1s² 2s²p⁶ 3s²p⁶d¹⁰ 4s²p⁶d¹⁰f¹⁴ 5s²p⁶d¹⁰f⁴ 6s²p⁶d¹ 7s²

History

Neptunium (named for the planet Neptune, the next planet out from Uranus, after which uranium was named) was first discovered by Edwin McMillan and Philip 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.

Edwin McMillan

Characteristics

Neptunium is prepared by the reduction of NpF₃ with barium or lithium vapor at about 1200^oC.  Neptunium metal has a silvery appearance, is chemically reactive, and exists in at least three structural modifications: alpha-neptunium, orthorhombic, density 20.25 g/cm³, beta-neptunium (above 280^oC), tetragonal, density (313^oC) 19.36 g/cm³, gamma-neptunium (577^oC), cubic, density (600^oC) 18.0 g/cm. Neptunium has four ionic oxidation states 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).  These latter oxygenated species are in contrast to the rare earths which exhibit only simple ions of the (II), (III), and (IV) oxidation states in aqueous solution.  The element 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.

Silvery in appearance, neptunium 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:

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 by-product in plutonium production.

Applications

²³⁷Np is irradiated with neutrons to create ²³⁸Pu, a rare and valuable isotope 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.  US officials in March 2004, planned to move the nation's supply of enriched neptunium to a site in Nevada.

Compounds

Neptunium forms tri- and tetrahilides 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 like other actinides 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.

Isotopes

Although the neptunium on which the characterization work was done was synthesized in a cyclotron, we now know that minute amounts of the element exist in the environment (the longest-lived isotope has a half-life of about 2 million years). All isotopes of the metal are radioactive.

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-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 betta emission.  The primary decay products before ²³⁷Np are uranium-92 isotopes (alpha emission produces protactium-91, however) and the primary products after are plutonium-94 isotopes.

²³⁷Np eventually decays to form bismuth, unlike most other common heavy nuclei which decay to make lead.

Nuclear Synthesis

When a 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.

Precautions

Neptunium plays no role in living things and has never been encountered outside nuclear facilities or research laboratories.  Most of the neptunium that is retained in the body deposits in the bones. Some is also retained in the liver. Several studies report "relatively high concentrations" of neptunium in adrenal glands of laboratory animals.

No health effects specific to exposure from neptunium "have been observed" in human beings. Roy C. Thompson, Biology Department of Battelle Pacific Northwest Laboratory in Richland, conducted an extensive review of studies involving neptunium. This review included Russian studies that found an increase in the number of bone tumors in animals receiving bone doses as low as a few rad. Thompson concluded that "there can be little doubt" that neptunium can cause cancer in bone.

In 1984, a team of German scientists reported preliminary results of an experiment with mice designed to measure the combined effect of having neptunium-239 deposit in bone and decay into plutonium-239. These initial results found evidence that the buildup of plutonium-239 (as the neptunium decayed) increased the number of bone tumors compared to those expected from exposure to neptunium alone.