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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 221.pm. 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):
2.39
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
InChI Key: LFNLGNPSGWYGGD-UHFFFAOYSA-N
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)
225Np225.03391(8)3# ms [>2 µs]a to 221Pa9/2-#N/A1685.41-
226Np226.03515(10)#35(10) msa to 222PaN/AN/A1693.49-
227Np227.03496(8)510(60) msa to 223Pa; ß- to 227U5/2-#N/A1701.57-
228Np228.03618(21)#61.4(14) sß- to 228U; a to 224PaN/AN/A1709.64-
229Np229.03626(9)4.0(2) mina to 225Pa; ß- to 229U5/2+#N/A1717.72-
230Np230.03783(6)4.6(3) minß- to 230U; a to 226PaN/AN/A1725.8-
231Np231.03825(5)48.8(2) minß- to 231U; a to 227Pa(5/2)(+#)N/A1733.88-
232Np232.04011(11)#14.7(3) minß- to 232U; a to 228Pa(4+)N/A1732.64-
233Np233.04074(5)36.2(1) minß- to 233U; a to 229Pa(5/2+)N/A1740.72-
234Np234.042895(9)4.4(1) dEC to 234U(0+)N/A1748.8-
235Np235.0440633(21)396.1(12) dEC to 235U; a to 231Pa5/2+N/A1756.88-
236Np236.04657(5)154(6)E+3 yEC to 236U; a to 232Pa; ß- to 236Pu(6-)N/A1764.96-
237Np237.0481734(20)2.144(7)E+6 ya to 233Pa; SF 5/2+3.141773.04-
238Np238.0509464(20)2.117(2) dß- to 238Pu2+N/A1771.8-
239Np239.0529390(22)2.356(3) dß- to 239Pu5/2+N/A1779.88-
240Np240.056162(16)61.9(2) minß- to 240Pu(5+)N/A1787.96-
241Np241.05825(8)13.9(2) minß- to 241Pu(5/2+)N/A1796.03-
242Np242.06164(21)2.2(2) minß- to 242Pu(1+)N/A1794.8-
243Np243.06428(3)#1.85(15) minß- to 243Pu(5/2-)N/A1802.88-
244Np244.06785(32)#2.29(16) minß- to 244Pu(7-)N/A1810.95-
Neptunium Elemental Symbol

Recent Research & Development for Neptunium

  • Analysis of trace neptunium in the vicinity of underground nuclear tests at the Nevada National Security Site.2014 NovZhao P, Tinnacher RM, Zavarin M, Kersting AB. J Environ Radioact. 2014 Nov
  • Understanding the interactions of neptunium and plutonium ions with graphene oxide: scalar-relativistic DFT investigations.2014 Nov 6Wu QY, Lan JH, Wang CZ, Zhao YL, Chai ZF, Shi WQ. J Phys Chem A. 2014 Nov 6
  • Comparison of sample preparation methods for reliable plutonium and neptunium urinalysis using automatic extraction chromatography.2014 OctQiao J, Xu Y, Hou X, Miró M. Talanta. 2014 Oct
  • Quantum chemistry study of uranium(VI), neptunium(V), and plutonium(IV,VI) complexes with preorganized tetradentate phenanthrolineamide ligands.2014 Oct 20Xiao CL, Wu QY, Wang CZ, Zhao YL, Chai ZF, Shi WQ. Inorg Chem. 2014 Oct 20
  • In situ spectroscopic identification of neptunium(V) inner-sphere complexes on the hematite-water interface.2015 Feb 17Müller K, Gröschel A, Rossberg A, Bok F, Franzen C, Brendler V, Foerstendorf H. Environ Sci Technol. 2015 Feb 17
  • Molecular toolbox for genetic manipulation of the stalked budding bacterium Hyphomonas neptunium.2015 JanJung A, Eisheuer S, Cserti E, Leicht O, Strobel W, Möll A, Schlimpert S, Kühn J, Thanbichler M. Appl Environ Microbiol. 2015 Jan
  • Intrinsic formation of nanocrystalline neptunium dioxide under neutral aqueous conditions relevant to deep geological repositories.2015 Jan 25Husar R, Hübner R, Hennig C, Martin PM, Chollet M, Weiss S, Stumpf T, Zänker H, Ikeda-Ohno A. Chem Commun (Camb). 2015 Jan 25
  • Formation of neptunium(IV)-silica colloids at near-neutral and slightly alkaline pH.2015 Jan 6Husar R, Weiss S, Hennig C, Hübner R, Ikeda-Ohno A, Zänker H. Environ Sci Technol. 2015 Jan 6
  • A Linear Trans-bis(imido) Neptunium(V) Actinyl Analog: Np<sup>V</sup>(NDipp)<sub>2</sub>(<sup>t</sup>Bu<sub>2</sub>bipy)<sub>2</sub>Cl (Dipp = 2,6-<sup>i</sup>Pr<sub>2</sub>C<sub>6</sub>H<sub>3</sub>).2015 Jul 22Brown JL, Gaunt AJ, Batista ER, Boncella JM, Reilly SD, Scott BL, Tomson NC. J Am Chem Soc. 2015 Jul 22
  • XAS and TRLIF spectroscopy of uranium and neptunium in seawater.2015 Mar 28Maloubier M, Solari PL, Moisy P, Monfort M, Den Auwer C, Moulin C. Dalton Trans. 2015 Mar 28