Neptunium, Radioactive

Other Properties

There are 23 isotopes and isomers of neptunium(1). Trace quantities of the element are actually found in nature due to transmutation reactions in uranium ores produced by the neutrons which are present(1). All neptunium isotopes are radioactive(2).
Atomic number 93; valence: 3, 4, 5, 6; twenty-three isotopes and isomers are recognized; neptunium metal is chemically reactive
Neptunium-237: Atomic weight = 237.048166; half-life = 2.14x10+6 years; alpha decay; 4.957 MeV; spontaneous fission, 2.1x10-10 MeV
Neptunium-239: Atomic weight = 239.052931; half-life = 2.117 days; beta(-) decay; 1.292 MeV
First synthetic transuranium element; no stable nuclides; known isotopes (mass numbers): 227-242; silvery metal; develops a thin oxide layer upon exposure to air for short periods; reacts with air at high temperatures to form NpO2; exhibits 3 allotropic modifications: othrorhombic alpha-form; density = 20.45; transforms to beta-form at 280 deg C; tetragonal beta-form: density = 19.36 transforms to gamma-form at 577 deg C; cubic gamma-form: transforms to liquid at melting point, 637 deg C
DECAY PATHWAY: Neptunium-237, half-life 2,144,000 years, decays via alpha emission, 4.959 MeV, to protactinium-233, half-life 26.967 days. Protactinium-233 decays via beta emission, 0.571 MeV, to uranium-233, half-life 159,200 years.
DECAY PATHWAY: Neptunium-239, half-life 2.3565 days, decays via beta(-) emission, 0.722 MeV, to plutonium-239, half-life 24,110 years. Plutonium-239 decays via alpha emission, 5.245 MeV, to uranium-235, half-life 703,800,000 years.
Metallic neptunium forms a protective oxide layer in air at room temperature, but it rapidly oxidizes at higher temperatures. It dissolves readily in HCl and H2SO4.
Five binary oxides or oxide hydrates of neptunium: NpO2, Np2O5, Np3O8, NpO3.2H2O and NpO3.H2O. Anhydrous Np(VI) oxide has not been prepared. Neptunium dioxide, NpO2, is the most stable of the neptunium oxides. It crystallizes with the fluorite structure of all the actinide dioxides, with a crystalline density of 11.14 g/cu cm. ... High-fired NpO2 can be dissolved in hot concentrated nitric acid containing small amounts of fluoride. The mixed oxide Np3O8 is structurally analogous to U3O8. Above 500 deg C it decomposes to NpO2. /Neptunium oxides/
In aqueous solution neptunium exists in the five oxidation states Np(III), Np(IV), Np(V), Np(VI), and Np(VII), although the heptavalent Np(VII) is stable only in alkaline solutions. In the absence of complexing agents the first four oxidation states exist as Np+3, Np+4, NpO2+, and NpO2+2, usually in the hydrated form. ... Pentavalent neptunium is the most stable state in solution. ... Hexavalent neptunium is much less stable in solution than is hexavalent plutonium; in fact, hexavalent neptunium is a strong oxidizing agent and is easily reduced in the presence of oxidizable substances, such as those present in ion-exchange and solvent extraction separations. ..Trivalent neptunium is stable only in the absence of oxygen, being oxidized to Np(IV) in aqueous solutions exposed to air. Tetravalent neptunium forms strong complexes with anions, but Np(V) forms only weak complexes. ...
Silvery-white metal; exhibits three crystalline modifications: an orthorhombic alpha form, stable at ordinary temperatures and density 20.45 g/cm3; the alpha-form transforms to a tetragonal beta allotrope of density 19.36 g/cm3 when heated at 280 deg C; the beta form converts to a body-centered cubic crystalline gamma modification at 577 deg C, having a density 18.0 g/cm3. The metal melts at 644 deg C; boils at 3,902 deg C (estimated); dissolves in hydrochloric acid. /Neptunium metal forms/
Neptunium metal reacts with hydrogen under milder conditions at 50 deg C and one atmospheric pressure, forming hydrides of varying stoichiometric compositions. The metal combines with carbon at 1,200 deg C, forming two carbides, NpC and Np2C3. Heating the trifluoride, NpF3 with silicon at 1,500 deg C forms neptunium silicide, NpSi2.Many other neptunium compounds have been prepared and their crystal structures determined. These include the black orthorhombic sulfide, Np2S3, and the tetragonal oxysulfide, NpOS, and the pink hexagonal oxofluoride, NpO2F2. Neptunium also is known to form many intermetallic compounds with aluminum, beryllium and other metals. In solution, neptunium oxidizes to Np3+ and Np4+ ions, the salts of which are pink and greenish-yellow, respectively. Unlike its rare earth analog promethium, neptunium also forms oxoions, such as, NpO+ (blue green) and NpO2+ (light pink).
Neptunium solubility is strongly dependent upon oxidation state. The +3 and +4 states form very insoluble fluorides, while the (V) and (VI) states are soluble. This property is an effective means of separation of neptunium from uranium. Neptunium (+4) may be carried on zirconium phosphate precipitate, indicating its insolubility as a phosphate only in that oxidation state. Neptunium forms two oxides, NpO2 and Np3O8, both of which are soluble in concentrated hydrochloric, perchloric and nitric acids. The most soluble of the neptunium compounds are Np(SO4)2, Np(C2O4)2, Np(NO3)5, Np(IO3)4, and (NH4)2Np2O7. Neptunium (+3) compounds are easily oxidized to Np+4 when exposed to air.
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