2007 Schools Wikipedia Selection. Related subjects: Chemical elements
|Name, Symbol, Number||neptunium, Np, 93|
|Group, Period, Block||n/a, 7, f|
|Atomic mass||(237) g/mol|
|Electron configuration||[Rn] 5f4 6d1 7s2|
|Electrons per shell||2, 8, 18, 32, 22, 9, 2|
|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|
|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|
|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|
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.
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 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.
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.
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.
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.