Americium
Americium is a synthetic radioactive metal best known for an unexpectedly everyday role: a tiny amount of it sits inside most household ionization smoke detectors. First made during the Manhattan Project, it's one of the few elements on this list most people have, without realizing it, had in their home.
- Group · Period
- — · 7
- At room temp
- solid
- Melts at
- 1449 K
- Density
- 13.69 g/cm³
- Discovered
- 1944
Uses
Americium’s best-known use, by far, is as the radioactive source inside ionization smoke detectors, where americium-241 ionizes air between two electrodes, allowing a small steady current to flow; smoke entering the chamber disrupts that current and sets off the alarm. This application has made americium one of the most widely distributed radioactive materials in the world, even though most people never realize it’s there. Beyond smoke detectors, americium-241 is also used in industrial thickness and density gauges, where its steady, predictable radiation output is used to measure materials like paper, plastic film, and other manufactured sheeting as they’re produced.
History
Americium was first produced in 1944 by Glenn Seaborg’s team, working under wartime secrecy as part of the Manhattan Project, by bombarding plutonium with neutrons. Because of that secrecy, the discovery wasn’t published in a scientific journal when it happened — it was instead revealed to the public for the first time on a children’s radio quiz program in November 1945. It was named after the Americas, following the same logic used for europium, which sits directly above it on the periodic table and was named after Europe.
Fun facts
- Americium is the one element on this list you've likely had in your own home — a tiny amount sits inside most ionization smoke detectors.
- Its discovery wasn't announced in a scientific journal first, but revealed publicly on a children's radio quiz program in 1945.
- It's named after the Americas, echoing how europium — the element above it in the same column — was named after Europe.
Frequently asked questions
Is the americium in my smoke detector dangerous?
In normal use, no. Smoke detectors contain an extremely small amount of americium-241 sealed inside a metal chamber, and the alpha radiation it emits doesn't travel far enough to penetrate skin or the detector's casing. The main risk would come from breaking one open and inhaling or ingesting the material, which is why old detectors should be disposed of properly rather than pried apart.
How does americium actually detect smoke?
It emits alpha particles that ionize the air in a small chamber, letting a tiny steady electric current flow across it. When smoke particles enter the chamber, they disrupt that ionized air and reduce the current, which triggers the alarm.
Does americium have any other uses?
Yes, in smaller numbers — americium-241 is also used in industrial gauges that measure the thickness or density of materials like paper and plastic sheeting, relying on the same reliable, steady radioactive emission that makes it useful in smoke detectors.
Compounds
2 notable compounds containing Am
- AmO2oxide
Americium dioxide
A black solid with the fluorite structure typical of tetravalent actinide oxides, formed by heating americium compounds in oxygen. It's the most stable oxide form of americium under ambient conditions.
Used for: Encapsulated radiation source in ionization-type smoke detectors
- Am2O3oxide
Americium(III) oxide
A reddish-brown solid holding americium in the +3 state, its most stable oxidation state in aqueous solution and solid compounds alike. It forms when americium dioxide is heated under reducing conditions.
Isotopes
20 known isotopes
Swipe to see all columns →
| Isotope | Natural abundance | Half-life | Decay mode | Mass excess | Binding energy / nucleon | Decay width |
|---|---|---|---|---|---|---|
| Am-240 | — | 50.8 h | Electron capture (100%), Alpha decay (0.00019%) | 51,510.11 keV | 7,547.01 keV | 2.49 × 10-21 eV |
| Am-241 | — | 432.6 Y | Alpha decay (100%), Spontaneous fission (3.6e-10%) | 52,934.34 keV | 7,543.28 keV | 3.34 × 10-26 eV |
| Am-243 | — | 7364 Y | Alpha decay (100%), Spontaneous fission (3.7e-9%) | 57,175.01 keV | 7,530.17 keV | 1.96 × 10-27 eV |
Show all 20 isotopes
| Am-223 | — | 5 ms | Alpha decay (100%) | 42,700 keV | 7,547 keV | 9.12 × 10-14 eV |
| Am-229 | — | 0.9 s | Alpha decay (91%), Electron capture / beta-plus decay (9%) | 42,180.41 keV | 7,562.57 keV | 5.07 × 10-16 eV |
| Am-230 | — | 31 s | Electron capture (100%), Spontaneous fission | 42,872 keV | 7,562 keV | 1.47 × 10-17 eV |
| Am-232 | — | 79 s | Electron capture (97%), Alpha decay (3%), ECSF (0.069%) | 43,420 keV | 7,564 keV | 5.78 × 10-18 eV |
| Am-233 | — | 3.2 m | Electron capture / beta-plus decay (95.5%), Alpha decay (4.5%) | 43,285 keV | 7,567 keV | 2.38 × 10-18 eV |
| Am-234 | — | 2.32 m | Electron capture (100%), Alpha decay | 44,462 keV | 7,564 keV | 3.28 × 10-18 eV |
| Am-235 | — | 10.3 m | Electron capture (99.6%), Alpha decay (0.4%) | 44,624.6 keV | 7,565.16 keV | 7.38 × 10-19 eV |
| Am-236 | — | 3.6 m | Electron capture (100%), Alpha decay (0.004%) | 46,041 keV | 7,561 keV | 2.11 × 10-18 eV |
| Am-237 | — | 73.6 m | Electron capture / beta-plus decay (99.975%), Alpha decay (0.025%) | 46,570 keV | 7,561 keV | 1.03 × 10-19 eV |
| Am-238 | — | 98 m | Electron capture / beta-plus decay (100%), Alpha decay (0.0001%) | 48,421.42 keV | 7,555.59 keV | 7.76 × 10-20 eV |
| Am-239 | — | 11.9 h | Electron capture (99.99%), Alpha decay (0.01%) | 49,390.36 keV | 7,553.69 keV | 1.06 × 10-20 eV |
| Am-242 | — | 16.01 h | Beta-minus decay (83%), Electron capture (17%), Alpha decay (0.0001%) | 55,468.01 keV | 7,534.99 keV | 7.92 × 10-21 eV |
| Am-244 | — | 10.1 h | Beta-minus decay (100%) | 59,879.14 keV | 7,521.31 keV | 1.25 × 10-20 eV |
| Am-245 | — | 2.05 h | Beta-minus decay (100%) | 61,900.42 keV | 7,515.3 keV | 6.18 × 10-20 eV |
| Am-246 | — | 39 m | Beta-minus decay (100%) | 64,994 keV | 7,505 keV | 1.95 × 10-19 eV |
| Am-247 | — | 23.0 m | Beta-minus decay (100%) | 67,153 keV | 7,499 keV | 3.31 × 10-19 eV |
| Am-248 | — | — | Unknown | 70,563 keV | 7,487 keV | — |