Americium
Atomic Data
| Atomic Number | 95 |
| Symbol | Am |
| Atomic Weight | 243 u |
| Density (STP) | 13.67 g/cm³ |
| Melting Point | 1175.85 °C (1449 K) |
| Boiling Point | 2606.85 °C (2880 K) |
| Electronegativity | 1.3 (Pauling) |
| Electron Config. | 1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p6 4d10 4f14 5s2 5p6 5d10 5f7 6s2 6p6 7s2 |
| Oxidation States | +2, +3, +4, +5, +6 |
| Phase at STP | Solid |
| Category | Actinoid |
| Period / Group | 7 / None |
| CAS Number | 7440-35-9 |
Electron Configuration
[Rn] 5f7 7s2
| Shell | n | Subshell | Electrons | Cumulative |
|---|---|---|---|---|
| K | 1 | 1s | 2 | 2 |
| L | 2 | 2s | 2 | 4 |
| L | 2 | 2p | 6 | 10 |
| M | 3 | 3s | 2 | 12 |
| M | 3 | 3p | 6 | 18 |
| M | 3 | 3d | 10 | 28 |
| N | 4 | 4s | 2 | 30 |
| N | 4 | 4p | 6 | 36 |
| N | 4 | 4d | 10 | 46 |
| N | 4 | 4f | 14 | 60 |
| O | 5 | 5s | 2 | 62 |
| O | 5 | 5p | 6 | 68 |
| O | 5 | 5d | 10 | 78 |
| O | 5 | 5f | 7 | 85 |
| P | 6 | 6s | 2 | 87 |
| P | 6 | 6p | 6 | 93 |
| Q | 7 | 7s | 2 | 95 |
| Total | 95 | 95 | ||
Isotopes of Americium
Americium has two naturally occurring stable isotopes. The most abundant is ²⁴¹Am, comprising None% of all naturally occurring Americium.
| Isotope | Symbol | Protons | Neutrons | Abundance | Stability |
|---|---|---|---|---|---|
| Americium-241 | ²⁴¹Am | 95 | 146 | trace | Stable |
| Americium-243 | ²⁴³Am | 95 | 148 | trace | Stable |
Abundance & Occurrence
Americium is present in Earth's crust at approximately trace amounts by mass and at approximately trace amounts by mass throughout the universe.
Earth's Crust (ppm by mass)
Universe (ppm by mass)
Discovery & History
Read more about the discovery of the periodic table of elements →
Safety & Handling
- Alpha radiation: Americium-241 (t½ = 432 years, alpha/gamma emitter) is a significant internal dose hazard if inhaled or ingested; it concentrates in bone and liver: all americium work requires glove boxes and air monitoring.
- Smoke detector source: Am-241 in household smoke detectors contains only ~1 µCi (37 kBq): insufficient to pose a radiation hazard during normal use; however, smoke detectors must not be dismantled or incinerated and must be disposed of through authorised collection.
- Gamma emissions: Am-241 emits 59.5 keV gamma radiation in addition to alphas; shielding is required for higher-activity sources used in industrial applications (gauges, level sensors).
- Criticality potential: Large accumulated quantities of Am-241 could approach criticality in specific geometries; mass controls are required in licensed research and production facilities.
Americium in the Real World
Real-World Uses
- Smoke detectors: Americium-241 (t½ = 432.2 yr) alpha particles ionise air between two electrodes in ionisation-type smoke detectors; smoke particles interrupt the ion current, triggering the alarm. Virtually every ionisation smoke detector manufactured since the 1970s contains approximately 1 μCi (37 kBq) of Am-241.
- Industrial gauging: Am-241 gamma sources (59.5 keV) are used in gamma-ray density gauges, moisture meters, and thickness gauges for measuring the density and thickness of soil, asphalt, paper, and plastics in industrial quality control.
- Medical research: Americium compounds are studied as potential radiopharmaceutical agents and as alpha-radiation sources for radiation biology experiments; Am-241 is a calibration standard for alpha-particle detectors and gamma spectrometers.
- Production of heavier actinides: Am-241 and Am-243 irradiated in high-flux research reactors are the starting materials for producing curium, berkelium, californium, and heavier synthetic elements in milligram to microgram quantities.
Downloadable Resources
Free periodic table reference sheets for classrooms, study sessions, and laboratory use.
Frequently Asked Questions
What is americium used for?
Americium-241 is used in ionisation-type smoke detectors: a tiny amount (about 1 microcurie) ionises the air between two electrodes, allowing a small current to flow. Smoke particles disrupt this current, triggering the alarm. Hundreds of millions of homes have smoke detectors containing americium. Am-241 is also used in portable moisture and density gauges used in road construction and as an X-ray source in some thickness gauges.
Is the americium in smoke detectors dangerous?
No. The americium in an ionisation smoke detector is a tiny amount (about 0.9 micrograms) sealed inside a metal chamber. The alpha particles it emits cannot penetrate the casing and have a range of only a few centimetres in air. The detector poses no radiation hazard in normal use. Smoke detectors should be disposed of according to local regulations: not simply thrown in the trash: but they do not require special radiation precautions during use.
How was americium discovered?
Americium was synthesised in 1944 by Glenn Seaborg, Ralph James, Leon Morgan, and Albert Ghiorso at the Metallurgical Laboratory of the University of Chicago as part of the Manhattan Project. It was produced by neutron bombardment of plutonium-239. The discovery was kept secret until 1945. Seaborg named it americium after the Americas, following the pattern of naming actinide elements after their continent of discovery (analogous to europium for Europe).
How does a smoke detector use americium?
An ionisation smoke detector contains a small chamber with two electrodes and a tiny amount of Am-241. The americium continuously emits alpha particles, which ionise the air molecules between the electrodes, allowing a small electrical current to flow. When smoke enters the chamber, smoke particles attach to the ionised air molecules, reducing the ionisation and interrupting the current. The detector circuit recognises this drop in current as smoke and sounds the alarm.