Neptunium
Neptunium is a chemical element with symbol Np and atomic number 93. A radioactive actinide metal, neptunium is the first transuranic element. Its position in the periodic table just after uranium, named after the planet Uranus, led to it being named after Neptune, the next planet beyond Uranus.
Identity
- Atomic mass
- [237] u
- Category
- Actinides
- Group
- 3
- Period
- 7
- Block
- f
- Oxidation States
- +3, +4, +5, +6, +7
- Phase (25 °C)
- Solid
- Appearance
- silvery metallic
Electronic Structure
- Electron configuration
- 1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p6 5s2 4d10 5p6 6s2 4f14 5d10 6p6 7s2 5f4 6d1
- Short configuration
- [Rn] 5f4 6d1 7s2
- Electrons per shell
- 2, 8, 18, 32, 22, 9, 2
- Electronegativity (Pauling)
- 1.36
- Electron affinity
- 45.9 kJ/mol
- 1st ionization energy
- 605 kJ/mol
Physical properties
- Melting point
- 638.9 °C (912 K)
- Boiling point
- 4173.9 °C (4447 K)
- Density
- 20.45 g/cm³
- Molar heat capacity
- 29.5 J/(mol·K)
Chemical behavior of neptunium
Neptunium has five outer electrons ([Rn] 5f⁴ 6d¹ 7s²) and is the actinide with the most varied oxidation states: +3, +4, +5, +6, and +7, with +5 (the neptunyl ion NpO₂⁺) the most stable in solution. Its electronegativity is low (1.36) and it is dense, malleable, and radioactive. It oxidizes in air and is pyrophoric as fine powder; its redox chemistry dyes solutions from green to pink.
Reactivity
4 Np + 3 O₂ → 2 Np₂O₃
In air it coats itself with dark oxides; when heated it forms NpO₂, the stable form that is vitrified in waste. Fine powder can burn spontaneously.
Np + 4 HCl → NpCl₄ + 2 H₂
It dissolves in acids to give green neptunium(IV) salts. With oxidants it passes to neptunyl(V), pink-violet, the species separated in PUREX.
NpO₂ + 4 HF → NpF₄ + 2 H₂O
The dioxide converts to the tetrafluoride, the precursor of the metal by reduction with calcium and of targets for producing Pu-238.
2 NpF₆ → 2 NpF₄ + F₂
The volatile hexafluoride decomposes readily: unlike UF₆ it is useless for enrichment and complicates handling.
Notable compounds
| Compound | Formula | Use or importance |
|---|---|---|
| Neptunium dioxide | NpO₂ | Waste vitrification and targets |
| Neptunium tetrafluoride | NpF₄ | Metal production and Pu-238 |
| Neptunyl nitrate | NpO₂NO₃ | PUREX fuel separation |
| Np-U mixed oxide | Np-UO₂ | Advanced fuel studies |
| Np-237 detector | ²³⁷Np | Fast-neutron dosimetry |
Electron shell distribution
View in the atom builderIsotopes
No neptunium isotope is stable. Neptunium-237 (²³⁷Np, 2.14 million years) dominates the reactor inventory and is the precursor of Pu-238. Neptunium-239 (²³⁹Np, 2.36 days) is the bridge between U-239 and Pu-239 in every reactor. Neptunium-238 (²³⁸Np, 2.1 days) decays into space-grade Pu-238.
Occurrence and production
It barely exists in nature: only traces from cosmic neutron capture in uranium. All useful neptunium is bred in reactors, when U-238 captures neutrons and decays via Np-239 into Pu-239. It is extracted from spent fuel by nuclear reprocessing: each tonne contains about 500 g of Np-237.
History and origin of the name
Edwin McMillan and Philip Abelson isolated it in 1940 by bombarding uranium with neutrons in Berkeley and named it neptunium, after Neptune, beyond Uranus: the next planet and the next element. It was the first transuranium element. Its daughter Pu-238 would later carry heat to the Voyager probes.
Applications and relevance of neptunium
Uses of the element
As Np-237 it feeds targets that, when irradiated, produce Pu-238 for space RTGs (Voyager, Perseverance). As a fissile detector it measures fast neutrons in reactors and weapons.
Uses of its compounds
The dioxide is vitrified with high-activity waste; the nitrate is separated in PUREX. Its solutions calibrate actinide spectroscopy and migration studies in geological repositories.
Health, safety and environment
Biological role
It has no biological function and is highly radiotoxic: it accumulates in bone and liver with a long biological half-life. Inhalation of particles is the most dangerous route because of its long-lived alpha emission.
Safety
It is handled only in nuclear facilities, in glove boxes with shielding and monitors. Metallic powder can be pyrophoric. It does not exist in consumer products.
Environment
Np-237, mobile and extremely long-lived, shapes the design of geological repositories over hundreds of thousands of years. Research explores transmuting it into shorter-lived isotopes or immobilizing it in ceramics.
Frequently asked questions about Neptunium
What is neptunium-237 used for?
Neptunium-237 is used mainly in nuclear research and in high-energy neutron detectors. It also serves as starting material to produce plutonium-238, an isotope used in radioisotope thermoelectric generators for space probes and rovers. Its long half-life and its ability to undergo fission make it a relevant material for nuclear fuel studies and radioactive waste management.
Is neptunium dangerous?
Yes. Neptunium is radioactive, toxic, and can be pyrophoric as finely divided metal. Its most stable isotope, neptunium-237, has a half-life of about 2.14 million years and can accumulate in bones if it enters the body. The main risks are inhalation or ingestion of particles and prolonged exposure. For this reason it is handled only in nuclear facilities with strict radiation protection.
How is neptunium used to produce plutonium-238?
Neptunium-237 can capture a neutron and become neptunium-238, which decays by beta emission into plutonium-238. This plutonium emits heat steadily for decades and is used in radioisotope thermoelectric generators for space missions. Neptunium thus acts as a strategic precursor of an energy source used in probes and rovers that operate far from the Sun.
Does neptunium have any common commercial use?
No. Neptunium has hardly any commercial applications and is used mainly in research, neutron detectors, and plutonium-238 production. Its radiotoxicity, long half-life, and the complexity of its separation make it impractical for general industrial uses. Most neptunium generated in reactors is considered nuclear waste and must be managed as long-lived radioactive material.
Where does neptunium come from?
Neptunium is produced mainly as a byproduct in nuclear reactors, when the uranium in the fuel absorbs neutrons and transforms into new elements. It also occurs in extremely small amounts in uranium minerals, formed by natural reactions with neutrons. To obtain useful quantities it is extracted from spent nuclear fuel through radiochemical separation processes.
Why is neptunium a problem in nuclear waste?
Neptunium-237 has a very long half-life and some environmental mobility, so it can remain radiotoxic for hundreds of thousands of years. This complicates the design of deep geological repositories for high-activity waste. Research studies ways to separate it, immobilize it, or transmute it into shorter-lived isotopes. Its management is one of the challenges of the nuclear fuel cycle.
What is the difference between neptunium and uranium?
Both are radioactive actinides, but uranium occurs naturally in significant amounts and is the conventional nuclear fuel, whereas neptunium is produced almost entirely artificially in reactors. Uranium-235 is naturally fissile; neptunium-237 can also fission, but its use is far more limited. In addition, neptunium is a decay and irradiation product of uranium, with more complex waste management.
Main sources consulted: PubChem, Royal Society of Chemistry, NIST, IUPAC. Last revision: November 2026.