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General Info
Atomic Symbol Atomic symbol::Am
Atomic Number Atomic number::95
Atomic Weight Atomic weight::243 g/mol
Chemical series Actinides
Appearance Silvery White
Group, Period, Block none, 7th, 5f
Electron configuration [Rn] 5f7, 7s2
Electrons per shell 2,8,18,32,25,8,2
Electron shell americium.png
CAS number CAS number::7440-35-9
Physical properties
Phase Solid
Density Density::13.67 g/ml
Melting point Melting point::1449 K
Boiling point Boiling point::2880 K
Isotopes of Americium
iso NA half-life DT DE (MeV) DP
230Am SYN ≈ 17s α No Data
231Am SYN No Data α No Data
232Am SYN 79s α No Data 228Np
232Am SYN 79 s ε 7.200 232Pu
233Am SYN ≈ 2m ε 4.17 234Pu
233Am ≈ 2m α 6.870 230Np
234Am SYN 2.32m α No Data
235Am SYN 9.9m α No Data
236Am SYN 3.6m ε 3.280 236Pu
236Am SYN 3.6m α 6.450 232Np
237Am SYN 73.0m ε 1.730
237Am SYN 73.0m α 99.98
238Am SYN 98m ε 2.260 238Pu
238Am SYN 98m α 6.040 234Np
239Am SYN 11.9h ε 0.0803 234Pu
239Am SYN 11.9h ε 5.924 235Np
240Am SYN 50.8h α 5.710 236Np
240Am SYN 50.8h ε 1.379 240Pu
241Am SYN 432.2y SF No Data
241Am SYN 432.2y α 5.638 237Np
242Am SYN 16.02h β- 0.665 242Cm
242Am SYN 16.02h ε 0.751 242Pu
242Am SYN 141y IT 0.049
242Am SYN 141y α 5.637 248Np
242Am SYN 141y SF No Data
242Am SYN 14ms SF No Data
242Am SYN 14ms α 7.788 238Np
242Am SYN 14ms IT 2.200
243Am SYN 7370y SF 242Am
243Am SYN 7370y α 5.438 239Np
244Am SYN 10.1h β- 1.428 244Cm
244mAm SYN ≈ 26m β- 1.526 244Cm
244Am SYN ≈ 26m ε 0.164 244Pu
245Am SYN 2.05h β- 242Am
246Am SYN 39m β- 2.376 246Cm
246mAm SYN 25m IT 242Am 246Am
246Am SYN 25m β- 2.376 246Cm
247Am SYN 23.0m β- 1.700 247Cm
248Am SYN ≈ 10m β- 3.100 248Cm
249Am SYN No Data [1]
All properties are for STP unless otherwise stated.

Americium is a chemical element classified as an Actinide and known by the chemical symbol Am. It is a synthetic (man-made) metal that only occurs in radioactive isotopes. Discovered in 1944 by scientists in the Manhattan project, americium was a bi-product of experiments with plutonium. The name americium comes from the element above it in the lanthanide series, europium. Just as europium takes its name from the continent Europe, americium is named after America.

During nuclear decay Americium 241 (241Am) emits both alpha particles as well as gamma rays, giving it several uses. For example, traces of americium are used in household smoke detectors. Americium can also occasionally be used as a portable source of gamma radiation.


A picture of americium 243

Americium, a member of the actinides, is a silver-white metal.[2] With an atomic number of ninety-five and a mass number of two hundred forty-three, this element only occurs in unstable, radioactive isotopes. As there are no stable forms of Americium, each form has a half life between five nanoseconds and 7370 years.[1] With a melting point of 1176 oC (1449.15 K) and a boiling point of 2607 oC (2880 K), americium is found in a solid state in room temperature.[3] Similar to many other metals, Americium tarnishes very slowly when exposed to air. When placed in acids, this element dissolves.[4] All isotopes of this element are radioactive, but the most common forms emit alpha particles. Because of the intense radiation, americium can be extremely dangerous to anyone without proper protective equipment and adequate knowledge of how to handle it.[1]


Americium, like other transuranium elements does not occur naturally. All forms of this element are synthetically produced.[5] Americium 241 (241Am) was first produced through a process of bombarding plutonium 239 (239Pu) with neutrons, resulting in plutonium 241 (241Pu). This new, unstable isotope decays through beta decay into americium 241 (241Am). [6]


Americium 241 is a common Isotope of Americium that is used in household smoke detectors.

Americium is one of the few radioactive elements that one should find in a common household object- the smoke detector. Most smoke detectors contain a small amount of americium 241 (241Am) in the compound americium dioxide. This form of americium undergoes radioactive decay primarily through alpha decay (α), but does emit small amounts of gamma rays as well.[4] The amount of radioactive material within smoke detectors is minimal. In fact, the amount of radioactive material found in a single smoke detector is so minuscule that about three million detectors could be made with a single gram of americium dioxide.[2] Even though such a small amount of radioactive material is found within smoke detectors, and most of the gamma rays emitted harmlessly escape, tampering with the device is not advisable and could be harmful.[4]

Inside the smoke detector, the americium is located within an ionization chamber where the emitted alpha particles collide with air particles to produce ions. A battery produces a small electric current that runs across these ions. When smoke enters the ionization chamber, its particles neutralize the charge of the ions and interfere with the flow of electricity, and therefore setting off the alarm.[4]

As well as its use in smoke detectors, americium has also previously used as a source of radiation in the field of radiography and a portable source of gamma rays.[1] [6]


In 1944, scientists in the Manhattan Project, Glenn Seaborg, Ralph James, Leon Morgan, and Albert Ghiorso successfully created the first samples of americium in the University of Chicago's metallurgical laboratory. As stated before, the first samples of the ninety-fifth element were created with neutron capture reactions of plutonium 239. This process involves the bombardment of the isotope with neutrons. Initially, the discovery of americium was considered classified as a discovery of the Manhattan project, but was later declassified. The public announcement of this new element by Glenn Seaborg occurred five days before the expected presentation at the American Chemistry Society. On a radio show, a child asked if there were any newly discovered transuranium elements other than plutonium and neptunium. Seaborg responded with his discovery of americium. [7]

Following the discovery of americium, Seaborg found that it and another recently discovered element, curium, both chemically behaved like the lanthanide series. This discovery brought issues with the periodic table, and lead Seaborg to revise it. This new revision included the new actinide series.[7]


This video explains some of the basic properties of americium and describes how it is used in household smoke detectors.


  1. 1.0 1.1 1.2 1.3 Barbalance, Kenneth. Periodic Table of Elements Element Americium - Am Web. Accessed 11 Oct 2014.
  2. 2.0 2.1 Americium Web. Author Unknown Published 15 October 2012.
  3. Americium Royal Society of Chemistry. Web. Author Unknown. Accessed 11 Oct 2014.
  4. 4.0 4.1 4.2 4.3 Smoke Detectors and Americium World Nuclear Association. Web. Author Unknown. Updated July 2014.
  5. Americium Chemistry Explained. Web. Author Unknown. Accessed October 25, 2014.
  6. 6.0 6.1 The Element Americium Jefferson Lab. Web. Author Unknown. Accessed 12 October 2014.
  7. 7.0 7.1 Live Science Staff. Facts About Americium Live Science. Web. Published September 23, 2013.