Plutonium
Plutonium is a transuranic radioactive chemical element with symbol Pu and atomic number 94. It is an actinide metal of silvery-gray appearance that tarnishes when exposed to air, and forms a dull coating when oxidized. The element normally exhibits six allotropes and four oxidation states.
Identity
- Atomic mass
- [244] u
- Category
- Actinides
- Group
- 3
- Period
- 7
- Block
- f
- Oxidation States
- +3, +4, +5, +6
- Phase (25 °C)
- Solid
- Appearance
- silvery white, tarnishing to dark gray in air
Electronic Structure
- Electron configuration
- 1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p6 5s2 4d10 5p6 6s2 4f14 5d10 6p6 7s2 5f6
- Short configuration
- [Rn] 5f6 7s2
- Electrons per shell
- 2, 8, 18, 32, 24, 8, 2
- Electronegativity (Pauling)
- 1.28
- Electron affinity
- -48.3 kJ/mol
- 1st ionization energy
- 585 kJ/mol
Physical properties
- Melting point
- 639.4 °C (913 K)
- Boiling point
- 3231.8 °C (3505 K)
- Density
- 19.816 g/cm³
- Molar heat capacity
- 35.5 J/(mol·K)
Chemical behavior of plutonium
Plutonium has six outer electrons ([Rn] 5f⁶ 7s²) and shows +3, +4, +5, and +6 states, with +4 the most stable in solution. Its electronegativity is low (1.28) and it is dense, brittle, and radioactive: it heats its own compounds and changes phase with temperature. Its solutions color from blue to pink depending on the state; Pu-238 glows with heat.
Reactivity
4 Pu + 3 O₂ → 2 Pu₂O₃
In air it tarnishes rapidly to olive-green PuO₂, the ceramic form used in RTGs and MOX. Fine powder can burn spontaneously.
Pu + 4 HF → PuF₄ + 2 H₂
With hydrofluoric acid it gives the tetrafluoride, the precursor of the metal by reduction with calcium and of targets for space isotopes.
PuO₂ + 4 HNO₃ → Pu(NO₃)₄ + 2 H₂O
The dioxide dissolves in concentrated nitric acid for PUREX: this is how plutonium is separated from uranium and fission products.
²³⁸Pu → ²³⁴U + α + heat
Pu-238 decays emitting alphas and 0.56 W/g of steady heat for decades: the battery that powers Voyager and Perseverance.
Notable compounds
| Compound | Formula | Use or importance |
|---|---|---|
| Plutonium-238 dioxide | ²³⁸PuO₂ | RTGs for probes and rovers |
| MOX fuel | UO₂-PuO₂ | Recycling in reactors |
| Plutonium tetrafluoride | PuF₄ | Metal production |
| Plutonium nitrate | Pu(NO₃)₄ | PUREX reprocessing |
| Pu-Ga alloy | Pu-Ga | Military phase stabilization |
Electron shell distribution
View in the atom builderIsotopes
No plutonium isotope is stable. Plutonium-239 (²³⁹Pu, 24,100 years) is fissile and fuels weapons and MOX. Plutonium-238 (²³⁸Pu, 87.7 years) generates heat for space. Plutonium-240, 241, and 242 accompany spent fuel and complicate its handling with neutrons and heat.
Occurrence and production
It barely exists in nature: only traces from neutron capture in uranium. All useful plutonium is bred in reactors, when U-238 captures neutrons and decays via Np-239 into Pu-239. It is separated from spent fuel by nuclear reprocessing: each tonne contains about 10 kg. Space-grade Pu-238 is bred from Np-237.
History and origin of the name
Glenn Seaborg and his team synthesized it in 1940 by bombarding uranium with deuterons in Berkeley and named it plutonium, after Pluto, beyond Neptune: the next planet and the next element. Pu-239 powered Nagasaki and the Chicago reactor; Pu-238 has kept Voyager warm since 1977.
Applications and relevance of plutonium
Uses of the element
As Pu-238 in ceramic pellets it feeds RTGs that deliver hundreds of watts for decades to Voyager, Cassini, and Perseverance. As Pu-239 it forms the core of weapons and, diluted, the MOX of French and Japanese reactors.
Uses of its compounds
Encapsulated ceramic dioxide survives reentry without dispersing; MOX turns military surpluses into electricity. Nitrates calibrate IAEA safeguards.
Health, safety and environment
Biological role
It has no biological function and is highly radiotoxic: inhaled it accumulates in lung, bone, and liver with a biological half-life of decades. A single inhaled microgram raises the risk of lung cancer.
Safety
It is handled only in nuclear facilities, in inert glove boxes with shielding and monitors. The dust is pyrophoric and the neutrons from Pu-240 require additional shielding. It does not exist in consumer products.
Environment
Military surpluses and spent fuel are vitrified or stored dry under safeguards. Every space RTG travels certified against dispersal. Its management shapes geological repositories over tens of thousands of years.
Frequently asked questions about Plutonium
What is plutonium-238 used for in space?
Plutonium-238 is used in radioisotope thermoelectric generators, which convert the heat of its decay into electricity. These systems power probes, rovers, and observatories that operate too far from the Sun to rely on solar panels. Its heat output is stable for decades, enabling long-duration missions. It is used in the form of encapsulated ceramic oxide, designed to reduce risks in case of an accident.
Is plutonium dangerous?
Yes. Plutonium is radioactive and toxic, especially when inhaled as fine particles, because it can deposit in the lungs and accumulate in bones and liver. Its alpha radiation barely penetrates the skin, but is very harmful if the material enters the body. For this reason its handling requires containment, shielding, ventilation, and strict radiation protection protocols.
Is plutonium used in nuclear power plants?
Yes, although it is not the main fuel in most countries. Plutonium generated in reactors can remain in the spent fuel or be recovered through reprocessing. In some countries it is used in MOX fuel, a mixture of uranium and plutonium oxides, to produce electricity. Its use depends on energy policy, reprocessing capacity, and the nuclear regulation of each country.
What is MOX fuel?
MOX fuel is a mixture of uranium and plutonium oxides used in some nuclear reactors. It allows plutonium recovered from spent fuel to be reused and reduces the amount of plutonium stored as waste. Although it can generate electricity, its manufacture and handling are complex because plutonium is fissile and radiotoxic. Its use is limited to countries with reprocessing infrastructure and advanced nuclear regulation.
What is the difference between plutonium-238 and plutonium-239?
Plutonium-238 emits mainly alpha radiation and generates steady heat, so it is used in nuclear batteries and space missions. Plutonium-239 is fissile: it can sustain a chain reaction and is used in nuclear weapons and in some fuels or reprocessing programs. In addition, plutonium-238 has a half-life of about 88 years, while plutonium-239's is about 24,100 years.
Where does plutonium come from?
Plutonium is produced mainly in nuclear reactors, when uranium-238 absorbs neutrons and transforms into plutonium-239. It can then be separated from spent fuel through chemical reprocessing, such as the PUREX process. Plutonium-238, used in space, is obtained through specific nuclear routes from neptunium-237 or other irradiated materials.
Why was plutonium used in pacemakers?
Plutonium-238 generates steady heat for decades, and that heat could be converted into electricity by a thermoelectric generator. This made it possible to build nuclear pacemakers with far greater autonomy than the batteries of the era. However, they were replaced by lithium batteries that are safer, lighter, and easier to manage. The plutonium pacemakers that still exist require special monitoring and radiological disposal.
Main sources consulted: PubChem, Royal Society of Chemistry, NIST, IUPAC. Last revision: November 2026.