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93 uraniumneptuniumplutonium


Periodic Table - Extended Periodic Table
Name, Symbol, Number neptunium, Np, 93
Chemical series actinides
Group, Period, Block n/a, 7, f
Appearance silvery metallic
Atomic mass (237) g/mol
Electron configuration [Rn] 5f4 6d1 7s2
Electrons per shell 2, 8, 18, 32, 22, 9, 2
Physical properties
Phase solid
Density (near r.t.) 20.2 g·cm−3
Melting point 910  K
(637 ° C, 1179 ° F)
Boiling point 4273 K
(4000 ° C, 7232 ° F)
Heat of fusion 3.20 kJ·mol−1
Heat of vaporization 336 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
Oxidation states 6, 5, 4, 3
( amphoteric oxide)
Electronegativity 1.36 (Pauling scale)
Ionization energies 1st: 604.5 kJ/mol
Atomic radius 175 pm
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

Neptunium ( IPA: /ˌnɛpˈt(j)uːniəm/) is an 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, 237Np, 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 20.25 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

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 tetra halides such as NpF3, NpF4, NpCl4, NpBr3, NpI3, and oxides of the various compositions such as are found in the uranium-oxygen 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

Please also see Actinides in the environment


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. 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 enriched uranium, discovering that the critical mass of neptunium is less than previously predicted. US officials in March 2004, planned to move the nation's supply of enriched neptunium to a site in Nevada.


Neptunium (named for the planet Neptune) was first discovered by Edwin McMillan and Philip Abelson in 1940. Initially predicted by Walter Russell's "spiral" organization of the periodic table, it was found at the Berkeley Radiation Laboratory of the University of California, Berkeley where the team produced the neptunium isotope 239Np (2.4 day half-life) by bombarding uranium with slow moving neutrons. It was the first transuranium element produced synthetically and the first actinide series transuranium element discovered.


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

Nuclear synthesis

When an 235U atom captures a neutron, it is converted to an excited state of 236U. About 81% of the excited 236U nuclei undergo fission, but the remainder decay to the ground state of 236U by emitting gamma radiation. Further neutron capture creates 237U which has a half-life of 7 days and thus quickly decays to 237Np. 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.


19 neptunium radioisotopes have been characterized, with the most stable being 237Np with a half-life of 2.14 million years, 236Np with a half-life of 154,000 years, and 235Np 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 236mNp (t½ 22.5 hours).

The isotopes of neptunium range in atomic weight from 225.0339 u (225Np) to 244.068 u (244Np). The primary decay mode before the most stable isotope, 237Np, is electron capture (with a good deal of alpha emission), and the primary mode after is beta emission. The primary decay products before 237Np are element 92 (uranium) isotopes (alpha emission produces element 91, protactinium, however) and the primary products after are element 94 (plutonium) isotopes.

237Np eventually decays to form bismuth, unlike most other common heavy nuclei which decay to make lead.

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