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Neptunium in the periodic table
silvery metallic
General properties
Name, symbol, number neptunium, Np, 93
Pronunciation UK /nɛpˈtjniəm/
US /nɛpˈtniəm/
Element category actinide
Group, period, block n/a, 7, f
Standard atomic weight (237)
Electron configuration [Rn] 5f4 6d1 7s2
2, 8, 18, 32, 22, 9, 2
Physical properties
Phase solid
Density (near r.t.) (alpha) 20.45[1] g·cm−3
Density (near r.t.) (accepted standard value) 19.38 g·cm−3
Melting point 912 K, 639 °C, 1182 °F
Boiling point 4447 K, 4174 °C, 7545 (extrapolated) °F
Heat of fusion 5.19 kJ·mol−1
Heat of vaporization 336 kJ·mol−1
Molar heat capacity 29.46 J·mol−1·K−1
Vapor pressure
P (Pa) 1 10 100 1 k 10 k 100 k
at T (K) 2194 2437        
Atomic properties
Oxidation states 7, 6, 5, 4, 3
(amphoteric oxide)
Electronegativity 1.36 (Pauling scale)
Ionization energies 1st: 604.5 kJ·mol−1
Atomic radius 155 pm
Covalent radius 190±1 pm
Crystal structure orthorhombic
Neptunium has a orthorhombic crystal structure
Magnetic ordering paramagnetic[2]
Electrical resistivity (22 °C) 1.220 µΩ·m
Thermal conductivity 6.3 W·m−1·K−1
CAS registry number 7439-99-8
Naming after planet Neptune, itself named after Roman god of the sea Neptune
Discovery Edwin McMillan and Philip H. Abelson (1940)
Most stable isotopes
Main article: Isotopes of neptunium
iso NA half-life DM DE (MeV) DP
235Np syn 396.1 d α 5.192 231Pa
ε 0.124 235U
236Np syn 1.54×105 y ε 0.940 236U
β 0.940 236Pu
α 5.020 232Pa
237Np trace 2.144×106 y α 4.959 233Pa
239Np trace 2.356 d β 0.218 239Pu
· references

Neptunium is a chemical element with the 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 its being named after Neptune, the next planet beyond Uranus. A neptunium atom has 93 protons and 93 electrons, of which seven are valence electrons. Neptunium metal is silvery and tarnishes when exposed to air. It occurs in three allotropic forms. The element normally exhibits five oxidation states, ranging from +3 to +7.

In the 1870s, Dmitri Mendeleev first predicted the existence of neptunium, and many false claims of its discovery were made over the years. The element was first synthesized by Edwin McMillan and Philip H. Abelson at the Berkeley Radiation Laboratory in 1940. Most neptunium is produced by bombarding uranium with neutrons in nuclear reactors; neptunium is also generated as a by-product in conventional nuclear reactors. Though neptunium has no commercial uses at present, it is widely used as a precursor for the formation of plutonium-238, used in radioisotope thermal generators which are used to power some spacecraft. Neptunium itself can be used in detectors of high-energy neutrons.

The most stable isotope of neptunium, neptunium-237, is a by-product of nuclear reactors and plutonium production, and it can be used as a component in neutron detection equipment. The isotopes neptunium-237 and neptunium-239 are also found in trace amounts in uranium ores due to neutron capture reactions and beta decay.[3]



Even before its discovery, the existence of neptunium had been predicted several times. The periodic table of Dmitri Mendeleev published in the 1870s showed a " — " in place after uranium similar to several other places for at that point undiscovered elements. Also, a 1913 publication of the known radioactive isotopes by Kasimir Fajans shows the empty place after uranium.[4]

The search for element 93 in minerals was encumbered by the fact that the predictions on the chemical properties of element 93 were based on a periodic table which lacked the actinide series, and therefore placed thorium below hafnium, protactinium below tantalum, and uranium below tungsten. This periodic table suggested that element 93, at that point often named eka-rhenium, should be similar to manganese or rhenium. With this misconception it was impossible to isolate element 93 from minerals, although neptunium was later found in uranium ore, in 1952.[5]

In 1934, Odolen Koblic extracted a small amount of material from the wash water of roasted pitchblende. He assumed the sample was element 93, and called it bohemium, but after being analyzed, it turned out that the sample was a mixture of tungsten and vanadium.[6]

In 1938, Horia Hulubei, a Romanian physicist; and Yvette Cauchois, a French chemist; claimed to have discovered element 93 via spectroscopy in minerals. They named their element sequanium, but the claim was opposed at the time because neptunium was thought to occur exclusively artificially. As neptunium does occur in nature, it is possible that Hulubei and Cauchois did discover neptunium.[6]

Enrico Fermi believed that bombarding uranium with neutrons and subsequent beta decay would lead to the formation of element 93. Chemical separation of the new formed elements from the uranium yielded material with low half-life, and, therefore, Fermi announced the discovery of a new element in 1934,[7] though this was soon found to be mistaken. Soon it was speculated[8] and later proven[9] that most of the material is created by nuclear fission of uranium by neutrons. Small quantities of neptunium had to be produced in Otto Hahn's experiments in late 1930s as a result of decay of 239U. Hahn and his colleagues experimentally confirmed production and chemical properties of 239U, but were unsuccessful at isolating and detecting neptunium.[10]


In 1939, Edwin McMillan did some preliminary work leading to the eventual discovery of element 93. Uranium trioxide was placed on paper together with aluminium or paper foils and the result was irradiated with neutrons from a cyclotron. Examination revealed the presence of two components, both beta decaying: one turned out to be uranium-239 (half-life 23 minutes), while the other could not be characterized firmly. Emilio Segrè contradictorily labelled the second component as having atomic number 93 but not being a transuranic element, also noting that it behaved similarly to the rare earth elements. This lack of confidence in labelling the product as transuranic was from the absence of any observed activity from the beta decay product of element 93.[11]

Neptunium (named for the planet Neptune, the next planet out from Uranus, after which uranium was named) was finally convincingly discovered by Edwin McMillan and Philip H. Abelson in 1940 at the Berkeley Radiation Laboratory of the University of California, Berkeley. 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.[12]

\mathrm{^{238}_{\ 92}U\ +\ ^{1}_{0}n\ \longrightarrow \ ^{239}_{\ 92}U\ \xrightarrow[23 \ min]{\beta^-} \ ^{239}_{\ 93}Np\ \xrightarrow[2.355 \ d]{\beta^-} \ ^{239}_{\ 94}Pu}


The most stable isotope of neptunium is 237Np, with a half-life of two million years. Thus, all primordial neptunium should have decayed by now. Trace amounts of the neptunium isotopes neptunium-237 through neptunium-240, are found naturally as decay products from transmutation reactions in uranium ores.[3][6]

Artificial 237Np is produced through a reaction of 237NpF3 with liquid barium or lithium at around 1200 °C and is most often extracted from spent nuclear fuel rods in kilogram amounts as a by-product in plutonium production.[6]

2 NpF
+ 3 Ba → 2 Np + 3 BaF

By weight, neptunium-237 discharges are about 5% as great as plutonium discharges and about 0.05% of spent nuclear fuel discharges.[13] This amounts to more than fifty tons per year.[14]


Silvery in appearance, neptunium metal is chemically fairly reactive and is found in at least three allotropes:[3]

  • α-neptunium, orthorhombic, density 20.45 g/cm3[15]
  • β-neptunium (above 280 °C), tetragonal, density (313 °C) 19.36 g/cm3[15]
  • γ-neptunium (above 577 °C), cubic, density (600 °C) 18 g/cm3[15]

Neptunium has the largest liquid range of any element, 3363 K, between the melting point and boiling point. It is the densest of all the actinides and the fifth-densest of all naturally occurring elements.[16] Neptunium has no biological role. It is not absorbed by the digestive tract. When injected into the body, it accumulates in bones, from which it is slowly released.


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-lives that are less than 4.5 days, and the majority of these have half-lives that are less than 50 minutes. This element also has 4 meta states, with the most stable being 236mNp (t½ 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 beta emission. The primary decay products before 237Np are element 92 (uranium) isotopes (alpha emission produces element 91, protactinium, however) and the primary products after are element 94 (plutonium) isotopes.

237Np is fissionable.[17] 237Np 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.


Chemically, neptunium is prepared by the reduction of NpF3 with barium or lithium vapor at about 1200 °C.[3] Most Np is produced in nuclear reactions:

  • When an 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 through beta decay. During beta decay, the excited 237U emits an electron, while the atomic weak interaction converts a neutron to a proton, thus creating 237Np.
\mathrm{^{235}_{\ 92}U\ +\ ^{1}_{0}n\ \longrightarrow \ ^{236}_{\ 92}U_m\ \xrightarrow[120 \ ns]{} \ ^{236}_{\ 92}U\ +\ \gamma}
\mathrm{^{236}_{\ 92}U\ +\ ^{1}_{0}n\ \longrightarrow \ ^{237}_{\ 92}U\ \xrightarrow[6.75 \ d]{\beta^-} \ ^{237}_{\ 93}Np}
  • 237U is also produced via an (n,2n) reaction with 238U. This only happens with very energetic neutrons.
  • 237Np is the product of alpha decay of 241Am.

Heavier isotopes of neptunium decay quickly, and lighter isotopes of neptunium cannot be produced by neutron capture, so chemical separation of neptunium from cooled spent nuclear fuel gives nearly pure 237Np.


Neptunium ions in solution.

This element has five ionic oxidation states while in solution:

  • Np3+ (pale purple), analogous to the rare earth ion Pm3+
  • Np4+ (yellow-green)
  • NpO+
  • NpO2+
    (pale pink)
  • NpO3−
    (green). This may better be labelled as a hydroxo species [NpO

Neptunium(III) hydroxide is not soluble in water and does not dissolve in excess alkali. Neptunium(III) is susceptible to oxidation in contact to air forming neptunium(IV).[18][19]

Neptunium 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.

Neptunium hexafluoride, NpF6, is volatile like uranium hexafluoride.

Neptunium, like protactinium, uranium, plutonium, and americium readily forms a linear dioxo neptunyl core (NpO2n+), in its 5+ and 6+ oxidation states, which readily complexes with hard O-donor ligands such as OH, NO2, NO3, and SO42– to form soluble anionic complexes which tend to be readily mobile with low affinities to soil. Neptunium is very reactive when in contact with oxygen, steam, or acid. However it is not attacked by alkalis.[6]

  • NpO2(OH)2
  • NpO2(CO3)
  • NpO2(CO3)23–
  • NpO2(CO3)35–


Precursor in plutonium-238 production[edit]

237Np is irradiated with neutrons to create 238Pu, an alpha emitter for radioisotope thermal generators for spacecraft and military applications. 237Np will capture a neutron to form 238Np and beta decay with a half-life of two days to 238Pu.[20]

\mathrm{^{237}_{\ 93}Np\ +\ ^{1}_{0}n\ \longrightarrow \ ^{238}_{\ 93}Np\ \xrightarrow[2.117 \ d]{\beta^-} \ ^{238}_{\ 94}Pu}

238Pu also exists in sizable quantities in spent nuclear fuel but would have to be separated from other isotopes of plutonium.

Weapons applications[edit]

Neptunium is fissionable, and could theoretically be used as fuel in a fast neutron reactor or a nuclear weapon, with a critical mass of around 60 kilograms.[14] In 1992, the U.S. Department of Energy declassified the statement that neptunium-237 "can be used for a nuclear explosive device".[21] It is not believed that an actual weapon has ever been constructed using neptunium. As of 2009, the world production of neptunium-237 by commercial power reactors was over 1000 critical masses a year, but to extract the isotope from irradiated fuel elements would be a major industrial undertaking.

In September 2002, researchers at the Los Alamos National Laboratory briefly created the first known nuclear critical mass using neptunium in combination with shells of enriched uranium (U-235), discovering that the critical mass of a bare sphere of neptunium-237 "ranges from kilogram weights in the high fifties to low sixties,"[1] showing that it "is about as good a bomb material as U-235."[17] The United States Federal government made plans in March 2004 to move America's supply of separated neptunium to a nuclear-waste disposal site in Nevada.

Physics applications[edit]

237Np is used in devices for detecting high-energy (MeV) neutrons.[22]

Role in nuclear waste[edit]

Neptunium-237 is the most mobile actinide in the deep geological repository environment.[23] This makes it and its predecessors such as americium-241 candidates of interest for destruction by nuclear transmutation.[24] Neptunium accumulates in commercial household ionization-chamber smoke detectors from decay of the (typically) 0.2 microgram of americium-241 initially present as a source of ionizing radiation. With a half-life of 432 years, the americium-241 in a smoke detector includes about 3% neptunium after 20 years, and about 15% after 100 years.

Due to its long half-life, neptunium becomes the major contributor of the total radiation in 10,000 years. As it is unclear what happens to the containment in that long time span, an extraction of the neptunium would minimize the contamination of the environment if the nuclear waste could be mobilized after several thousand years.[25][26]

Biological role and precautions[edit]

Neptunium does not have any biological role. It is absorbed via the digestive tract. When injected it concentrates in the bones, from which it is slowly released.[6]


  1. ^ a b Criticality of a 237Np Sphere
  2. ^ Magnetic susceptibility of the elements and inorganic compounds, in Handbook of Chemistry and Physics 81st edition, CRC press.
  3. ^ a b c d C. R. Hammond (2004). The Elements, in Handbook of Chemistry and Physics 81st edition. CRC press. ISBN 0-8493-0485-7. 
  4. ^ Fajans, Kasimir (1913). "Die radioaktiven Umwandlungen und das periodische System der Elemente". Berichte der deutschen chemischen Gesellschaft 46: 422. doi:10.1002/cber.19130460162. 
  5. ^ Peppard, D. F.; Mason, G. W.; Gray, P. R.; Mech, J. F. (1952). Journal of the American Chemical Society 74 (23): 6081. doi:10.1021/ja01143a074. 
  6. ^ a b c d e f Emsley, John (2001). Nature's Building Blocks: An A-Z Guide to the Elements (1st ed.). New York, NY: Oxford University Press. pp. 345–7. ISBN 978-019-850340-8. 
  7. ^ Fermi, E. (1934). "Possible Production of Elements of Atomic Number Higher than 92". Nature 133 (3372): 898. Bibcode:1934Natur.133..898F. doi:10.1038/133898a0. 
  8. ^ Ida Noddack (1934). "Über das Element 93". Zeitschrift für Angewandte Chemie 47 (37): 653. doi:10.1002/ange.19340473707. 
  9. ^ Meitner, Lise; Frisch, O. R. (1939). "Disintegration of Uranium by Neutrons: a New Type of Nuclear Reaction". Nature 143 (3615): 239. Bibcode:1939Natur.143..239M. doi:10.1038/143239a0. 
  10. ^ Otto Hahn (1958). "Discovery of fission". Scientific American. 
  11. ^ Yoshida et al., pp. 699–700
  12. ^ Mcmillan, Edwin; Abelson, Philip (1940). "Radioactive Element 93". Physical Review 57 (12): 1185. Bibcode:1940PhRv...57.1185M. doi:10.1103/PhysRev.57.1185.2. 
  13. ^ "Separated Neptunium 237 and Americium" (PDF). Retrieved 2009-06-06. 
  14. ^ a b
  15. ^ a b c Lee, J; Mardon, P; Pearce, J; Hall, R (1959). "Some physical properties of neptunium metal II: A study of the allotropic transformations in neptunium". Journal of Physics and Chemistry of Solids 11 (3–4): 177. Bibcode:1959JPCS...11..177L. doi:10.1016/0022-3697(59)90211-2. 
  16. ^ Theodore Gray. The Elements. Page 215
  17. ^ a b Weiss, P. (October 26, 2002). "Little-studied metal goes critical – Neptunium Nukes?". Science News. Retrieved 2006-09-29. 
  18. ^ Burney, G. A; Harbour, R. M; Subcommittee On Radiochemistry, National Research Council (U.S.); Technical Information Center, U.S. Atomic Energy Commission (1974). Radiochemistry of neptunium. 
  19. ^ Nilsson, Karen (1989). The migration chemistry of neptunium. ISBN 978-87-550-1535-7. 
  20. ^ Lange, R; Carroll, W (2008). "Review of recent advances of radioisotope power systems". Energy Conversion and Management 49 (3): 393–401. doi:10.1016/j.enconman.2007.10.028. 
  21. ^ "Restricted Data Declassification Decisions from 1946 until Present", accessed Sept 23, 2006
  22. ^ Dorin N Poenaru, Walter Greiner (1997). Experimental techniques in nuclear physics. Walter de Gruyter. p. 236. ISBN 3-11-014467-0. 
  23. ^ "Yucca Mountain". Retrieved 2009-06-06. 
  24. ^ Rodriguez, C; Baxter, A.; McEachern, D.; Fikani, M.; Venneri, F. (2003). "Deep-Burn: making nuclear waste transmutation practical". Nuclear Engineering and Design 222 (2–3): 299. doi:10.1016/S0029-5493(03)00034-7. 
  25. ^ Yarris, Lynn (2005-11-29). "Getting the Neptunium out of Nuclear Waste". Berkeley laboratory, U.S. Department of Energy. Retrieved 05-12-2008. 
  26. ^ J. I. Friese; E. C. Buck; B. K. McNamara; B. D. Hanson; S. C. Marschman (January 6, 2003). "Existing Evidence for the Fate of Neptunium in the Yucca Mountain Repository". Pacific northwest national laboratory, U.S. Department of Energy. Retrieved 05-12-2008. 


  • Yoshida, Zenko; Johnson, Stephen G.; Kimura, Takaumi; Krsul, John R. (2006). "Neptunium". In Morss, Lester R.; Edelstein, Norman M.; Fuger, Jean. The Chemistry of the Actinide and Transactinide Elements 3 (3rd ed.). Dordrecht, the Netherlands: Springer. pp. 699–812. doi:10.1007/1-4020-3598-5_6. 


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