Neptunium

93 uranium ← neptunium → plutonium Pm ↑ Np ↓ (Uqt) periodic table
General
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)
Vapor pressure P/Pa 1 10 100 1 k 10 k 100 k at T/K 2194 2437
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
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 154×103 y ε 0.940 236U β- 0.940 236Pu α 5.020 232Pa 237Np syn 2.144×106 y SF & α 4.959 233Pa

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 (t_½ 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". 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.

See also

• Chemical element
• Inner transition metal
• Metal
• Periodic table
• Radioactive decay
• Uranium

References

ISBN 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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