Facts About Plutonium (Pu or Atomic Number 94)

You probably know that plutonium is an element and that plutonium is radioactive, but what else do you know about it? Learn more with these fascinating facts.

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Plutonium Facts (Pu or Atomic Number 94)

By Anne Marie Helmenstine, Ph.D.

Facts About Plutonium

Here are 21 useful and interesting facts about plutonium:

1. The element symbol for plutonium is Pu, rather than Pl, because this was a more amusing, easily remembered symbol. The element was synthetically 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. The researchers submitted news of the discovery and the proposed name and symbol to the journal Physical Review but withdrew it when it became apparent plutonium could be used for an atomic bomb. The element's discovery was kept secret until after World War II.
2. Pure plutonium is a silvery-white metal, although it quickly oxidizes in air to a dull finish.
3. The atomic number of plutonium is 94, meaning all atoms of plutonium have 94 protons. It has an atomic weight around 244, a melting point of 640 degrees C (1183 degrees F), and a boiling point of 3228 degrees C (5842 degrees F).
4. Plutonium oxide forms on the surface of plutonium exposed to air. The oxide is pyrophoric, so pieces of plutonium might glow like embers as the outer coating burns. Plutonium is one of a handful of radioactive elements that "glows in the dark," although the glow is from heat.
5. Ordinarily, there are six allotropes, or forms, of plutonium. A seventh allotrope exists at high temperatures. These allotropes have different crystal structures and densities. Changes in environmental conditions readily cause plutonium to shift from one allotrope to another, making plutonium a difficult metal to machine. Alloying the element with other metals (e.g., aluminum, cerium, gallium) helps make it possible to work and weld the material.
6. Plutonium displays colorful oxidation states in aqueous solution. These states tend not to be stable, so plutonium solutions may spontaneously change oxidation states and colors. The colors of the oxidation states are as follows:
7. Pu(III) is lavender or violet.​
8. Pu(IV) is golden brown.
9. Pu(V) is pale pink.
10. Pu(VI) is orange-pink.
11. Pu(VII) is green. Note this oxidation state is uncommon. The 2+ oxidation state also occurs in complexes.
12. Unlike most substances, plutonium increases in density as it melts. The increase in density is about 2.5%. Near its melting point, liquid plutonium also exhibits higher-than-usual viscosity and surface tension for a metal.
13. Plutonium is used in radioisotope thermoelectric generators, which are used to power spacecraft. The element has been used in nuclear weapons, including the Trinity test and the bomb that was dropped on Nagasaki. Plutonium-238 was once used to power heart pacemakers.
14. Plutonium and its compounds are toxic and accumulate in bone marrow. Inhalation of plutonium and its compounds increases the risk of lung cancer, although many people have inhaled substantial amounts of plutonium yet didn't develop lung cancer. Inhaled plutonium is said to have a metallic taste.
15. Criticality accidents involving plutonium have occurred. The amount of plutonium required for critical mass is about one-third that necessary for uranium-235. Plutonium in solution is more likely to form critical mass than solid plutonium because the hydrogen in water acts as a moderator.
16. Plutonium is not magnetic. Other members of the element group stick to magnets, but plutonium can have a variable number of electrons in its valence shell, which makes it difficult for the unpaired electrons to align in a magnetic field.
17. The element name follows the trend of uranium and neptunium being named for planets outward from the Sun. Plutonium is named for the dwarf planet Pluto.
18. Plutonium is not a good conductor of electricity or heat, unlike some metals.
19. The alpha form of plutonium is hard and brittle, while the delta form is soft and ductile.
20. Plutonium occurs naturally in the Earth's crust in uranium ores, but it is very rare. The main source of the element is synthesis in reactors from uranium-238.
21. Plutonium is a member of the actinide element group, which makes it a type of transition metal.