uranium ← neptunium → plutonium Pm
Appearance silvery metallic
General properties Name, symbol, number neptunium, Np, 93 Pronunciation //
Element category actinide Group, period, block n/a, 7, f Standard atomic weight (237) Electron configuration [Rn] 7s2 6d1 5f4 Electrons per shell 2, 8, 18, 32, 22, 9, 2 (Image) Physical properties Phase solid Density (near r.t.) 20.45  g·cm−3 Melting point 910 K, 637 °C, 1179 °F Boiling point 4273 K, 4000 °C, 7232 °F Heat of fusion 3.20 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
Electronegativity 1.36 (Pauling scale) Ionization energies 1st: 604.5 kJ·mol−1 Atomic radius 155 pm Covalent radius 190±1 pm Miscellanea Crystal structure note 3 forms: orthorhombic,
tetragonal and cubic
Magnetic ordering paramagnetic Electrical resistivity (22 °C) 1.220 µΩ·m Thermal conductivity 6.3 W·m−1·K−1 CAS registry number 7439-99-8 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 syn 2.144×106 y SF & α 4.959 233Pa 239Np trace 2.356 d β− 0.218 239Pu nep-tew-nee-əm) is a chemical element with the symbol Np and atomic number 93. A radioactive metal, neptunium is the first transuranic element and belongs to the actinide series. Its most stable isotope, 237Np, is a by-product of nuclear reactors and plutonium production and it can be used as a component in neutron detection equipment. Neptunium is also found in trace amounts in uranium ores due to transmutation reactions.
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 publication of the known radioactive isotopes by Kasimir Fajans shows the empty place after uranium.
At least three times, discoveries of the element 93 were falsely reported, as bohemium and ausonium in 1934 and then sequanium in 1939.
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 actinides 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 later neptunium was found in uranium ore in 1952.
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, though this was soon found to be mistaken. Soon it was speculated and later proven 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.
Neptunium (named for the planet Neptune, the next planet out from Uranus, after which uranium was named) was 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.
Trace amounts of neptunium are found naturally as decay products from transmutation reactions in uranium ores. Artificial 237Np is produced through the reduction of 237NpF3 with barium or lithium vapor at around 1200 °C and is most often extracted from spent nuclear fuel rods as a by-product in plutonium production.
- 2 NpF3 + 3 Ba → 2 Np + 3 BaF2
- α-neptunium, orthorhombic, density 20.45 g/cm3
- β-neptunium (above 280 °C), tetragonal, density (313 °C) 19.36 g/cm3
- γ-neptunium (above 577 °C), cubic, density (600 °C) 18 g/cm3
Neptunium has the largest liquid range of any element, 3363 K, between the melting point and boiling point. It is the densest element of all actinoids.
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.
Chemically, neptunium is prepared by the reduction of NpF3 with barium or lithium vapor at about 1200 °C. 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.
- 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.
This element has four ionic oxidation states while in solution:
- Np3+ (pale purple), analogous to the rare earth ion Pm3+
- Np4+ (yellow green)
- NpO2+ (green blue)
- NpO22+ (pale pink)
Neptunium(VI) fluoride, 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.
Precursor in plutonium-238 production
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.
Neptunium is fissionable, and could theoretically be used as fuel in a fast neutron reactor or a nuclear weapon. In 1992, the U.S. Department of Energy declassified the statement that neptunium-237 "can be used for a nuclear explosive device". 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 University of California's 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," showing that it "is about as good a bomb material as U-235." 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.
237Np is used in devices for detecting high-energy (MeV) neutrons.
Role in nuclear waste
Neptunium-237 is the most mobile actinide in the deep geological repository environment. This makes it and its predecessors such as americium-241 candidates of interest for destruction by nuclear transmutation. 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.
- ^ a b Criticality of a 237Np Sphere
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- WebElements.com – Neptunium (also used as a reference)
- Lab builds world's first neptunium sphere, U.S. Department of Energy Research News
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- C&EN: It's Elemental: The Periodic Table – Neptunium
Periodic table H He Li Be B C N O F Ne Na Mg Al Si P S Cl Ar K Ca Sc Ti V Cr Mn Fe Co Ni Cu Zn Ga Ge As Se Br Kr Rb Sr Y Zr Nb Mo Tc Ru Rh Pd Ag Cd In Sn Sb Te I Xe Cs Ba La Ce Pr Nd Pm Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Hf Ta W Re Os Ir Pt Au Hg Tl Pb Bi Po At Rn Fr Ra Ac Th Pa U Np Pu Am Cm Bk Cf Es Fm Md No Lr Rf Db Sg Bh Hs Mt Ds Rg Cn Uut Uuq Uup Uuh Uus Uuo Alkali metals Alkaline earth metals Lanthanides Actinides Transition metals Other metals Metalloids Other nonmetals Halogens Noble gases Unknown chem. properties Large version Neptunium compounds Chemical elements named after places Named after terrestrial places Named after astronomical objects
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Neptunium — Uranium ← Neptunium → Plutonium Pm … Wikipédia en Français
NEPTUNIUM — Élément chimique de numéro atomique 93, le neptunium (symbole Np) est le premier membre de la série d’éléments lourds artificiels: les transuraniens. Comme tous ces éléments, il a de nombreux isotopes qui sont tous radio actifs et produits… … Encyclopédie Universelle
Neptunium — Nep*tu ni*um, n. [NL.] A radioactive metallic element of atomic number 93, produced in nuclear reactors from Plutonium or Uranium. Symbol Np; The atomic weight of the most stable isotope is 237.0482. [PJC] … The Collaborative International Dictionary of English
NEPTUNIUM — alias Navale Antiatum, oppid. ditionis Pontificiae cum portu tantisper capaci, in Latio, in ora maris Tyrrheni alias sub dominio utili Columnarum, medium inter Terracinam ad Ortum et ostia Tiberina ad Occasum, 25. milliar. utrinque, versus… … Hofmann J. Lexicon universale
neptunium — Symbol: Np Atomic number: 93 Atomic weight: (237) Radioactive metallic transuranic element, belongs to the actinoids. Np 237, the most stable isotope, has a half life of 2.2*10^6 years and is a by product of nuclear reactors. The other known… … Elements of periodic system
neptunium — (n.) 1941, from NEPTUNE (Cf. Neptune) + element ending ium. Named for its relative position in the periodic table, next after URANIUM (Cf. Uranium), as the planet Neptune is one beyond Uranus. Cf. also PLUTONIUM (Cf. plutonium) … Etymology dictionary
neptunium — ☆ neptunium [nep to͞o′nē əm, neptyo͞o′nē əm ] n. [ModL < Neptunus, NEPTUNE + IUM: so named by MCMILLAN Edwin MattisonEdwin MattisonEdwin MattisonEdwin MattisonEdwin MattisonEdwin Mattison & P. Abelson (1913 2004), U.S. phys i cists, because… … English World dictionary
Neptunium — Eigenschaften … Deutsch Wikipedia
neptunium — /nep tooh nee euhm, tyooh /, n. Chem., Physics. a transuranic element produced in nuclear reactors by the neutron bombardment of U 238: decays rapidly to plutonium and then to U 235. Symbol: Np; at. no.: 93. [1940 45; NEPTUNE + IUM] * * * ▪… … Universalium
Neptunium — Nep|tu|ni|um 〈n.; s; unz.; chem. 〉 künstliches radioaktives Element, Ordnungszahl 93 [neulat., nach dem Planeten Neptun] * * * Nep|tu|ni|um [nach dem Planeten Neptun, der nach dem Uranus (↑ Uran) entdeckt wurde; ↑ ium (1)], das; s; Symbol: Np: in … Universal-Lexikon