Americium

Americium is the most common synthetic radioactive element found in everyday life, a tiny amount of americium-241 is in every domestic smoke detector. Named after the Americas, it was the third transuranic element produced and the first to find a widespread consumer application.

• Atomic Number9595 protons, 95 electrons
• Atomic Mass243.061380 u95× heavier than hydrogen
• State at Room TempSolidSolid
• Density13.69 g/cm³
• Melting / Boiling1175.8°C / 2010.8°C
• Discovered1944

What is Americium?

Americium has 95 protons. Am-241 (half-life 432 years) emits alpha particles that ionise the air between two electrodes in a smoke detector. When smoke enters, it reduces the ionisation current and triggers the alarm. About 0.3 micrograms of Am-241 sits in each detector. Produced in 1944 by Glenn Seaborg, Ralph James, Leon Morgan and Albert Ghiorso at Chicago.

Fact Americium is element 95, symbol Am. As an actinide, it is part of the f-block of the periodic table: one of 15 radioactive elements from actinium (89) to lawrencium (103). All actinides are radioactive; none has a stable isotope. They are produced in nuclear reactors and by decay of heavier elements, and their chemistry has been studied using only microgram or nanogram quantities.

Where you find Americium

On Earth

Americium does not occur in significant natural abundance. It is produced only artificially, by bombarding heavier actinide targets with neutrons or lighter ions in nuclear reactors or particle accelerators. World production is measured in micrograms or milligrams per year.

How we use Americium

Americium has 95 protons Future applications in targeted cancer radiotherapy are being investigated using alpha-emitting actinide isotopes bound to tumour-targeting molecules.

How it was discovered

Americium has 95 protons. Am-241 (half-life 432 years) emits alpha particles that ionise the air between two electrodes in a smoke detector. When smoke enters, it reduces the ionisation current and triggers the alarm. About 0.3 micrograms of Am-241 sits in each detector. Produced in 1944 by Glenn Seaborg, Ralph James, Leon Morgan and Albert Ghiorso at Chicago.

Deeper dive: americium and the actinide series

The actinides (elements 89-103) form the lower of the two rows below the main body of the periodic table. They represent the filling of the 5f electron subshell. Unlike the lanthanides (the upper row), the actinides show greater variety in their chemistry because the 5f, 6d and 7s orbitals are close in energy. The early actinides, thorium through neptunium, can show many different oxidation states (e.g. uranium from +3 to +6). The heavier actinides increasingly resemble the lanthanides in preferring the +3 state.

All actinides beyond bismuth (83) are radioactive. The lightest, thorium, protactinium and uranium, have long enough half-lives to survive from the formation of the solar system. Neptunium and beyond are almost entirely synthetic, produced in nuclear reactors or accelerators. The transuranic elements were created at remarkable facilities including Oak Ridge National Laboratory, the Berkeley Cyclotron, the GSI in Darmstadt and JINR in Dubna.

Moving to 96 protons brings us to the next element on the periodic table.