Neptunium
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General | ||||||||||||||||||||||||||||||||||
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Name, Symbol, Number | neptunium, Np, 93 | |||||||||||||||||||||||||||||||||
Chemical series | actinides | |||||||||||||||||||||||||||||||||
Group, Period, Block | n/a, 7, f | |||||||||||||||||||||||||||||||||
Appearance | silvery metallic | |||||||||||||||||||||||||||||||||
Atomic mass | (237) g/mol | |||||||||||||||||||||||||||||||||
Electron configuration | [Rn] 5f4 6d1 7s2 | |||||||||||||||||||||||||||||||||
Electrons per shell | 2, 8, 18, 32, 22, 9, 2 | |||||||||||||||||||||||||||||||||
Physical properties | ||||||||||||||||||||||||||||||||||
Phase | solid | |||||||||||||||||||||||||||||||||
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) | |||||||||||||||||||||||||||||||||
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Atomic properties | ||||||||||||||||||||||||||||||||||
Crystal structure | 3 forms: orthorhombic, tetragonal and cubic | |||||||||||||||||||||||||||||||||
Oxidation states | 6, 5, 4, 3 (amphoteric oxide) | |||||||||||||||||||||||||||||||||
Electronegativity | 1.36 (Pauling scale) | |||||||||||||||||||||||||||||||||
Ionization energies | 1st: 604.5 kJ/mol | |||||||||||||||||||||||||||||||||
Atomic radius | 175 pm | |||||||||||||||||||||||||||||||||
Miscellaneous | ||||||||||||||||||||||||||||||||||
Magnetic ordering | ? | |||||||||||||||||||||||||||||||||
Electrical resistivity | (22 °C) 1.220 µΩ·m | |||||||||||||||||||||||||||||||||
Thermal conductivity | (300 K) 6.3 W/(m·K) | |||||||||||||||||||||||||||||||||
CAS registry number | 7439-99-8 | |||||||||||||||||||||||||||||||||
Notable isotopes | ||||||||||||||||||||||||||||||||||
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Neptunium (chemical symbol Np, atomic number 93) is a silvery radioactive metallic element. It is the first transuranic element and belongs to the actinide series. Its most stable isotope, 237Np, is a byproduct 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.
Occurrence
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 1200 ° C and is most often extracted from spent nuclear fuel rods as a byproduct in plutonium production.
Etymology and history
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 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.
Nuclear synthesis
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. 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.
Notable characteristics
Neptunium is an inner transition metal (or actinide) that lies 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:
- alpha-neptunium, orthorhombic, density 20.25 Mg/m3,
- beta-neptunium (above 280 °C), tetragonal, density (313 °C) 19.36 Mg/m3, and
- gamma-neptunium (above 577 °C), cubic, density (600 °C) 18 Mg/m3
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).
Neptunium forms tri- and tetrahalides such as NpF3, NpF4, NpCl4, NpBr3, 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 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
Isotopes
Masny 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 (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 eventually decays to form bismuth, unlike most other common heavy nuclei which decay to make lead.
Uses
Precursor in plutonium-238 production
The isotope 237Np can be irradiated with neutrons to create 238Pu, a rare plutonium isotope useful for spacecraft and military applications.
Weapons 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".[1] 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[2]. In March 2004, U.S. officials planned to move the nation's supply of enriched neptunium to a site in Nevada.
See also
Notes
- ↑ "Restricted Data Declassification Decisions from 1946 until Present", accessed Sept 23, 2006
- ↑ Weiss, P. (October 26, 2002). Little-studied metal goes critical - Neptunium Nukes?. Science News. Retrieved 2006-09-29.
ReferencesISBN links support NWE through referral fees
- Emsley, John. 2001. Nature's Building Blocks: An A–Z Guide to the Elements. Oxford: Oxford Univ. Press. ISBN 0198503407 and ISBN 978-0198503408.
- Greenwood, N.N., and A. Earnshaw. 1998. Chemistry of the Elements 2nd ed. Oxford, UK; Burlington, MA: Butterworth-Heinemann. ISBN 0750633654. Online version.
- Hampel, Clifford A. 1968. The Encyclopedia of the Chemical Elements. New York: Reinhold Book Corp. ISBN 0442155980 and ISBN 978-0442155988.
- Morss, Lester R., Norman M. Edelstein, and Jean Fuger, eds. 2006. The Chemistry of the Actinide and Transactinide Elements. 3rd ed. 5 vols. Joseph J. Katz, adapter. Dordrecht: Springer. ISBN 1402035551 and ISBN 978-1402035555.
- Stwertka, Albert. 1998. Guide to the Elements. Rev. ed. Oxford: Oxford University Press. ISBN 0-19-508083-1.
- "Neptunium" Los Alamos National Laboratory, Chemistry Division. Retrieved March 14, 2007.
External links
- "Neptunium" WebElements.com. Retrieved March 14, 2007.
- Lab builds world's first neptunium sphere, U.S. Department of Energy Research News
- NLM Hazardous Substances Databank – Neptunium, Radioactive
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