Neptunium



93 plutonium
Pm
↑
Np
↓
(Uqt)
General
Number neptunium, Np, 93
actinides
Block f
Appearance silvery metallic
Standard atomic weight (237)  g·mol−1
Rn] 5f4 6d1 7s2
shell 2, 8, 18, 32, 22, 9, 2
Physical properties
Phase solid
r.t.) 20.2  g·cm−3
F)
F)
kJ·mol−1
kJ·mol−1
Heat capacity (25 °C) 29.46  J·mol−1·K−1
Vapor pressure
P(Pa) 1 10 100 1 k 10 k 100 k
at T(K) 2194 2437        
Atomic properties
Crystal structure 3 forms: orthorhombic,
tetragonal and cubic
amphoteric oxide)
Electronegativity 1.36 (Pauling scale)
Ionization energies 1st: 604.5 kJ/mol
Atomic radius 175  pm
Miscellaneous
Magnetic ordering  ?
Electrical resistivity (22 °C) 1.220 µΩ·m
Thermal conductivity (300 K) 6.3  W·m−1·K−1
CAS registry number 7439-99-8
Selected isotopes
Main article: Isotopes of neptunium
iso NA half-life DM DE (MeV) DP
235Np syn 396.1 d α 5.192 231Pa
ε 0.124 235U
236Np syn 154×103 y ε 0.940 236U
β- 0.940 236Pu
α 5.020 232Pa
237Np syn 2.144×106 y SF & α 4.959 233Pa
References

Neptunium (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 20.26 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

Compounds

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 tetraoxygen 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.

  • NpO2(OH)2-1
  • NpO2(CO3)-1
  • NpO2(CO3)2-3
  • NpO2(CO3)3-5
See also: Actinides in the environment

Uses

Precursor in Plutonium-238 Production

237Np is irradiated with neutrons to create 238Pu, a rare and valuable isotope for spacecraft and military applications.

Weapons 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".[1] 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 critical mass of neptunium is less than previously predicted[2]. US officials in March 2008, planned to move the nation's supply of separated neptunium to a site in Nevada.

History

Neptunium (named for the planet Neptune, the next planet out from Uranus, after which actinide series transuranium element discovered.

Occurrence

Trace amounts of neptunium are found naturally as plutonium production.

By weight, neptunium-237 discharges are about five percent as great as plutonium discharges and about 0.05 percent of spent nuclear fuel discharges. [3]

Nuclear synthesis

  • When an neutron capture creates 237U which has a half-life of 7 days and thus quickly decays to 237Np.
  • 237U is also produced via an (238U.
  • 237Np is the decay product of 241Am.

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.

Role in nuclear waste

Neptunium-237 is the most mobile deep geological repository environment.[1] This makes it and its predecessors such as americium-241 candidates of interest for destruction by nuclear transmutation.[2]

Isotopes

Main article: isotopes of neptunium

19 neptunium 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 (t½ 22.5 hours).

The isotopes of neptunium range in plutonium) isotopes.

237Np is both fissionable and fissile[2]. 237Np eventually decays to form lead.

Neptunium in Popular Culture

  • When Confederate scientists in Harry Turtledove's Southern Victory Series of alternate-history novels, discover the 93rd element they dub it "saturnium". When American and British scientists learn of the element, they call it "Neptunium" and "Mosleyium" (After Oswald Mosley) respectively.

References

  1. ^ "Restricted Data Declassification Decisions from 1946 until Present", accessed Sept 23, 2006
  2. ^ a b Weiss, P. (October 26, 2002). Little-studied metal goes critical - Neptunium Nukes?. Science News. Retrieved on 2006-09-29.
  3. ^ http://www.isis-online.org/publications/fmct/book/New%20chapter%205.pdf
  • 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
 
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