|Name, Symbol, Number||neptunium, Np, 93|
|Group, Period, Block||n/a, 7, f|
|Atomic mass||(237) g/mol|
|Electron configuration||[Rn] 5f4 6d1 7s2|
|Electrons per shell||2, 8, 18, 32, 22, 9, 2|
|Density (near r.t.)||20.2 g/cm³|
|Melting point||910 K
(637 °C, 1179 °F)
|Boiling point||4273 K
(4000 °C, 7232 °F)
|Heat of fusion||3.20 kJ/mol|
|Heat of vaporization||336 kJ/mol|
|Heat capacity||(25 °C) 29.46 J/(mol·K)|
|Crystal structure||3 forms: orthorhombic,
tetragonal and cubic
|Oxidation states||6, 5, 4, 3
|Electronegativity||1.36 (Pauling scale)|
|Ionization energies||1st: 604.5 kJ/mol|
|Atomic radius||175 pm|
|Electrical resistivity||(22 °C) 1.220 µΩ·m|
|Thermal conductivity||(300 K) 6.3 W/(m·K)|
|CAS registry number||7439-99-8|
Neptunium (chemical symbol Np, atomic number 93) is a silvery radioactive metallic element, belonging to the actinide series. It is the first transuranic element and the first member of this group to be produced synthetically. Its most stable isotope, 237Np, is a byproduct of nuclear reactors and plutonium production. It is also found in trace amounts in uranium ores. It can be used as a component in neutron detection equipment, and could potentially be used as fuel for a nuclear reactor or to create a nuclear weapon.
Trace amounts of neptunium are found naturally as decay products from transmutation reactions in uranium ores. The isotope 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 byproduct in plutonium production.
Neptunium was named for the planet Neptune, the next planet out from Uranus, after which uranium was named. It was first discovered by Edwin McMillan and Philip H. 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. The research 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.
When an atom of 235U captures a neutron, it is converted to an excited state of 236U. About 81 percent 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 seven 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.
Neptunium is an inner transition metal of the actinide series, located in period 7 of the periodic table, between uranium and plutonium. Silvery in appearance, this metal is fairly chemically reactive and is found in at least three structural modifications:
This element has four ionic oxidation states while in solution:
Many neptunium radioisotopes have been characterized. The most stable of these are 237Np, with a half-life (t½) 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 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 four meta states, with the most stable being 236mNp (t½ 22.5 hours).
The isotopes of neptunium range in atomic weight from 225.0339 atomic mass units (amu) (225Np) to 244.068 amu (244Np). The isotope 237Np eventually decays to form bismuth, unlike most other common heavy nuclei, which decay to produce lead.
Neptunium forms tri- and tetrahalides such as NpF3, NpF4, NpCl4, NpBr3, and NpI3. It also forms oxides of various compositions, such as are found in the uranium-oxygen system, including Np3O8 and NpO2.
Like other actinides, neptunium readily forms a dioxide neptunyl core (NpO2). In the environment, this neptunyl core readily complexes with carbonate as well as other oxygen-containing ionic groups, such as OH-, NO2-, NO3-, and SO42-). In so doing, it forms charged complexes that tend to be readily mobile with low affinities to the soil. Examples of these complexes are:
The isotope 237Np can be irradiated with neutrons to create 238Pu, a rare plutonium isotope useful 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. In March 2004, U.S. officials planned to move the nation's supply of enriched neptunium to a site in Nevada.
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