Americium
Actinide · Period 7 · f-block
The synthetic element quietly guarding millions of homes
- Atomic mass
- 243 u
- State at 25 °C
- Solid
- Electronegativity
- 1.13
- Melting point
- 1,175.9 °C
- Boiling point
- 2,606.9 °C
- Configuration
- [Rn] 5f7 7s2
Nucleus drawn at reduced count for clarity. Drag to rotate.
Americium, explained
Video lesson in production
Bhavya’s explainer for this element will play here.
Overview
Americium is a silvery, radioactive actinide metal, softer and less dense than plutonium. With the configuration 5f7 7s2, it is the point in the series where the 5f electrons become localised, so americium behaves much more like the lanthanides than uranium or plutonium do. Its most stable oxidation state in water is +3, and Am3+ is very hard to separate from the chemically similar lanthanide ions in nuclear waste. Americium can also be oxidised to +4, +5 and +6 under strong conditions.
Americium-241, with a half-life of 432 years, forms in reactors when plutonium-241 beta-decays. It emits alpha particles and a soft 59.5 keV gamma ray. In an ionisation smoke detector, a tiny americium source about 0.3 micrograms ionises the air between two plates so a small current flows; smoke particles absorb the ions and the drop in current sets off the alarm. Mixed with beryllium, americium-241 also makes a portable neutron source used to survey oil wells.
Discovery
Glenn Seaborg, Ralph James, Leon Morgan and Albert Ghiorso identified americium in late 1944 at the Metallurgical Laboratory in Chicago, after bombarding plutonium with neutrons in a reactor. Because it sits below europium, named for Europe, in the periodic table, they named it after the Americas. Seaborg first revealed americium and curium on a children's radio quiz show, Quiz Kids, in November 1945, days before the formal announcement.
- Where it’s found
- Americium does not occur naturally in significant amounts; it is recovered from aged plutonium and spent nuclear fuel, where plutonium-241 decays into it.
- What it’s used for
- Smoke detectors
- Oil-well neutron logging
- Industrial thickness gauges
- Spacecraft power research
- Production of heavier elements
- In living things
- Americium has no biological role; if absorbed it collects in bone and the liver and is radiotoxic.
- Safety
- Sealed smoke detector sources are safe in normal use and should not be opened. Unsealed americium is a serious internal radiation hazard, especially if inhaled.
How it compares
Where Americium sits among all 118 elements on the trends that shape the table.
- Electronegativity1.13Pauling
#83 of 100 elements with data
- Ionization energy578kJ/mol
#80 of 104 elements with data
- Atomic radius175pm
#25 of 95 elements with data
- Electron affinity9.93kJ/mol
#83 of 109 elements with data
- Density12g/cm³
#33 of 114 elements with data
- Melting point1,450K
#42 of 110 elements with data
- Density
- 12 g/cm³
- Atomic radius
- 175 pm
- Covalent radius
- 166 pm
- Molar heat
- 62.7 J/(mol·K)
- Appearance
- silvery white
- Discovered by
- Glenn T. Seaborg
Inside the atom
95 electrons arranged in 7 shells: 2 · 8 · 18 · 32 · 25 · 8 · 2.
Orbital diagram
1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p6 5s2 4d10 5p6 6s2 4f14 5d10 6p6 7s2 5f7 · shorthand [Rn] 5f7 7s2
- 1s2
- 2s2
- 2p6
- 3s2
- 3p6
- 4s2
- 3d10
- 4p6
- 5s2
- 4d10
- 5p6
- 6s2
- 4f14
- 5d10
- 6p6
- 7s2
- 5f7
Each box is an orbital holding up to two electrons of opposite spin. Highlighted subshells hold the valence electrons that decide how Americium bonds.
Signature reactions
The chemistry Americium is known for.
Nuclear · Beta decay
Formation from plutonium-241
Plutonium-241 made in reactors beta-decays to americium-241. That is why stored plutonium slowly accumulates americium, which can be chemically extracted.
Conditions: spontaneous; half-life 14.3 years
Nuclear · Alpha decay
Alpha decay in smoke detectors
Each americium-241 nucleus emits a 5.5 MeV alpha particle and becomes neptunium-237. In a smoke detector, the alpha particles ionise air so that a small current flows.
Conditions: spontaneous; half-life 432 years
Nuclear · (alpha, n) reaction
Americium–beryllium neutron source
Alpha particles from americium knock neutrons out of beryllium nuclei, leaving carbon-12. The resulting sealed Am–Be sources are used to measure rock porosity and moisture down boreholes.
Conditions: Am-241 alpha particles striking beryllium powder
Where you’ll meet it
Smoke detector
Ionisation smoke detectors use a tiny americium-241 source to sense smoke.
Photo: Wikimedia Commons
Glenn T. Seaborg
Glenn Seaborg co-discovered americium and announced it on a children's radio show.
Photo: Wikimedia Commons
Metallurgical Laboratory
Americium was identified at the University of Chicago's wartime Metallurgical Laboratory.
Photo: Wikimedia Commons
Oil well
Americium–beryllium neutron sources are lowered into oil wells to measure the surrounding rock.
Photo: Wikimedia Commons
Research
No research on Americium yet
When Bhavya publishes a project that studies Americium, it will be linked here.