Chemical element, symbol Np and atomic number 93 / From Wikipedia, the free encyclopedia

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Neptunium is a chemical element; it has symbol Np and atomic number 93. A radioactive actinide metal, neptunium is the first transuranic element. Its position in the periodic table just after uranium, named after the planet Uranus, led to it being named after Neptune, the next planet beyond Uranus. A neptunium atom has 93 protons and 93 electrons, of which seven are valence electrons. Neptunium metal is silvery and tarnishes when exposed to air. The element occurs in three allotropic forms and it normally exhibits five oxidation states, ranging from +3 to +7. Like all actinides, it is radioactive, poisonous, pyrophoric, and capable of accumulating in bones, which makes the handling of neptunium dangerous.

Quick facts: Neptunium, Pronunciation, Appearance, Mass nu...
Neptunium, 93Np
Pronunciation/nɛpˈtjniəm/ (nep-TEW-nee-əm)
Appearancesilvery metallic
Mass number[237]
Neptunium in the periodic table


Atomic number (Z)93
Groupf-block groups (no number)
Periodperiod 7
Block  f-block
Electron configuration[Rn] 5f4 6d1 7s2
Electrons per shell2, 8, 18, 32, 22, 9, 2
Physical properties
Phase at STPsolid
Melting point912±3 K (639±3 °C, 1182±5 °F)
Boiling point4447 K (4174 °C, 7545 °F) (extrapolated)
Density (near r.t.)alpha: 20.45 g/cm3[1]
accepted standard value: 19.38 g/cm3
Heat of fusion5.19 kJ/mol
Heat of vaporization336 kJ/mol
Molar heat capacity29.46 J/(mol·K)
Vapor pressure
P (Pa) 1 10 100 1 k 10 k 100 k
at T (K) 2194 2437
Atomic properties
Oxidation states+2, +3, +4,[2] +5, +6, +7 (an amphoteric oxide)
ElectronegativityPauling scale: 1.36
Ionization energies
  • 1st: 604.5 kJ/mol
Atomic radiusempirical: 155 pm
Covalent radius190±1 pm
Color lines in a spectral range
Spectral lines of neptunium
Other properties
Natural occurrencefrom decay
Crystal structure orthorhombic
Orthorhombic crystal structure for neptunium
Thermal conductivity6.3 W/(m⋅K)
Electrical resistivity1.220 µΩ⋅m (at 22 °C)
Magnetic orderingparamagnetic[3]
CAS Number7439-99-8
Namingafter planet Neptune, itself named after Roman god of the sea Neptune
DiscoveryEdwin McMillan and Philip H. Abelson (1940)
Isotopes of neptunium
Main isotopes[4] Decay
abun­dance half-life (t1/2) mode pro­duct
235Np synth 396.1 d α 231Pa
ε 235U
236Np synth 1.54×105 y ε 236U
β 236Pu
α 232Pa
237Np trace 2.144×106 y α 233Pa
239Np trace 2.356 d β 239Pu
Symbol_category_class.svg Category: Neptunium
| references

Although many false claims of its discovery were made over the years, the element was first synthesized by Edwin McMillan and Philip H. Abelson at the Berkeley Radiation Laboratory in 1940.[5] Since then, most neptunium has been and still is produced by neutron irradiation of uranium in nuclear reactors. The vast majority is generated as a by-product in conventional nuclear power reactors. While neptunium itself has no commercial uses at present, it is used as a precursor for the formation of plutonium-238, and in radioisotope thermal generators to provide electricity for spacecraft. Neptunium has also been used in detectors of high-energy neutrons.

The longest-lived isotope of neptunium, neptunium-237, is a by-product of nuclear reactors and plutonium production. This isotope, and the isotope neptunium-239, are also found in trace amounts in uranium ores due to neutron capture reactions and beta decay.[6]

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