Neptunium

Neptunium is a synthetic element in the periodic table that has the symbol Np and atomic number 93. A silvery radioactive metallic element, neptunium is the first transuranic element and belongs to the actinide series. Its most stable isotope, neptunium-237 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.

Notable characteristics

Silvery in appearance, neptunium metal is fairly chemically reactive and is found in at least three structural modifications:

• alpha-neptunium, orthorhombic, density 20,250 kg/m³,
• beta-neptunium (above 280oC), tetragonal, density (313 ^oC) 19,360 kg/m³, and
• gamma-neptunium (above 577^oC), cubic, density (600^oC) 18,000 kg/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, NpC_l4, NpBr₃, NpI₃, and oxides of the various compositions such as are found in the uranium-oxygen system, including Np₃O₈ and NpO₂.

History

Neptunium (named for the planet Neptune) was first discovered by Edwin McMillan and Philip Abelson in 1940. The discovery was made at the Berkeley Radiation Laboratory of the University of California, Berkeley where the team produced the neptunium isotope Np-239 (2.4 day half-life) by bombarding uranium with cyclotron-accelerated neutrons. It was the first transuranium element produced synthetically and the first actinide series transuranium element discovered.

Occurrence

Trace amounts of neptunium are found naturally as decay products from transmutation reactions in uranium ores. Np-237 is produced through the reduction of NpF₃ 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.

Isotopes

19 neptunium radioisotopes have been characterized, with the most stable being Np-237 with a half-life of 2.14 million years, Np-236 with a half-life of 154,000 years, and Np-235 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 Np-236m (t_½ 22.5 hours).

The isotopes of neptunium range in atomic weight from 225.0339 u (Np-225) to 244.068 u (Np-244). The primary decay mode before the most stable isotope, Np-237, is electron capture (with a good deal of alpha emission), and the primary mode after is beta emission. The primary decay products before Np-237 are element 92 (uranium) isotopes (alpha emission produces element 91, protactinium, however) and the primary products after are element 93 (plutonium) isotopes.

Weapons applications

In September, 2002, researchers for the University of California conducting research for a United States of America weapons of mass destruction development program created the first nuclear critical nuclear mass using neptunium rather than plutonium or uranium. US officials in March, 2004, planned to move the nation's supply of enriched neptunium to a site in Nevada.

References

• Los Alamos National Laboratory's Chemistry Division: Periodic Table - Neptunium
• Guide to the Elements - Revised Edition, Albert Stwertka, (Oxford University Press; 1998) ISBN 0-19-508083-1

External links

• WebElements.com - Neptunium (also used as a reference)
• EnvironmentalChemistry.com - Neptunium (also used as a reference)