Name: Neptunium
Symbol: Np
Atomic Number: 93
Atomic Weight: 237.048200
Family: Rare Earth Elements
CAS RN: 7439-99-8
Description: A silvery metallic radioactive rare earth metal.
State (25C): Solid
Oxidation states: +3, +4, +5, +6

Molar Volume: 11.59 cm3/mole
Valence Electrons: 5f46d17s2

Boiling Point: 4175.15oK,  3902.0oC, 7055.6oF:
Melting Point: 913.15oK, 640.0oC, 1184.0oF:

Electrons Energy Level: 2, 8, 18, 32, 22, 9, 2
Isotopes: 20 + None Stable + 3 Meta States
Heat of Vaporization: unknown
Heat of Fusion:
5.19 kJ/mol
Density: 20.25 g/cm3 at 20oC
Specific Heat: 0.12 J/gK
Atomic Radius: 1.31 pm
Ionic Radius: 0.75
Electronegativity: 1.36 (Pauling),  1.22 (Allrod Rochow)

1s2 2s2p6 3s2p6d10 4s2p6d10f14 5s2p6d10f4 6s2p6d1 7s2


Neptunium (named for the planet Neptune, the next planet out from Uranus, after which uranium was named) was first discovered by Edwin McMillan and Philip Abelson in 1940.   Initially predicted by Walter Russell's "spiral" organization of the periodic table, it was found at the Berkeley Radiation Laboratory of the University of California, Berkeley where the team produced the neptunium isotope  239Np (2.4 day half-life) by bombarding uranium with slow moving neutrons. It was the first transuranium element produced synthetically and the first actinide series transuranium element discovered.

mcmillan2.gif (12282 bytes)

Edwin McMillan


Neptunium is prepared by the reduction of NpF3 with barium or lithium vapor at about 1200oC.  Neptunium metal has a silvery appearance, is chemically reactive, and exists in at least three structural modifications: alpha-neptunium, orthorhombic, density 20.25 g/cm3, beta-neptunium (above 280oC), tetragonal, density (313oC) 19.36 g/cm3, gamma-neptunium (577oC), cubic, density (600oC) 18.0 g/cm. Neptunium has four ionic oxidation states in solution: Np+3 (pale purple), analogous to the rare earth ion Pm+3, Np+4 (yellow green); NpO2+ (green blue): and NpO2++ (pale pink).  These latter oxygenated species are in contrast to the rare earths which exhibit only simple ions of the (II), (III), and (IV) oxidation states in aqueous solution.  The element forms tri- and tetrahalides such as NpF3, NpF4, NpCl4, NpBr3, NpI3, and oxides of the various compositions such as are found in the uranium-oxygen system, including Np3O8 and NpO2.

2s2 2p6
3s2 3p6 3d10
4s2 4p6 4d10 4f14
5s2 5p6 5d10 5f4
6s2 6p6 6d1

Silvery in appearance, neptunium metal is fairly chemically reactive and is found in at least three structural modifications:

This element has four ionic oxidation states while in solution:

  • Np+3 (pale purple), analogous to the rare earth ion Pm+3,
  • Np+4 (yellow green);
  • NpO2+ (green blue): and
  • NpO2++ (pale pink).


Trace amounts of neptunium are found naturally as decay products from transmutation reactions in uranium ores.   237Np is produced through the reduction of 237NpF3 with barium or lithium vapor at around 1200C and is most often extracted from spent nuclear fuel rods as a by-product in plutonium production.


237Np is irradiated with neutrons to create 238Pu, a rare and valuable isotope for spacecraft and military applications.

Neptunium is fissionable, and could theoretically be used as reactor fuel or to create a nuclear weapon. In 1992, the U.S. Department of Energy declassified the statement that Np-237 "can be used for a nuclear explosive device".  It is not believed that an actual weapon has ever been constructed using neptunium.

In September 2002, researchers at the University of California Los Alamos National Laboratory created the first known nuclear critical mass using neptunium in combination with enriched uranium, discovering that the critical mass of neptunium is less than previously predicted.  US officials in March 2004, planned to move the nation's supply of enriched neptunium to a site in Nevada.


Neptunium forms tri- and tetrahilides such as NpF3, NpF4, NpCl4, NpBr3, NpI3, and oxides of the various compositions such as are found in the uranium-oxygen system, including Np3O8 and NpO2.

Neptunium like other actinides readily forms a dioxide neptunyl core (NpO2).   In the environment, this neptunyl core readily complexes with carbonate as well as other oxygen moieties (OH-, NO2-, NO3-, and SO4-2) to form charged complexes which tend to be readily mobile with low affinities to soil.

  • NpO2(OH)2-1
  • NpO2(CO3)-1
  • NpO2(CO3)2-3
  • NpO2(CO3)3-5


Although the neptunium on which the characterization work was done was synthesized in a cyclotron, we now know that minute amounts of the element exist in the environment (the longest-lived isotope has a half-life of about 2 million years). All isotopes of the metal are radioactive.

19 neptunium radioisotopes have been characterized, with the most stable being 237Np with a half-life of 2.14 million years, 236Np with a half-life of 154,000 years, and 235Np with a half-life of 396.1 days. All of the remaining radioactive isotopes have half-lifes that are less than 4.5 days, and the majority of these have half lifes that are less than 50 minutes. This element also has 4 meta states, with the most stable being 236mNp (t 22.5 hours).

The isotopes of neptunium range in atomic weight from 225.0339 u (225Np) to 244.068 u (244Np). The primary decay mode before the most stable isotope, 237Np, is electron capture (with a good deal of alpha emission), and the primary mode after is betta emission.  The primary decay products before 237Np are uranium-92 isotopes (alpha emission produces protactium-91, however) and the primary products after are plutonium-94 isotopes.

237Np eventually decays to form bismuth, unlike most other common heavy nuclei which decay to make lead.

Nuclear Synthesis

When a 235U atom captures a neutron, it is converted to an excited state of 236U.   About 81% of the excited 236U nuclei undergo fission, but the remainder decay to the ground state of 236U by emitting gamma radiation.  Further neutron capture creates 237U which has a half-life of 7 days and thus quickly decays to 237Np. 237U is also produced via an n,2n reaction with 238U.   Since nearly all neptunium is produced in this way or consists of isotopes which decay quickly, one gets nearly pure 237Np by chemical separation of neptunium.

atom.gif (700 bytes)

Atomic Mass
225Np 225.03391 ~3 ms
226Np 226.03515 35 ms
227Np 227.03496 510 ms
228Np 228.03618 61.4 seconds
229Np 229.03626 4.0 minutes
230Np 230.03783 4.6 minutes
231Np 231.03825 48.8 minutes
232Np 232.04011 14.7 minutes
233Np 233.04074 36.2 minutes
234Np 234.042895 4.4 days
235Np 235.0440633 396.1 days
236Np 236.04657 154 X 103 years
236mNp   22.5 hours
237Np 237.0481734 2.144 x 106 years
238Np 238.0509464 2.117 days
239Np 239.0529390 2.356 days
240Np 240.056162 61.9 minutes
240mNp   7.22 minutes
241Np 241.05825 13.9 minutes
242Np 242.06164 2.2 minutes
242mNp   5.5 minutes
243Np 243.06428 1.85 minutes
244Np 244.06785 2.29 minutes


Neptunium plays no role in living things and has never been encountered outside nuclear facilities or research laboratories.  Most of the neptunium that is retained in the body deposits in the bones. Some is also retained in the liver. Several studies report "relatively high concentrations" of neptunium in adrenal glands of laboratory animals.

No health effects specific to exposure from neptunium "have been observed" in human beings. Roy C. Thompson, Biology Department of Battelle Pacific Northwest Laboratory in Richland, conducted an extensive review of studies involving neptunium. This review included Russian studies that found an increase in the number of bone tumors in animals receiving bone doses as low as a few rad. Thompson concluded that "there can be little doubt" that neptunium can cause cancer in bone.

In 1984, a team of German scientists reported preliminary results of an experiment with mice designed to measure the combined effect of having neptunium-239 deposit in bone and decay into plutonium-239. These initial results found evidence that the buildup of plutonium-239 (as the neptunium decayed) increased the number of bone tumors compared to those expected from exposure to neptunium alone.

atom.gif (700 bytes)

Neptunium Data

Atomic Structure

  • Atomic Radius: 1.31 pm
  • Atomic Volume: 11.62cm3/mol
  • Covalent Radius:
  • Cross Section (Thermal Neutron Capture) Barns: 180
  • Crystal Structure: Orthorhombic
  • Electron Configuration:
    1s2 2s2p6 3s2p6d10 4s2p6d10f14 5s2p6d10f4 6s2p6d1 7s2
  • Electrons per Energy Level: 2, 8, 18, 32, 22, 9, 2
  • Ionic Radius: 0.75
  • Filling Orbital: 5f4
  • Number of Electrons (with no charge): 93
  • Number of Neutrons (most common/stable nuclide): 144
  • Number of Protons: 93
  • Oxidation States: 6, 5, 4, 3
  • Valence Electrons: 5f4 6d1 7s2

Chemical Properties

  • Electrochemical Equivalent: 1.7689 g/amp-hr
  • Electron Work Function:
  • Electronegativity: 1.36 (Pauling); 1.22 (Allrod Rochow)
  • Heat of Fusion: 5.19 kJ/mol
  • Incompatibilities:
  • Ionization Potential
    • First: 6.19
  • Valence Electron Potential (-eV): 96

Physical Properties

  • Atomic Mass Average: 237.0482
  • Boiling Point: 4175K, 3902C, 7056F
  • Coefficient of Lineal Thermal Expansion/K-1: 27.5E-6
  • Conductivity
    Electrical: 0.00822 106/cm
    Thermal: 0.063 W/cmK
  • Density: 20.25  g/cm3 @ 300K
  • Description:
    A radioactive silvery metal that occurs naturally in uranium ores but in extremely minute amounts.
  • Enthalpy of Fusion: 3.2 kJ/mole
  • Flammablity Class:
  • Freezing Point: see melting point
  • Heat of Vaporization: kJ/mol
  • Melting Point: 913K, 640C, 1184F
  • Molar Volume: 11.59 cm3/mole
  • Physical State (at 20C & 1atm): Solid
  • Specific Heat: 0.12 J/gK

Regulatory / Health

  • CAS Number
    • 7439-99-8
  • NFPA 704
    • Health:
    • Fire:
    • Reactivity:
    • Special Hazard: Radioactive<
    • OSHA Permissible Exposure Limit (PEL)
      • No limits set by OSHA
    • OSHA PEL Vacated 1989
      • No limits set by OSHA
    • NIOSH Recommended Exposure Limit (REL)
      • No limits set by NIOSH
    • Levels In Humans:
      Note: this data represents naturally occuring levels of elements in the typical human, it DOES NOT represent recommended daily allowances.
      • Blood/mg dm-3: N/A
      • Bone: N/A
      • Liver: N/A
      • Muscle: N/A
      • Daily Dietary Intake: N/A
      • Total Mass In Avg. 70kg human: N/A

    Who / Where / When / How

    • Discoverer: Edwin M. McMillan, Philip H. Abelson
    • Discovery Location: Berkeley California United States
    • Discovery Year: 1940
    • Name Origin:
      From planet Neptune.
    • Abundance:
      • Earth's Crust: N/A
      • Seawater: N/A
      • Atmosphere: N/A
      • Sun (Relative to H=1E12): N/A
    • Sources:
      Produced by bombarding uranium with slow neutrons. Neptunium is present in extremely small quantitles in Uranium ores and is created when a neutron that is emitted by one uranium atom undergoing fission is captured in the nucleus of another uranium atom. Although it is present in uranium rich ores, it is not extracted from this source.
    • Uses:
      Used in neutron detection instruments.
    • Additional Notes:
      Normally neptunium is never encountered outside of nuclear facilities and nuclear research facilities.


Fusion Heat: 9.6 kJ/mol and/or 5.19kJ/mol
Evaporation Heat: 336 kJ/mol
Lattice Constant: 4.720