Neptunium

Neptunium

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melting point: 640°C
boiling point: 3,930°C
density: 20.45 g/cm ³
most common ions: Np ³⁺, Np ⁴⁺, or NpO ₂⁺, NpO ₂²⁺, NpO ₃⁺

Neptunium was discovered by the U.S. physicists Edwin M. McMillan and Philip Abelson, in 1940, via the bombardment of ²³⁸U with neutrons. The name of the element is related to the planet Neptune. Neptunium-237 occurs as a product of ²³⁸U fission , and appears in uranium fuel elements.

Neptunium is used to produce plutonium (²³⁸Pu), via the irradiation of NpO₂ with neutrons. The isotope ²³⁸₉₄Pu is used as a power source for satellites.

Neptunium has several valence states: Np³⁺ (purplish in solution), stable in water, is easily oxidized in air to Np⁴⁺ (yellow-green), and then slowly oxidized in air to the stable ion, NpO₂²⁺ (pink); NpO₂⁺ (green) is obtained by the oxidation of Np⁴⁺ with hot nitric acid. NpO₂²⁺ can also be obtained by the oxidation of lower state ions with Ce⁴⁺, MnO⁴⁻, O₃, and BrO₃⁻. The production of ²³⁷Np (as NpO₂⁺) involves the oxidation of Np³⁺ by nitric acid, followed by the extraction of the NpO₂⁺ compound with tributyl-phosphate in kerosene.

The oxide NpO₂, isostructural with UO₂, is obtained by heating neptunium nitrates or hydroxides in air. The oxide Np₃O₈, isomorphous with U₃O₈, is also obtained by heating such compounds in air at specific temperatures. The oxidation of Np⁴⁺ hydroxide compounds with ozone gives the hydrated trioxides NpO₃ · 2H₂O (brown) and NpO₃ · H₂O (red-gold). The fluorides NpF₃ and NpF₄ are precipitated from aqueous solutions .

Other neptunium halides are: NpF₃ (purple-black); NpCl₃ (white); NpBr₃ (green); NpI₃ (brown); NpF₄ (green); NpCl₄ (red-brown); NpBr₄ (red-brown); and NpF₆ (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.

see also Actinium; Berkelium; Einsteinium; Fermium; Lawrencium; Mendelevium; Nobelium; Plutonium; Protactinium; Rutherfordium; Thorium; Uranium.

Lea B. Zinner

Geraldo Vicentini

Bibliography

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.