Neptunium (Np)
actinideSolid
Standard Atomic Weight
[237]Electron configuration
[Rn] 7s2 5f4 6d1Melting point
643.85 °CBoiling point
3901.85 °CDensity
2.025e+4 kg/m³Oxidation states
+2, +3, +4, +5, +6, +7Electronegativity (Pauling)
1.36Ionization energy (1st)
6.265608 eVDiscovery year
1940Atomic radius
175 pmDetails
Neptunium is a radioactive actinide and the first transuranium element. It is produced mainly by neutron reactions involving uranium in nuclear reactors and occurs naturally only in minute amounts from neutron capture and decay processes in uranium ores. Its chemistry bridges uranium and plutonium, with several accessible oxidation states in solution. The isotope ²³⁷Np is the most important for handling and study because of its long half-life and availability as a reactor by-product.
Neptunium metal buttons (photo courtesy Lawrence Berkeley National Laboratory)
Neptunium was first produced by Edwin M. McMillian and Philip H. Abelson, working at the University of California, Berkeley, in 1940. They produced neptunium-239, an isotope of neptunium with a half-life of about 2.4 days, by bombarding uranium with slow moving neutrons.
Named for the planet Neptune (named after the Roman god of the sea), the next planet out from the Sun after Uranus. There were many early false reports of the discovery of neptunium. The most significant was by Enrico Fermi who believed that bombarding uranium with neutrons followed by beta decay would lead to the formation of element 93. In 1934, he bombarded uranium atoms with neutrons and reported that he had produced elements 93 and 94. As it turned out, Fermi had actually fissioned or split uranium atoms into many fragment radioisotopes. The explanation and announcement of the discovery of fission was later published by Hahn and Strassman, although it was their co-worker Lisa Meitner who had correctly interpreted the results of the experiments. In 1940, with excitement about fission reaching the University of California at Berkeley, Professor Edwin McMillan and graduate student Philip Abelson bombarded uranium with cyclotron-produced moderated (slow) neutrons, resulting not in “fission” but "fusion" of the reactants forming the new element 93, which they named "neptunium":
23892U + 10n → 23992U → 23993Np + β-
Neptunium-239 was the first transuranium element produced synthetically and the first actinide series transuranium element discovered. This isotope has a beta-decay half-life of 2.3565 days, which forms daughter product plutonium-239 with a half-life of 24,000 years.
Metallic neptunium has been prepared in visible quantities. Fresh metal is silvery, dense, and chemically reactive, but it tarnishes in air as oxide layers form. It has several solid allotropes, and bulk physical measurements are limited by radioactivity, scarcity, and self-heating.
Neptunium has no broad commercial use as an element. ²³⁷Np is used in research on actinide chemistry, nuclear fuel cycles, and long-lived radioactive waste behavior. It is also the principal target material for producing ²³⁸Pu by neutron irradiation; ²³⁸Pu is then used in radioisotope heat sources and power systems. Small quantities of neptunium compounds have been used as analytical standards and in radiochemical method development.
Neptunium's most stable isotope, neptunium-237, has a half-life of about 2,144,000 years. It decays into protactinium-233 through alpha decay. Neptunium-237, which is produced in gram quantities as a by-product of the production of plutonium in nuclear reactors, is used in neutron detectors.
Once considered to be completely artificial, extremely small amounts of neptunium are produced naturally in uranium ores through the interaction of atoms of uranium in the ore with neutrons produced by the decay of other atoms of uranium in the ore.
Isotopes in Industry
237Np (with a half-life of 2.14×106 years) is fissionable, meaning that neptunium can be bombarded with neutrons and, as a result, create more neutrons that are free to interact with nearby material and can be used in fast neutron reactors or in nuclear weapons (Fig. IUPAC.93.1) [75] J. Peterson, M. McDonell, L. Haroun, F. Monette, R. D. Hildebrand, A. Taboas. Radiological and Chemical Fact Sheets to Support Health Risk Analyses for Contaminated Areas, Prepared by Argonne National Laboratory Environmental Science Division in collaboration with U.S. Department of Energy, Richland Operations Office and Chicago Operations Office (2014), Feb. 22; http://www.remm.nlm.gov/ANL_ContaminantFactSheets_All_070418.pdf., [603] P. Weiss. Science News.162, 259 (2002)., [604] T. Kenna. J. Anal. At. Spectrom.17, 1471 (2002).. 237Np is used in neutron detection instruments [75] J. Peterson, M. McDonell, L. Haroun, F. Monette, R. D. Hildebrand, A. Taboas. Radiological and Chemical Fact Sheets to Support Health Risk Analyses for Contaminated Areas, Prepared by Argonne National Laboratory Environmental Science Division in collaboration with U.S. Department of Energy, Richland Operations Office and Chicago Operations Office (2014), Feb. 22; http://www.remm.nlm.gov/ANL_ContaminantFactSheets_All_070418.pdf..
Isotopes Used as a Source of Radioactive Isotope(s)
237Np is used in the production of 238Pu (with a half-life of 87.7 years), which is an emitter of alpha particles used in thermoelectric generators and radioisotope-heater units. When 237Np captures a neutron, it becomes 238Np, with a half-life of 2.117 days, which decays to 238Pu [75] J. Peterson, M. McDonell, L. Haroun, F. Monette, R. D. Hildebrand, A. Taboas. Radiological and Chemical Fact Sheets to Support Health Risk Analyses for Contaminated Areas, Prepared by Argonne National Laboratory Environmental Science Division in collaboration with U.S. Department of Energy, Richland Operations Office and Chicago Operations Office (2014), Feb. 22; http://www.remm.nlm.gov/ANL_ContaminantFactSheets_All_070418.pdf..
Neptunium forms compounds in oxidation states from +3 to +7, with +4, +5, and +6 especially important in aqueous chemistry. The neptunyl ion NpO₂⁺ is a characteristic +5 species and is often relatively mobile in oxidizing waters, while NpO₂²⁺ represents +6 chemistry. Neptunium dioxide NpO₂ is a stable refractory oxide and an important solid phase. Halides such as neptunium tetrachloride NpCl₄ and neptunium hexafluoride NpF₆ illustrate the element’s actinide-like coordination and redox behavior.
See more information at the Neptunium compound page.
All neptunium isotopes are radioactive, and hazards depend strongly on isotope, chemical form, and exposure route. ²³⁷Np emits mainly alpha radiation with associated gamma emissions from decay products, so ingestion, inhalation, or contamination of wounds is the principal concern. Finely divided metal and soluble compounds require strict radiological containment. External dose rates can be significant for some samples because of accompanying gamma-emitting impurities or daughters.
Natural neptunium is extremely scarce, but anthropogenic neptunium occurs in spent nuclear fuel, reprocessing wastes, and fallout residues. Environmental behavior is controlled by oxidation state, complexation, mineral sorption, and reducing conditions. Np(V) as NpO₂⁺ can migrate more readily than many tetravalent actinides, whereas Np(IV) tends to form less soluble solids or strongly sorbed species. Its long half-life makes it important in repository performance assessments.
Neptunium is not traded as a commodity and has no ordinary industrial supply chain. Recoverable ²³⁷Np is generated as a by-product in irradiated uranium fuel and can be separated only in specialized nuclear facilities under heavy regulation. Demand is small and centered on research inventories and on target preparation for ²³⁸Pu production. Costs are dominated by radiochemical separation, safeguards, licensing, waste management, and secure storage rather than by ore mining or conventional refining.
Produced by bombarding uranium with slow neutrons.
Neptunium has no stable isotopes, so any primordial neptunium has decayed away. It can be formed in trace amounts by neutron capture in uranium-bearing materials and in nuclear explosions or reactors, and transient neptunium isotopes may occur in astrophysical rapid neutron-capture processes. Its extraterrestrial abundance is effectively negligible compared with long-lived actinides such as uranium and thorium.
- Neptunium was named after Neptune, following uranium’s naming after Uranus.
- ²³⁷Np has a half-life of about 2.14 million years.
- Neptunium was the first element beyond uranium to be identified.
- NpF₆ is volatile, a property relevant to actinide fluoride chemistry.
- The common aqueous Np(V) ion is linear neptunyl, NpO₂⁺.
- Separated neptunium metal darkens on exposure to air.
Images
Properties
Physical
- Atomic radius (empirical)
- 175 pm Compare Atomic radius (empirical) of all elements →
- Covalent radius
- 190 pm Compare Covalent radius of all elements →
- Van der Waals radius
- 221 pm Compare Van der Waals radius of all elements →
- Density
- 2.025 × 104 kg/m³ Compare Density of all elements →
- Molar volume
- 0.0211 L/mol
- Phase at STP
- Solid Compare Phase at STP of all elements →
- Melting point
- 643.85 °C Compare Melting point of all elements →
- Boiling point
- 3901.85 °C Compare Boiling point of all elements →
- Crystal structure
- Orthorhombic Compare Crystal structure of all elements →
Chemical
- Electronegativity (Pauling)
- 1.36 Compare Electronegativity (Pauling) of all elements →
- Electron affinity
- 0.48 eV
- Ionization energy (1st)
- 6.265608 eV Compare Ionization energy (1st) of all elements →
- Ionization energy (2nd)
- 11.50004 eV Compare Ionization energy (2nd) of all elements →
- Ionization energy (3rd)
- 19.700068 eV Compare Ionization energy (3rd) of all elements →
- Ionization energy (4th)
- 33.800116 eV Compare Ionization energy (4th) of all elements →
- Ionization energy (5th)
- 48.000165 eV Compare Ionization energy (5th) of all elements →
- Oxidation states
- +2, +3, +4, +5, +6, +7 Compare Oxidation states of all elements →
- Valence electrons
- 3 Compare Valence electrons of all elements →
- Electron configuration
- [Rn] 7s2 5f4 6d1
Thermodynamic
- Heat of fusion
- 0.10364305 eV Compare Heat of fusion of all elements →
- Heat of vaporization
- 3.482407 eV Compare Heat of vaporization of all elements →
- Heat of sublimation
- 4.33228 eV
- Heat of atomization
- 4.33228 eV
- Atomization enthalpy
- 4.817329 eV
Nuclear
- Protons
- 93 Compare Protons of all elements →
- Neutrons
- 144 Compare Neutrons of all elements →
- Known isotopes
- 27 Compare Known isotopes of all elements →
- Stable isotopes
- 0 Compare Stable isotopes of all elements →
- Mass number (most stable)
- 237
- Most stable isotope
- Np-237
- Discovery year
- 1940
Abundance
N/A
Crystal Structure
- Lattice constant a
- 472 pm
Electronic Structure
- Electrons per shell
- 2, 8, 18, 32, 22, 9, 2 Compare Electrons per shell of all elements →
Identifiers
- CAS number
- 7439-99-8 Compare CAS number of all elements →
- Term symbol
- 6L11/2
- InChI
- InChI=1S/Np
- InChI Key
- LFNLGNPSGWYGGD-UHFFFAOYSA-N
Electron Configuration Measured
Np: 5f⁴ 6d¹ 7s²[Rn] 5f⁴ 6d¹ 7s²1s² 2s² 2p⁶ 3s² 3p⁶ 3d¹⁰ 4s² 4p⁶ 4d¹⁰ 5s² 5p⁶ 4f¹⁴ 5d¹⁰ 6s² 6p⁶ 5f⁴ 6d¹ 7s²Atomic model
Isotopes change neutron count, mass, and stability — not the electron configuration of a neutral atom.
Schematic atomic model, not to scale.
Atomic Fingerprint
Emission / Absorption Spectrum
Isotope Distribution
No stable isotopes.
| Mass number | Atomic mass (u) | Natural abundance | Half-life |
|---|---|---|---|
| 219 Radioactive | 219.03143 ± 0.00021 | N/A | 570 us |
| 227 Radioactive | 227.034957 ± 0.000078 | N/A | 510 ms |
| 222 Radioactive | 222.0333 ± 0.00021 | N/A | 480 ns |
| 235 Radioactive | 235.0440635 ± 0.0000021 | N/A | 396.1 days |
| 236 Radioactive | 236.04657 ± 0.000054 | N/A | 153 ky |
Phase / State
Reason: 618.9 °C below melting point (643.85 °C)
Schematic, not to scale
Phase transition points
Transition energies
Energy required to melt 1 mol at melting point
Energy required to vaporize 1 mol at boiling point
Energy required to sublime 1 mol at sublimation point
Density
At standard conditions
At standard conditions
Atomic Spectra
Showing 10 of 93. Sorted by ion charge (ascending).
Lines Holdings ?
| Ion | Charge | Total lines | Transition probabilities | Level designations |
|---|---|---|---|---|
| Np I | 0 | 96 | 0 | 0 |
Levels Holdings ?
| Ion | Charge | Levels |
|---|---|---|
| Np I | 0 | 2 |
| Np II | +1 | 2 |
| Np III | +2 | 2 |
| Np IV | +3 | 2 |
| Np V | +4 | 2 |
| Np VI | +5 | 2 |
| Np VII | +6 | 2 |
| Np VIII | +7 | 2 |
| Np IX | +8 | 2 |
| Np X | +9 | 2 |
Ionic Radii
| Charge | Coordination | Spin | Radius |
|---|---|---|---|
| +2 | 6 | N/A | 110.00000000000001 pm |
| +3 | 6 | N/A | 101 pm |
| +3 | 9 | N/A | 117.8 pm |
| +4 | 6 | N/A | 87 pm |
| +4 | 8 | N/A | 98 pm |
| +5 | 6 | N/A | 75 pm |
| +6 | 6 | N/A | 72 pm |
| +7 | 6 | N/A | 71 pm |
Compounds
Isotopes (5)
There are 25 known radioactive isotopes of neptunium ranging in atomic weights from 225 to 244 with 5 of those as metastable isotopes. The most stable are Np-237 with a half-life of 2.14 million years; Np-236 with a half-life of 154,000 years; and Np-235 with a half-life of 396 days. All of the remaining isotopes have half-lives less than 4.5 days, with most less than 50 minutes. The primary decay mode for isotopes lighter than 237Np is by electron capture with a great deal of alpha emission. The products are mostly isotopes of uranium. The primary decay mode for Np-237 is by alpha-decay forming protactinium. The primary decay mode for the isotopes heavier than Np-237 is by beta-decay, forming plutonium. Neptunium-237, after decaying to protactinium then to uranium, eventually decays to form bismuth-209 and thallium-205. Unlike most other common heavy nuclei which decay to make isotopes of lead this decay chain is known as the neptunium series.
| Mass number | Atomic mass (u) | Natural abundance | Half-life | Decay mode | |
|---|---|---|---|---|---|
| 219 Radioactive | 219.03143 ± 0.00021 | N/A | 570 us | α =100% | |
| 227 Radioactive | 227.034957 ± 0.000078 | N/A | 510 ms | α ≈100%β+ ? | |
| 222 Radioactive | 222.0333 ± 0.00021 | N/A | 480 ns | α =100% | |
| 235 Radioactive | 235.0440635 ± 0.0000021 | N/A | 396.1 days | ε =99.99740±1.3%α =0.00260±1.3% | |
| 236 Radioactive | 236.04657 ± 0.000054 | N/A | 153 ky | ε =86.3±0.8%β- =13.5±0.8%α =0.16±0.4% |
Extended Properties
Covalent Radii (Extended)
- Covalent radius (Pyykkö)
- 171 pm
- Covalent radius (Pyykkö, double)
- 136 pm
- Covalent radius (Pyykkö, triple)
- 116 pm
Van der Waals Radii
- Alvarez
- 282 pm
- UFF
- 342.4 pm
- MM3
- 252 pm
Atomic & Metallic Radii
- Atomic radius (Rahm)
- 281 pm
Numbering Scales
- Mendeleev
- 22
- Pettifor
- 44
- Glawe
- 37
Electronegativity Scales
- Ghosh
- 0
Polarizability & Dispersion
- Dipole polarizability
- 151 a.u.
- Dipole polarizability (unc.)
- 20 a.u.
Phase Transitions & Allotropes
| Melting point | 917.15 K |
Oxidation State Categories
Advanced Reference Data
Crystal Radii Detail (8)
| Charge | CN | Spin | rcrystal (pm) | Origin |
|---|---|---|---|---|
| 2 | VI | 124 | ||
| 3 | VI | 115 | from r^3 vs V plots, | |
| 4 | VI | 101 | from r^3 vs V plots, | |
| 4 | VIII | 112 | from r^3 vs V plots, | |
| 5 | VI | 89 | ||
| 6 | VI | 86 | from r^3 vs V plots, | |
| 7 | VI | 85 | Ahrens (1952) ionic radius, | |
| 3 | IX | — | 131.8 |
Isotope Decay Modes (44)
| Isotope | Mode | Intensity |
|---|---|---|
| 219 | A | 100% |
| 220 | A | 100% |
| 221 | A | — |
| 222 | A | 100% |
| 223 | A | 100% |
| 224 | A | 100% |
| 225 | A | 100% |
| 225 | B+ | — |
| 226 | A | 100% |
| 226 | B+ | — |
Additional Data
Estimated Crustal Abundance
The estimated element abundance in the earth's crust.
Not Applicable
References (1)
- [5] Neptunium https://education.jlab.org/itselemental/ele093.html
Estimated Oceanic Abundance
The estimated element abundance in the earth's oceans.
Not Applicable
References (1)
- [5] Neptunium https://education.jlab.org/itselemental/ele093.html
References
(9)
Data deposited in or computed by PubChem
The half-life and atomic mass data was provided by the Atomic Mass Data Center at the International Atomic Energy Agency.
Element data are cited from the Atomic weights of the elements (an IUPAC Technical Report). The IUPAC periodic table of elements can be found at https://iupac.org/what-we-do/periodic-table-of-elements/. Additional information can be found within IUPAC publication doi:10.1515/pac-2015-0703 Copyright © 2020 International Union of Pure and Applied Chemistry.
The information are cited from Pure Appl. Chem. 2018; 90(12): 1833-2092, https://doi.org/10.1515/pac-2015-0703.
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The periodic table at the LANL (Los Alamos National Laboratory) contains basic element information together with the history, source, properties, use, handling and more. The provenance data may be found from the link under the source name.
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This section provides all form of data related to element Neptunium.
The element property data was retrieved from publications.