We continue to review some of the most important materials in heat treatment and metallurgy.
Americium (chemical symbol: Am)
Americium is a radioactive, soft, silver-white, heavy metal that is denser than lead and slowly tarnishes in air. With a density of about 13.6 g/cc, it is almost twice as heavy as steel. Americium is a transuranic metal, which means it is located after uranium on the periodic table. Americium compounds have a colored appearance (i.e., the chloride is pink). Most americium is a by-product of nuclear fission and is produced by showering uranium or plutonium with neutrons in nuclear reactors. A ton of spent nuclear fuel contains about 100 grams of americium. Americium was named in honor of the American continent and is located just below europium (named after Europe) in the periodic table.
Americium was first produced in 1944 during the Manhattan Project by a group led by renowned American chemist Glenn T. Seaborg at the Metallurgical Laboratory of the University of Chicago (now known as Argonne National Laboratory). Seaborg was an accomplished researcher who advised 10 U.S. presidents on nuclear matters. His contribution to the synthesis and discovery of 10 trans-uranium elements earned him a share of the 1951 Nobel Prize in chemistry, and the element seaborgium (Sg) was named in his honor.
By far the most common use of americium is in home smoke detectors (Fig. 2). A small particle of americium oxide made with the americium-241 isotope is contained inside the detector. Americium-241 gives off alpha particles, which collide with air molecules, causing them to break down to form ions, which are electrically charged. These charged ions make the air inside the smoke detector electrically conductive, and a current is maintained from one side of the detector to the other. When smoke passes between the sender and receiver, the conductivity of the air changes and is detected, which then sets off the smoke detector. Despite americium being rare and expensive to produce, only a very small amount (0.29 micrograms) is used in each smoke detector. This is less than the weight of a grain of pollen.
Here are a few important facts about americium.
- Atomic number: 95
- Atomic weight: 243
- Melting point: 1449 K (1176°C or 2149°F)
- Boiling point: 2284 K (2011°C or 3652°F)
- Density: 13.69 grams per cubic centimeter
- Phase at room temperature: Solid
- Element classification: Metal
- Period number: 7
- Group number: none
- Group name: Actinide