Neptunium is a chemical element with the 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. It is radioactive, poisonous, pyrophoric, and capable of accumulating in bones, which makes the handling of neptunium dangerous.

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. 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.

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Atomic properties

Atomic mass(237.048 u)

Atomic radii

Radius (empirical)155 pm
Covalent radius190 ±1 pm

Atomic shell

Electron configurationRn 5f4 6d1 7s2
Ionization energy(1st) 6.26554 eV
(2nd) 11.5 eV
(3rd) 19.7 eV
(4th) 33.8 eV
(5th) 48 eV
Shell model

Physical properties

Density20.45 g·cm−3
Molar volume1.159·10-5 m3·mol−1


Melting point912 ±3 K
Boiling point(4,447 K)
Liquid range(3,535 K)


Melting enthalpy10 kJ·mol-1
Enthalpy of vaporization335 kJ·mol-1

Heat and conductivity

Thermal conductivity6.3 W·m-1·K-1 (300 K)

Electrical properties

Electrical conductivity820,000 S·m-1
Resistance1.2·10-6 Ωm



Chemical properties

Oxidation state+3, +4, +5, +6, +7
Standard potential-1.79 V (Np3+ + 3e- → Np)


Pauling scale1.36
Allred-Rochow scale1.22
Nagle scale1.09

Other properties

Natural occurrenceDecay product
Crystal structureOrthorhombic
Goldschmidt Classificationsynthetic
Superconductorwithout transition tempperature
Radioactivityslightly radioactive
Price/kg660,000 USD