Plutonium – facts and properties
- Symbol
- Pu
- Atomic number
- 94
- Atomic mass
- 244 u
- Period
- 7
- Category
- actinide
- Electron configuration
- [Rn] 5f⁶ 7s²
- Electronegativity
- 1.28
- Atomic radius
- 175 pm
- Valence electrons
- 8
- State at standard conditions
- solid
- Melting point
- 640 °C
- Boiling point
- 3228 °C
- Density
- 19.84 g/cm³
- Discovered
- 1940
Plutonium is a silvery-grey, radioactive metal with atomic number 94 that is very dangerous inside the body. Glenn T. Seaborg and his colleagues made it artificially in Berkeley in 1940 and named it after Pluto, which was still counted as a planet then and orbits beyond Neptune. Plutonium forms from uranium in nuclear reactors, serves as fuel in MOX fuel assemblies, and as plutonium-238 it powers space probes such as Voyager and Curiosity. In history it also played a part as a material for nuclear weapons.
Position in the periodic table
Occurrence and extraction
Plutonium practically does not occur in nature. Traces form in uranium ores when neutrons strike uranium-238, and of the longest-lived isotope, plutonium-244, with a half-life of roughly eighty million years, at most the tiniest remnants from the early Earth survive. Almost all the plutonium that exists today comes from nuclear reactors. There uranium-238 captures a neutron, becomes uranium-239 and turns via neptunium-239 into plutonium-239 through two beta decays. Part of it is split again in the reactor. Plutonium can be separated chemically from spent fuel, which is called reprocessing.
In 1940 Seaborg, McMillan, Joseph Kennedy and Arthur Wahl bombarded uranium with deuterons, the nuclei of heavy hydrogen, and so made plutonium-238 first. Early in 1941 they identified a new element chemically. Soon afterwards it turned out that plutonium-239 can be split. Because of the war the discovery was kept secret and announced only afterwards. The name continues the sequence Uranus, Neptune, Pluto. Seaborg is said to have suggested the symbol Pu as a joke, and nobody objected.
Properties and reactions
Plutonium is in period 7 and is the sixth actinoid, in the actinoid row of the table between neptunium on its left and americium on its right. Its electrons are arranged as [Rn] 5f⁶ 7s², with an electronegativity of 1.28 and an atomic radius of 175 pm. The density is 19.84 g/cm³ and the melting point 640 °C, which is low for such a heavy metal. The metal boils at 3228 °C. Its atomic mass is given as 244 u, the mass number of the longest-lived isotope, plutonium-244.
The metal tarnishes quickly in air and becomes covered with a yellowish to olive-green oxide layer. Finely divided, it may ignite spontaneously. Its structure is unusual: at normal pressure it forms six different crystal structures, and its density jumps as it is heated. In solution several oxidation states between plus three and plus six can exist side by side and colour it differently.
Plutonium-239 has a half-life of more than twenty thousand years, plutonium-238 only about ninety. Both emit alpha particles whose energy ends up as heat. A larger piece of plutonium therefore feels warm.
Plutonium is radiotoxic. Its alpha radiation cannot penetrate paper or skin, yet it acts strongly on tissue once the substance is inside the body. Inhaled dust settles in the lungs, and plutonium that is taken up settles in bone and liver, where it irradiates tissue for years and can cause cancer. On top of this comes the chemical toxicity of a heavy metal. It is handled only in sealed glove boxes and in quantities at which no chain reaction can start.
Reactor fuel, space probes and history
In reactors plutonium-239 supplies a considerable share of the energy. Some countries press separated plutonium and uranium into mixed oxide, MOX for short, to make fuel assemblies for light-water reactors.
Plutonium-238 emits mainly alpha particles, which turn into heat. A capsule of plutonium-238 oxide therefore produces heat for decades, and thermoelectric elements without moving parts convert it into electricity. Such radioisotope batteries power the two Voyager probes, which have been travelling since 1977, and the Mars rovers Curiosity and Perseverance, which can thus work independently of sunlight, night and dust storms. The capsules are built to stay intact even if a launch fails.
Plutonium also has a place in the history of nuclear weapons. During the Second World War it was made in large reactors in the USA. The first nuclear weapon test in July 1945 in New Mexico and the bomb dropped on Nagasaki in August 1945 relied on plutonium and its chain reaction. Large stockpiles built up during the Cold War, and disarmament treaties have tried to limit them since.
Quick question
What is the atomic number of plutonium (Pu)?
FAQ
Is plutonium poisonous?
Yes, mainly through its radiation. Outside the body skin shields the alpha radiation, but it damages tissue if plutonium dust is inhaled or taken up.
What is plutonium used for?
On a small scale as fuel in MOX fuel assemblies and as plutonium-238 in the radioisotope batteries of space probes and Mars rovers. Research is another use. Historically it was also a material for nuclear weapons.
How is plutonium made?
Almost entirely artificially. In nuclear reactors uranium-238 captures neutrons and turns via neptunium-239 into plutonium-239. In nature there are only traces.