Americium (Am)

Americium (symbol Am) is the chemical element with atomic number 95: an actinide, solid at 0 °C, radioactive, made artificially, with an atomic mass of 243 u. Discovered in 1944 (Glenn T. Seaborg et al.).

Americium is a synthetic metal made from plutonium and named after the Americas. It is one of the few synthetic elements found in consumer products: a minute pellet of americium-241 sits inside ionization smoke detectors. Its most stable isotope, americium-243, has a half-life of about 7,400 years.

Key facts

Atomic number: 95
Symbol: Am
French name: Américium
Atomic mass: 243 u
Molar mass: [243] g/mol
Family: Actinides
Position: period 7, f-block
State at 0 °C: Solid
Melting point: 1,175.9 °C
Boiling point: 2,606.9 °C
Density: 12 g/cm³
Electronegativity (Pauling): 1.13
First ionization energy: 578 kJ/mol
Atomic radius: 175 pm
Electron configuration: [Rn] 5f⁷ 7s²
Indicative price: 730,000 $/kg (order of magnitude)
Radioactive: yes
Discovery: 1944, Glenn T. Seaborg et al.

Origin of the name

From the Americas, by analogy with europium.

What does americium look like?

A shiny, silvery-white and fairly soft metal that tarnishes slowly in dry air. It emits low-energy gamma rays, and its samples give off heat.

Where is americium found?

It does not occur naturally in any significant amount. It appears in spent nuclear fuel and in aged plutonium, where plutonium-241 decays into americium-241.

How is americium produced?

It is extracted from spent fuel or old plutonium stocks by chemical separation. It is relatively “abundant” for a synthetic element, and a smoke detector needs only a few tenths of a microgram.

Top producers

United States
Russia

What is americium used for?

Ionization smoke detectors (americium-241)
Americium–beryllium neutron sources for probing oil wells
Industrial thickness and density gauges
Being studied by the European Space Agency to power future probes

In everyday life

Ionization smoke detectors (very common in the United States; in France, home detectors are optical and old ionization models are being withdrawn)
Some industrial measuring instruments

Role in living things

No biological role. Once absorbed, it settles in the bones and liver.

Hazards of americium

An alpha emitter, dangerous if inhaled or swallowed. The sealed amount in a smoke detector is safe in normal use, but the source should never be taken apart.

Cosmic origin

Human-made: Does not exist (or barely) in nature: synthesized in a reactor or accelerator.

History of americium

Glenn Seaborg, Ralph James, Leon Morgan and Albert Ghiorso identified it in late 1944 at the Metallurgical Laboratory in Chicago, in plutonium irradiated with neutrons. The name echoes europium, which sits just above it in the table.

Did you know?

Its discovery was revealed in November 1945 on a children's radio show, “Quiz Kids,” when a young listener asked Seaborg whether he had found any new elements — a few days before the official announcement.
In 1994, an American teenager, David Hahn, extracted americium from smoke detectors in an attempt to build a reactor in his mother's backyard shed; the site had to be decontaminated.
A smoke detector pellet contains about 0.3 micrograms of americium.

Isotopes of americium

No stable isotopes; americium is born in nuclear reactors. A tiny pellet of americium-241 hides inside many smoke detectors.

americium-241: radioactive, half-life 433 years. Ionizes the air inside smoke detectors; being considered for European space power generators.
americium-242m: radioactive, half-life 141 years. Highly fissile, it might one day propel nuclear rockets.
americium-243: radioactive, half-life 7,370 years. The longest-lived; a target for making superheavy elements such as moscovium.

Related elements

Same family (actinides): Actinium (Ac), Thorium (Th), Protactinium (Pa), Uranium (U), Neptunium (Np), Plutonium (Pu), Curium (Cm), Berkelium (Bk), Californium (Cf), Einsteinium (Es), Fermium (Fm), Mendelevium (Md), Nobelium (No), Lawrencium (Lr).