americium-243 now come in kilogram quantities, Los Alamos notes.

Why it matters

Ionising smoke detectors hold a small piece of americium-241 oxide, IUPAC notes. Its alpha particles strike molecules of air and ionise them, and the ions let a current flow between two plates in the detector. Smoke drifting between the plates disrupts that current, and the disruption sets off the alarm. Americium-241 is the isotope chosen because relatively pure samples of it are easier to produce, Jefferson Lab explains.

The same isotope gives off gamma rays at 60 keV (kiloelectronvolts), and these serve radiography and X-ray fluorescence analysis, Los Alamos notes. In flat-glass making, a gauge reads the radiation passing through the sheet: glass thicker than intended lets less through and is rejected, IUPAC explains. Its spectrum is nearly a single gamma peak, so it also calibrates gamma-ray spectrometers at low energies, Los Alamos notes.

Mixed with beryllium, americium-241 emits neutrons at a high rate, and such sources log the porosity of rock along a borehole and monitor oil production, in IUPAC’s account. Americium-241’s gamma rays once helped diagnose thyroid function, a use now obsolete, IUPAC notes. Americium-242m, with a 141-year half-life, has been proposed for nuclear batteries and spacecraft propulsion, but its scarcity and high price hold that back, according to Los Alamos.

Properties

Properties of americium Standard atomic weights from IUPAC/CIAAW (abridged, 2024); other values from PubChem's periodic table, kelvin converted to °C.
Property	Value	Unit	Source
Atomic number	95
Atomic mass	243.061380 Da	Da	PubChem, longest-lived isotope
Melting point	1176 °C	°C	Los Alamos: 1176 °C
Boiling point	2607 °C	°C	Los Alamos: 2607 °C
Density	13.69 g/cm³	g/cm³
Electronegativity (Pauling)	1.3
Electron configuration	1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹⁰ 4p⁶ 5s² 4d¹⁰ 5p⁶ 6s² 4f¹⁴ 5d¹⁰ 6p⁶ 7s² 5f⁷
Phase at 25 °C	solid

This element has no standard atomic weight; the atomic mass shown is that of its longest-lived isotope, as PubChem gives it.

Safety

Americium’s hazard is ionising radiation, and it is greatest once americium is inside the body. Americium-241 mainly emits alpha particles with some gamma rays, and it poses the larger risk when swallowed or inhaled, EPA says. Inside, it gathers in bone, liver and muscle and can remain for decades, raising cancer risk; Haz-Map names bone the critical organ and rates internal toxicity very high. The reference sheet is PubChem’s Laboratory Chemical Safety Summary for americium.

Questions

How was the discovery of americium announced?

On a children's radio programme. Glenn Seaborg revealed elements 95 and 96, americium and curium, on the US show 'The Quiz Kids' when a young listener asked whether any transuranium element besides plutonium and neptunium had turned up. According to Los Alamos, the broadcast came five days ahead of the talk he had scheduled for a November 1945 meeting of the American Chemical Society.

What is the half-life of americium-241?

Americium-241 halves in 432.6 years in the IAEA's evaluation and the Atomic Mass Data Center's figures, and in 432.2 years for Los Alamos, EPA and Jefferson Lab. It is 432 years in ATSDR's figure, 430 in Haz-Map's and 458 in that of HSDB (Hazardous Substances Data Bank). Alpha emission takes 100 per cent of decays, leaving neptunium-237, and spontaneous fission accounts for 3.6 × 10⁻¹⁰ per cent in the IAEA's evaluation.

Is the americium in a smoke detector dangerous?

Not while the detector stays intact. The americium in smoke detectors carries no health risk provided nobody tampers with the unit and it is used as intended, EPA says, and the americium-241 inside is a very small amount. The greater risk comes from americium that is swallowed or breathed in. Disposal goes through the maker's instructions or the local fire department, according to EPA.

What does americium metal look like?

Freshly prepared americium has a white lustre, more silvery than plutonium or neptunium made the same way, Los Alamos reports. It seems more malleable than neptunium or uranium, and it tarnishes only gradually in dry air. The first substantial amounts of the metal, up to 200 milligrams, came in 1951 from reducing americium(III) fluoride with barium. In potassium carbonate solution, americium(III) is light pink and the americium(IV) complex golden-brown.