Americium – facts and properties
- Symbol
- Am
- Atomic number
- 95
- Atomic mass
- 243 u
- Period
- 7
- Category
- actinide
- Electron configuration
- [Rn] 5f⁷ 7s²
- Electronegativity
- 1.3
- Atomic radius
- 175 pm
- Valence electrons
- 9
- State at standard conditions
- solid
- Melting point
- 1176 °C
- Boiling point
- 2011 °C
- Density
- 13.69 g/cm³
- Discovered
- 1944
Americium is a silvery-white, radioactive metal with atomic number 95. Glenn T. Seaborg, Ralph James, Leon Morgan and Albert Ghiorso made it in 1944 in Chicago as part of the Manhattan Project and named it after America, just as europium above it is named after Europe. Because tiny amounts of americium-241 sit inside ionisation smoke detectors, it is one of the few artificial elements found in everyday life.
Position in the periodic table
Occurrence and extraction
Americium does not occur in nature. All its isotopes are radioactive, and even the longest-lived, americium-243, has a half-life of only just over seven thousand years. Americium-241, the most important isotope, manages about 430 years. Both are much shorter than the age of the Earth.
It is made in nuclear reactors. Plutonium-239 captures two neutrons in succession and becomes plutonium-241, which turns into americium-241 by beta decay with a half-life of about fourteen years. The americium content of stored plutonium-bearing material therefore grows with time. In spent fuel, americium is among the components that keep the waste radioactive and warm for centuries. Technically it is obtained from plutonium-bearing material by chemical separation.
Seaborg's group made it in 1944 at the University of Chicago by bombarding plutonium with neutrons. The work mattered for the war and stayed secret. Seaborg revealed the new element in November 1945 on a radio quiz show for children, a few days before the official announcement. The name recalls the Americas, just as europium recalls Europe.
Properties and reactions
Americium is in period 7 and is the seventh actinoid, in the actinoid row of the table between plutonium on its left and curium on its right. Above it, in the lanthanoid row, stands europium. The atom has the electron configuration [Rn] 5f⁷ 7s², and its half-filled 5f shell matches the 4f shell of europium. Its electronegativity is 1.3 and its atomic radius is 175 pm. The density is 13.69 g/cm³, much lower than that of plutonium. The metal melts at 1176 °C. The boiling point of 2011 °C is uncertain; other sources give considerably higher values. Its atomic mass is given as 243 u, the mass number of the longest-lived isotope, americium-243.
Americium is a silvery-white, soft metal that tarnishes only slowly in air. In compounds the oxidation state plus three is the most stable, as with most lanthanoids. The Am³⁺ ions colour solutions pink. Plus four to plus six are rarer. While europium also occurs in the divalent state, americium hardly does.
Americium-241 emits alpha particles and a weak but penetrating gamma radiation. Skin already stops the alpha radiation, and thin layers of metal shield the gamma rays. The element becomes dangerous if it enters the body, where it settles in bone and liver and irradiates them for decades. Strict rules therefore apply to handling it.
Smoke detectors, gauges and neutron sources
Americium-241 sits inside ionisation smoke detectors. In a small measuring chamber the alpha particles ionise the air, so that a weak current flows between two electrodes. When smoke enters, ions attach to the smoke particles, the current drops, and the detector sounds the alarm. The amount is tiny, less than a millionth of a gram. The americium is firmly sealed in a metal foil, and the alpha radiation travels only a few centimetres. Optical detectors, which need no radioactive substance, are now common in many homes. Disposal rules differ by country, so local guidance decides where an old ionisation detector goes; it should never be opened.
Mixing americium-241 with beryllium gives a neutron source: the alpha particles strike beryllium nuclei, and neutrons emerge from the nuclear reaction. Such sources are used in the oil industry to survey boreholes and in instruments that measure moisture and density. The weak gamma rays are also used to gauge the thickness of sheet metal and paper.
In research, americium-243 is the starting material for curium. Researchers in Europe are also testing whether americium-241 could serve as the heat source of radioisotope batteries for space probes, since its long half-life would allow missions lasting many decades.
Quick question
What is the atomic number of americium (Am)?
FAQ
Why is there americium in smoke detectors?
The alpha particles of americium-241 make the air in a measuring chamber electrically conductive. Smoke reduces the current, and the detector triggers. The amount is tiny, and the alpha radiation stays inside the device.
Is americium dangerous?
It is radioactive, and it becomes dangerous mainly if it enters the body, where it settles in bone and liver. Sealed inside a smoke detector it gives off hardly any radiation to the outside.
Why is americium called americium?
It is named after America, just as europium, in the row above it, is named after Europe. The two elements resemble each other in the build-up of their electron shells, and the name underlines this kinship.