Plutonium Facts (Pu or Atomic Number 94)

Plutonium Facts
Plutonium is a radioactive metal with atomic number 94 and element symbol Pu.

You probably know some plutonium facts, like it’s a radioactive element used in nuclear devices and for powering spacecraft. Here is a collection of interesting plutonium facts you may not know, including its properties, uses, and sources.

Interesting Plutonium Facts

  1. Plutonium is a radioactive element with atomic number 94. This makes it the naturally-occurring element with the highest atomic number. (Technically, americium, curium, berkelium, and californium occur as decay products in uranium ores, but not in significant quantities.)
  2. All plutonium isotopes are radioactive. The most stable isotope is Pu-239, with a half-life of 24,360 years.
  3. The element symbol for plutonium is Pu instead of Pl because the element’s discoverers thought it was funny to say “Pee-You”. (Physicists have a sense of humor.)
  4. The element is named for the dwarf planet Pluto. The naming follows the trend set by the preceding elements, with uranium named for Uranus and neptunium named for Neptune.
  5. Although plutonium occurs in nature, it was first discovered as a synthetic element produced by Glenn T. Seaborg, Edwin M. McMillan, J.W. Kennedy, and A.C. Wahl at the University of California at Berkeley in 1940–1941. Seaborg’s team made plutonium by bombarding uranium-238 with deuterons in a cyclotron. However, scientists immediately recognized the element’s potential for making nuclear weapons, so its discovery remained a secret until after World War II.
  6. Plutonium belongs to the actinide group of elements on the periodic table. Like the other elements in the group, plutonium is a silver-colored, radioactive metal that quickly oxidizes in air so that it appeared tarnished.
  7. Plutonium glows in the dark, but not because it’s radioactive. The element is pyrophoric, which means it essentially burns in air. A chunk of plutonium in air glows reddish orange, like an ember. Unlike most metals, plutonium is a fire hazard.
  8. Plutonium is also warm to the touch. Partly the heat comes from the element’s pyrophoricity. But, plutonium is warm even in the absence of oxygen because of its radioactivity. It’s the alpha decay that produces the heat, releasing about 9.68 watts of power from a 5-kilogram sample.
  9. The high radioactivity leads self-irradiation, which fatigues the metal, changes its crystal structure over time, and anneals the metal. Basically, a piece of plutonium displays ever-changing properties.
  10. It’s dangerous to work with plutonium because it’s radioactive and liable to undergo spontaneous fission at any time. Plus, plutonium is a toxic heavy metal. Plutonium accumulates in bone marrow, where it releases alpha, beta, and gamma radiation. No one is known to have died from acute plutonium radiation poisoning, but the genetic mutations readily cause cancer. However, some people have inhaled plutonium and did not develop lung cancer. Inhaled plutonium reportedly has a metallic scent.
  11. Plutonium is responsible for some criticality accidents. It takes one-third less plutonium than uranium-235 for critical mass. Even though plutonium atoms are further apart when it’s dissolved in water, it’s more likely to go critical than the solid metal because the hydrogen in water acts as a moderator.
  12. Most metals are good thermal and electrical conductors, but plutonium poorly conducts heat and electricity. It also has a low melting point for a metal.
  13. Plutonium is unusual in that its density increases as it melts. A few compounds display this property, such as water and paraffin wax, but elements tend to be at their highest density as solids. Near its melting point, plutonium has higher viscosity and surface tension than most metals.
  14. Plutonium takes several forms or allotropes. The allotropes have different properties from one another. For example, the alpha allotrope is hard and brittle, while the delta form is soft and ductile. Plutonium spontaneously changes forms depending on conditions, making it a difficult metal to work.
  15. Plutonium has several uses, mainly relating to its radioactivity and heat generation. The Trinity nuclear test and Fat Man device dropped on Nagasaki used plutonium. But, it has peaceful applications, too. Plutonium-rich spent reactor fuel serves as a mixed oxide (MOX) fuel in reactors, recycling material that would otherwise be nuclear waste. Plutonium is a power source and heat source for spacecraft. The Cassini, Voyager, Galileo, and New Horizons missions as well as Curiosity and Perseverance Mars rovers use plutonium generators and heater units. Plutonium power and heat sources may also find use in deep-sea exploration. Researchers use plutonium to make heavier elements. For example, the discovery of flerovium involved plutonium.
  16. Like other actinides, plutonium has several oxidation states. These oxidation states are colorful in aqueous solutions. Pu(III) is lavender or violet, Pu(IV) is golden brown, Pu(V) is pale pink, Pu(VI) is orange-pink, and Pu(VII) is green. Plutonium atoms spontaneously change oxidation states, so a solution may change colors.
  17. Natural plutonium occurs in uranium ores. It forms from natural neutron irradiation of uranium, transmuting the element into plutonium. However, the element is relatively rare in the Earth’s crust. The primary source of plutonium is reactor synthesis from uranium-238.

Key Plutonium Facts

  • Name: Plutonium
  • Element Symbol: Pu
  • Atomic Number: 94
  • Atomic Mass: 244 (for the most stable isotope)
  • Electron Configuration: [Rn] 5f6 7s2
  • Element Group: Actinide
  • Appearance: Solid, silver-colored metal
  • Density (g/cm3): 19.84
  • Melting Point: 912.5 K ​(639.4 °C, ​1182.9 °F)
  • Boiling Point: 3505 K ​(3228 °C, ​5842 °F
  • Atomic Radius (pm): 151
  • Ionic Radius: 93 (+4e) 108 (+3e)
  • Heat of Fusion (kJ/mol): 2.8
  • Heat of Vaporization (kJ/mol): 343.5
  • Pauling Electronegativity: 1.28
  • First Ionization Energy (kJ/mol): 491.9
  • Oxidation States: +2, +3, +4, +5, +6, +7, +8
  • Crystal Structure: Monoclinic


  • Emsley, John (2011). Nature’s Building Blocks: An A-Z Guide to the Elements. Oxford University Press. ISBN 978-0-19-960563-7.
  • Greenwood, Norman N.; Earnshaw, Alan (1997). Chemistry of the Elements (2nd ed.). Butterworth-Heinemann. ISBN 978-0-08-037941-8.
  • Hammond, C. R. (2004). The Elements, in Handbook of Chemistry and Physics (81st ed.). CRC press. ISBN 978-0-8493-0485-9.
  • Seaborg, Glenn T. The Plutonium Story. Lawrence Berkeley Laboratory, University of California. LBL-13492, DE82 004551.
  • Weast, Robert (1984). CRC, Handbook of Chemistry and Physics. Boca Raton, Florida: Chemical Rubber Company Publishing. ISBN 0-8493-0464-4.