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Neptunium

2007 Schools Wikipedia Selection. Related subjects: Chemical elements


              93               uranium ← neptunium → plutonium
              Pm
             ↑
             Np
             ↓
             (Uqt)

                                  Periodic Table - Extended Periodic Table

                                                                   General
                                    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] 5f^4 6d^1 7s^2
                                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

   CAPTION: 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
                                                             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

                CAPTION: 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 154×10^3 y   ε      0.940     ^236U
                                                   β^-    0.940     ^236Pu
                                                   α      5.020     ^232Pa
                           ^237Np syn 2.144×10^6 y SF & α 4.959     ^233Pa

                                                                References

   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/m^3,
     * beta-neptunium (above 280 °C), tetragonal, density (313 °C) 19.36
       Mg/m^3, and
     * gamma-neptunium (above 577 °C), cubic, density (600 °C) 18 Mg/m^3

   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);
     * NpO[2]^+ (green blue): and
     * NpO[2]^++ (pale pink).

   Neptunium forms tri- and tetra halides such as NpF[3], NpF[4], NpCl[4],
   NpBr[3], NpI[3], and oxides of the various compositions such as are
   found in the uranium-oxygen system, including Np[3]O[8] and NpO[2].

   Neptunium like other actinides readily forms a dioxide neptunyl core
   (NpO[2]). In the environment, this neptunyl core readily complexes with
   carbonate as well as other oxygen moieties (OH^-, NO[2]^-, NO[3]^-, and
   SO[4]^-2) to form charged complexes which tend to be readily mobile
   with low affinities to soil.
     * NpO[2](OH)[2]^-1
     * NpO[2](CO[3])^-1
     * NpO[2](CO[3])[2]^-3
     * NpO[2](CO[3])[3]^-5

   Please also see 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. 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.

History

   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.

Occurrence

   Trace amounts of neptunium are found naturally as decay products from
   transmutation reactions in uranium ores. ^237Np is produced through the
   reduction of ^237NpF[3] 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.

Isotopes

   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 (t[½] 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.
   Retrieved from " http://en.wikipedia.org/wiki/Neptunium"
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