- Plutonium (Pu) is a dense, silvery-gray, radioactive metal with atomic number 94, belonging to the actinide series.
- Its electron configuration is [Rn] 5f⁶ 7s², and it exhibits multiple oxidation states, ranging from +3 to +7, with +4 and +6 being the most stable. The atomic structure contains ninety-four protons, most commonly one hundred and forty-four neutrons in plutonium-238 (²³⁸Pu) or one hundred and fifty-four neutrons in plutonium-244 (²⁴⁴Pu), and ninety-four electrons arranged in seven shells.
- Naturally occurring plutonium is extremely rare, found only in trace amounts in uranium ores due to neutron capture processes. The most important isotopes are plutonium-239 (²³⁹Pu), with a half-life of 24,100 years, used in nuclear weapons and reactors; plutonium-238 (²³⁸Pu), with a half-life of 87.7 years, used as a heat source in radioisotope thermoelectric generators (RTGs); and plutonium-244 (²⁴⁴Pu), with a half-life of 80 million years, which is primordial and extremely scarce.
- Plutonium was first produced and identified in 1940 by Glenn T. Seaborg, Edwin M. McMillan, Joseph W. Kennedy, and Arthur C. Wahl at the University of California, Berkeley, by bombarding uranium-238 with deuterons in a cyclotron, creating neptunium-238, which decayed into plutonium-238. The element was named after Pluto, the dwarf planet beyond Neptune, following the sequence uranium (Uranus), neptunium (Neptune), plutonium (Pluto). The discovery of plutonium was classified during World War II because of its central role in the Manhattan Project.
- Plutonium’s most notable application is in nuclear weapons, where the isotope ²³⁹Pu serves as a fissile core material. The “Fat Man” bomb dropped on Nagasaki in 1945 used a plutonium implosion design. In nuclear reactors, ²³⁹Pu is bred from uranium-238 through neutron capture and subsequent beta decay, forming a crucial part of the plutonium-uranium fuel cycle. Plutonium-238’s steady heat output makes it ideal for powering RTGs in spacecraft such as Voyager, Cassini, and the Mars rovers.
- Chemically, plutonium is highly reactive, tarnishing rapidly in air to form a dull oxide layer. It reacts with hydrogen, halogens, and acids but is resistant to alkalis. Its metallic form exhibits six allotropes, with varying densities and crystal structures, making its metallurgy complex and sensitive to temperature changes. Plutonium dioxide (PuO₂) is a stable ceramic used in mixed oxide (MOX) fuel.
- Biologically, plutonium is extremely toxic and radiologically dangerous, primarily through alpha particle emission. Inhalation of plutonium dust is the most hazardous exposure route, as it can lodge in the lungs and irradiate surrounding tissue, significantly increasing cancer risk. Once absorbed, plutonium tends to accumulate in bones and the liver.
- Environmentally, plutonium contamination is a long-term hazard due to its long half-lives and alpha radiation. It has entered the environment through atmospheric nuclear weapons testing, nuclear accidents, and improper waste disposal. Because of its persistence, plutonium requires secure containment for tens of thousands of years.
