BOILING POINT: 3,930°C
DENSITY : 20.45 g/cm 3
MOST COMMON IONS : Np 3+ , Np 4+ , or NpO 2 + , NpO 2 2+ , NpO 3 +
Neptunium was discovered by the U.S. physicists Edwin M. McMillan and Philip Abelson, in 1940, via the bombardment of 238 U with neutrons. The name of the element is related to the planet Neptune. Neptunium-237 occurs as a product of 238 U fission , and appears in uranium fuel elements.
Neptunium is used to produce plutonium ( 238 Pu), via the irradiation of NpO 2 with neutrons. The isotope 238 94 Pu is used as a power source for satellites.
Neptunium has several valence states: Np 3+ (purplish in solution), stable in water, is easily oxidized in air to Np 4+ (yellow-green), and then slowly oxidized in air to the stable ion, NpO 2 2+ (pink); NpO 2 + (green) is obtained by the oxidation of Np 4+ with hot nitric acid. NpO 2 2+ can also be obtained by the oxidation of lower state ions with Ce 4+ , MnO 4− , O 3 , and BrO 3 − . The production of 237 Np (as NpO 2 + ) involves the oxidation of Np 3+ by nitric acid, followed by the extraction of the NpO 2 + compound with tributyl-phosphate in kerosene.
The oxide NpO 2 , isostructural with UO 2 , is obtained by heating neptunium nitrates or hydroxides in air. The oxide Np 3 O 8 , isomorphous with U 3 O 8 , is also obtained by heating such compounds in air at specific temperatures. The oxidation of Np 4+ hydroxide compounds with ozone gives the hydrated trioxides NpO 3 · 2H 2 O (brown) and NpO 3 · H 2 O (red-gold). The fluorides NpF 3 and NpF 4 are precipitated from aqueous solutions .
Other neptunium halides are: NpF 3 (purple-black); NpCl 3 (white); NpBr 3 (green); NpI 3 (brown); NpF 4 (green); NpCl 4 (red-brown); NpBr 4 (red-brown); and NpF 6 (orange). The removal of highly radioactive neptunium solids or solutions must be performed via remote control. Neptunium is one of the actinides that is found in nuclear waste (in oxidate states +3, +4, +5, +6, and may be +7) and must be kept out of the environment.
Lea B. Zinner
Ball, M. C., and Norbury, A. H. (1974). Physical Data for Inorganic Chemists. London: Longman.
Cotton, Frank A., and Wilkinson, Geoffrey (1972). Advanced Inorganic Chemistry: A Comprehensive Text, 3rd edition. New York: Wiley Interscience.
Greenwood, Norman N., and Earnshaw, A. (1997). Chemistry of the Elements, 2nd edition. Boston: Butterworth-Heinemann.