Skip to Main Content

About Neptunium

Neptunium Bohr

Neptunium is a metallic solid and very reactive radioactive actinide that was long theorized to exist (Dmitri Mendeleev - 1870s) before finally being synthesized during the atomic age of 1940. Edwin McMillan and Philip H. Abelson created the first observable neptunium element via neutron bombardment of uranium at Berkeley Radiation Laboratory. Neptunium is extremely scarce in nature and can only be found in trace amounts in uranium ores within the Earth’s crust. For this reason, neptunium is primarily available only through neutron bombardment of uranium, but is also produced as a by-product in nuclear reactors. It was the first transuranium element to be synthesized. The half-life of its most stable isotope, 237Np, is over two million years and decays into thallium and bismuth; primarily through alpha decay.

Though neptunium has limited practical uses, Oak Ridge National Laboratory sells certain quantities for legal uses. It is widely used as a precursor step in the generation of 238Pu, an element often used in radioisotope thermoelectric generators to power deep-space spacecraft such as Voyager and Cassini-Huygens. Neptunium, as a standalone element, is primarily used in detectors of high-energy (MeV) neutrons. Recent research has shown that neptunium could be used as an atomic weapon with a critical mass around 60 kilograms – and with about 60,000 kilograms of neptunium being produced as a byproduct each year, the Federal government recently (2004) made plans to store isolated neptunium in nuclear-waste disposal sites. After 10,000 years, due to its long half-life, neptunium will become the dominant element remaining in nuclear waste being produced and stored today.

Due to its reactionary nature, neptunium is known to oxidize quickly and produce compounds with several other elements, including unusual metal-metal compounds with aluminum and beryllium. It is primarily reactive with oxygen, steam or acid but does not react to alkalis. There are no known uses for any of these compounds outside of fundamental scientific research. Nineteen radioisotopes of neptunium have been observed. Excluding 237Np, the half-life of these isotopes ranges from 3 milliseconds (225Np) to 154,000 years (236Np).

Neptunium Properties

Neptunium Bohr ModelNeptunium has the atomic symbol Np, atomic number 93, and atomic weight 237. It is a Block F, Group 3, Period 7 element. The number of electrons in each of Neptunium's shells is 2, 8, 18, 32, 22, 9, 2 and its electron configuration is [Rn] 5f4 6d1 7s2. The neptunium atom has a Van der Waals radius of In its elemental form, CAS 7439-99-8, neptunium has a silvery metallic appearance. The element was discovered by Edwin M. McMillan and P. H. Abelson in 1940. It was the first synthetic transuranium element of the actinide series discovered and was named for the planet Neptune.

Neptunium information, including properties, research, applications and other useful facts are discussed below. Scientific facts such as the atomic structure, ionization energy, abundance on Earth, conductivity and thermal properties are included.

Symbol: Np
Atomic Number: 93
Atomic Weight: 237
Element Category: Actinide
Group, Period, Block: n/a, 7, f
Color: silvery metallic
Other Names: Nettunio
Melting Point: 644°C, 1191.2°F, 917.15 K
Boiling Point: 644°C, 1191.2°F, 917.15 K
Density: 20450 kg·m3
Liquid Density @ Melting Point: N/A
Density @ 20°C: 20.45 g/cm3
Density of Solid: 20450 kg·m3
Specific Heat: N/A
Superconductivity Temperature: N/A
Triple Point: N/A
Critical Point: N/A
Heat of Fusion (kJ·mol-1): 9.46
Heat of Vaporization (kJ·mol-1): 336.6
Heat of Atomization (kJ·mol-1): 457
Thermal Conductivity: 6.3 W·m-1·K-1
Thermal Expansion: N/A
Electrical Resistivity: (22 °C) 1.220 µΩ·m
Tensile Strength: N/A
Molar Heat Capacity: 29.46 J·mol-1·K-1
Young's Modulus: N/A
Shear Modulus: N/A
Bulk Modulus: N/A
Poisson Ratio: N/A
Mohs Hardness: N/A
Vickers Hardness: N/A
Brinell Hardness: N/A
Speed of Sound: N/A
Pauling Electronegativity: 1.36
Sanderson Electronegativity: N/A
Allred Rochow Electronegativity: 1.22
Mulliken-Jaffe Electronegativity: N/A
Allen Electronegativity: N/A
Pauling Electropositivity: 2.64
Reflectivity (%): N/A
Refractive Index: N/A
Electrons: 93
Protons: 93
Neutrons: 144
Electron Configuration: [Rn] 5f4 6d1 7s2
Atomic Radius: 155 pm
Atomic Radius,
non-bonded (Å):
Covalent Radius: 190±1 pm
Covalent Radius (Å): 1.8
Van der Waals Radius: 221 pm
Oxidation States: 6, 5, 4, 3
Phase: Solid 
Crystal Structure: orthorhombic
Magnetic Ordering: paramagnetic
Electron Affinity (kJ·mol-1) Unknown
1st Ionization Energy: 604.5 kJ·mol-1
2nd Ionization Energy: N/A
3rd Ionization Energy: N/A
CAS Number: 7439-99-8
EC Number: N/A
MDL Number: N/A
Beilstein Number: N/A
SMILES Identifier: [Np]
InChI Identifier: InChI=1S/Np
PubChem CID: 23933
ChemSpider ID: 22375
Earth - Total: N/A
Mercury - Total: N/A
Venus - Total: N/A
Earth - Seawater (Oceans), ppb by weight: N/A
Earth - Seawater (Oceans), ppb by atoms: N/A
Earth -  Crust (Crustal Rocks), ppb by weight: N/A
Earth -  Crust (Crustal Rocks), ppb by atoms: N/A
Sun - Total, ppb by weight: N/A
Sun - Total, ppb by atoms: N/A
Stream, ppb by weight: N/A
Stream, ppb by atoms: N/A
Meterorite (Carbonaceous), ppb by weight: N/A
Meterorite (Carbonaceous), ppb by atoms: N/A
Typical Human Body, ppb by weight: N/A
Typical Human Body, ppb by atom: N/A
Universe, ppb by weight: N/A
Universe, ppb by atom: N/A
Discovered By: Edwin McMillan and Philip H. Abelson
Discovery Date: 1940
First Isolation: N/A

Neptunium Isotopes

Neptunium is an artificial element. It has no stable isotopes.

Nuclide Isotopic Mass Half-Life Mode of Decay Nuclear Spin Magnetic Moment Binding Energy (MeV) Natural Abundance
(% by atom)
225Np 225.03391(8) 3# ms [>2 µs] α to 221Pa 9/2-# N/A 1685.41 -
226Np 226.03515(10)# 35(10) ms α to 222Pa N/A N/A 1693.49 -
227Np 227.03496(8) 510(60) ms α to 223Pa; β- to 227U 5/2-# N/A 1701.57 -
228Np 228.03618(21)# 61.4(14) s β- to 228U; α to 224Pa N/A N/A 1709.64 -
229Np 229.03626(9) 4.0(2) min α to 225Pa; β- to 229U 5/2+# N/A 1717.72 -
230Np 230.03783(6) 4.6(3) min β- to 230U; α to 226Pa N/A N/A 1725.8 -
231Np 231.03825(5) 48.8(2) min β- to 231U; α to 227Pa (5/2)(+#) N/A 1733.88 -
232Np 232.04011(11)# 14.7(3) min β- to 232U; α to 228Pa (4+) N/A 1732.64 -
233Np 233.04074(5) 36.2(1) min β- to 233U; α to 229Pa (5/2+) N/A 1740.72 -
234Np 234.042895(9) 4.4(1) d EC to 234U (0+) N/A 1748.8 -
235Np 235.0440633(21) 396.1(12) d EC to 235U; α to 231Pa 5/2+ N/A 1756.88 -
236Np 236.04657(5) 154(6)E+3 y EC to 236U; α to 232Pa; β- to 236Pu (6-) N/A 1764.96 -
237Np 237.0481734(20) 2.144(7)E+6 y α to 233Pa; SF 5/2+ 3.14 1773.04 -
238Np 238.0509464(20) 2.117(2) d β- to 238Pu 2+ N/A 1771.8 -
239Np 239.0529390(22) 2.356(3) d β- to 239Pu 5/2+ N/A 1779.88 -
240Np 240.056162(16) 61.9(2) min β- to 240Pu (5+) N/A 1787.96 -
241Np 241.05825(8) 13.9(2) min β- to 241Pu (5/2+) N/A 1796.03 -
242Np 242.06164(21) 2.2(2) min β- to 242Pu (1+) N/A 1794.8 -
243Np 243.06428(3)# 1.85(15) min β- to 243Pu (5/2-) N/A 1802.88 -
244Np 244.06785(32)# 2.29(16) min β- to 244Pu (7-) N/A 1810.95 -
Neptunium Elemental Symbol

Recent Research & Development for Neptunium

  • Pan Zeng, Yuancheng Teng, Yi Huang, Lang Wu, Xiaohuan Wang, Synthesis, phase structure and microstructure of monazite-type Ce1-xPrxPO4 solid solutions for immobilization of minor actinide neptunium, Journal of Nuclear Materials, Volume 452, Issues 1–3, September 2014
  • Yingjie Zhang, Daniel J. Gregg, Gregory R. Lumpkin, Bruce D. Begg, Miodrag Jovanovic, The incorporation of neptunium and plutonium in thorutite (ThTi2O6), Journal of Alloys and Compounds, Volume 581, 25 December 2013
  • Hirokazu Hayashi, Masahide Takano, Masaki Kurata, Kazuo Minato, Syntheses of neptunium trichloride and measurements of its melting temperature, Journal of Nuclear Materials, Volume 440, Issues 1–3, September 2013
  • A. Filanovich, A. Povzner, The influence of phonon anharmonicity on thermal and elastic properties of neptunium, Journal of Nuclear Materials, Volume 437, Issues 1–3, June 2013
  • Akihiro Uehara, Takayuki Nagai, Toshiyuki Fujii, Osamu Shirai, Hajimu Yamana, Spectrophotometric and electrochemical study of neptunium ions in molten NaCl–CsCl eutectic, Journal of Nuclear Materials, Volume 437, Issues 1–3, June 2013
  • Daniel S. Alessi, Jennifer E.S. Szymanowski, Tori Z. Forbes, Andrew N. Quicksall, Ginger E. Sigmon, Peter C. Burns, Jeremy B. Fein, Mineralogic controls on aqueous neptunium(V) concentrations in silicate systems, Journal of Nuclear Materials, Volume 433, Issues 1–3, February 2013
  • Mathew S. Snow, Pihong Zhao, Zurong Dai, Annie B. Kersting, Mavrik Zavarin, Neptunium(V) sorption to goethite at attomolar to micromolar concentrations, Journal of Colloid and Interface Science, Volume 390, Issue 1, 15 January 2013
  • Florent Lebreton, Damien Prieur, Aurélien Jankowiak, Magaly Tribet, Caroline Leorier, Thibaud Delahaye, Louis Donnet, Philippe Dehaudt, Fabrication and characterization of americium, neptunium and curium bearing MOX fuels obtained by powder metallurgy process, Journal of Nuclear Materials, Volume 420, Issues 1–3, January 2012
  • Geng Bang Jin, Adam D. Raw, S. Skanthakumar, Richard G. Haire, L. Soderholm, James A. Ibers, Single-crystal structures of uranium and neptunium oxychalcogenides AnOQ (An=U, Np; Q=S, Se), Journal of Solid State Chemistry, Volume 183, Issue 3, March 2010
  • Edgar C. Buck, Matt Douglas, Rick S. Wittman, Verifying the presence of low levels of neptunium in a uranium matrix with electron energy-loss spectroscopy, Micron, Volume 41, Issue 1, January 2010
  • Marcus Walter, Joseph Somers, Daniel Bouëxière, Piotr Gaczynski, Boris Brendebach, Oxidation behaviour of uranium and neptunium in stabilised zirconia, Journal of Solid State Chemistry, Volume 182, Issue 12, December 2009
  • A. Seibert, T. Gouder, F. Huber, Reaction of neptunium with molecular and atomic oxygen: Formation and stability of surface oxides, Journal of Nuclear Materials, Volume 389, Issue 3, 1 June 2009
  • Grigory B. Andreev, Nina A. Budantseva, Ivan G. Tananaev, Boris F. Myasoedov, Interaction of transuranium elements with biologically relevant ligands. Synthesis, crystal structure, IR and electronic spectra of neptunium glycolate [NpO2(C2O3H3)(Bipy)] · 2.5H2O, Inorganic Chemistry Communications, Volume 12, Issue 2, February 2009
  • Tsuyoshi Nishi, Akinori Itoh, Masahide Takano, Masami Numata, Mitsuo Akabori, Yasuo Arai, Kazuo Minato, Thermal conductivity of neptunium dioxide, Journal of Nuclear Materials, Volume 376, Issue 1, 15 May 2008
  • Yingjie Zhang, Eric R. Vance, Diffuse reflectance spectroscopy of tetravalent neptunium and plutonium ions in ThO2, Journal of Nuclear Materials, Volume 374, Issues 1–2, 29 February 2008
  • A.B. Khasanova, St.N. Kalmykov, I.V. Perminova, S.B. Clark, Neptunium redox behavior and sorption onto goethite and hematite in the presence of humic acids with different hydroquinone content, Journal of Alloys and Compounds, Volumes 444–445, 11 October 2007
  • Yingjie Zhang, Eric R. Vance, Bruce D. Begg, Huijun Li, Diffuse reflectance spectroscopy of neptunium ions in polycrystalline ceramics designed for immobilization of HLW, Journal of Alloys and Compounds, Volumes 444–445, 11 October 2007
  • O.N. Batuk, St.N. Kalmykov, V.G. Petrov, E.V. Zakharova, Yu.A. Teterin, A.Yu. Teterin, V.I. Shapovalov, M.J. Haire, Neptunium interaction with uranium dioxide in aqueous solution, Journal of Nuclear Materials, Volume 362, Issues 2–3, 31 May 2007
  • T. Yamamura, N. Watanabe, Y. Shiokawa, Energy efficiency of neptunium redox battery in comparison with vanadium battery, Journal of Alloys and Compounds, Volumes 408–412, 9 February 2006
  • Tsuyoshi Sato, Toshiyuki Yamashita, Tsuneo Matsui, Phase equilibria and thermal expansion of CaTiO3 doped with neptunium, Journal of Nuclear Materials, Volume 344, Issues 1–3, 1 September 2005