Neptunium Element Facts

The chemical element neptunium is classed as an actinide metal. It was discovered in 1940 by Edwin McMillan and Philip H. Abelson.

Neptunium-237 sphere
Neptunium-237 sphere

Neptunium-237 sphere (6 kg) Photo credit: Los Alamos National Laboratory


Data Zone

Classification: Neptunium is an actinide metal
Color: silvery
Atomic weight: (237), no stable isotopes
State: solid
Melting point: 640 oC, 913 K
Boiling point: 3900 oC, 4173 K
Electrons: 93
Protons: 93
Neutrons in most abundant isotope: 144
Electron shells: 2,8,18,32,23,8,2
Electron configuration: [Rn] 5f4 6d1 7s2
Density @ 20oC: 20.45 g/cm3
Show more, including: Heats, Energies, Oxidation, Reactions, Compounds, Radii, Conductivities
Atomic volume: 11.62 cm3/mol
Structure: orthorhombic
Specific heat capacity 0.11 J g-1 K-1
Heat of fusion 3.2 kJ mol-1
Heat of atomization 337 kJ mol-1
Heat of vaporization
1st ionization energy 597 kJ mol-1
2nd ionization energy
3rd ionization energy
Electron affinity
Minimum oxidation number 0
Min. common oxidation no. 0
Maximum oxidation number 7
Max. common oxidation no. 5
Electronegativity (Pauling Scale) 1.3
Polarizability volume 24.8 Å3
Reaction with air
Reaction with 15 M HNO3
Reaction with 6 M HCl
Reaction with 6 M NaOH
Oxide(s) NpO, NpO2, Np2O5
Hydride(s) NpH2, NpH3
Chloride(s) NpCl3, NpCl4
Atomic radius 175 pm
Ionic radius (1+ ion)
Ionic radius (2+ ion) 124 pm
Ionic radius (3+ ion) 115 pm
Ionic radius (1- ion)
Ionic radius (2- ion)
Ionic radius (3- ion)
Thermal conductivity 6.3 W m-1 K-1
Electrical conductivity 0.8 x 106 S m-1
Freezing/Melting point: 640 oC, 913 K

Neptunium Radioactive

Neptunium is Radioactive.

Discovery of Neptunium

Dr. Doug Stewart

Neptunium was the first synthetic transuranium element (elements after uranium) of the actinide series to be discovered.

Neptunium was first produced by Edwin McMillan and Philip H. Abelson in 1940 at Berkeley Radiation Laboratory of the University of California.

McMillan and Abelson bombarded uranium-238 with neutrons and they were able to show chemically that they had produced neptunium-239, which has a half-life of just 2.3 days.

A longer lived isotope neptunium-237 was discovered in 1942. Scientists A. C. Wahl and Glenn T. Seaborg bombarded uranium-238 with fast neutrons in the Berkeley 60-inch cyclotron. They isolated several hundred milligrams of neptunium and made a thorough study of its properties. (1)

The element is named after the planet Neptune, continuing the theme started by Martin Klaproth when he named uranium after the planet Uranus. This theme was to continue with plutonium, which follows neptunium in the actinide series.

Appearance and Characteristics

Harmful effects:

Neptunium is harmful due to its radioactivity.


Neptunium is a silvery radioactive synthetic metal. (Miniscule quantities of neptunium-237 and neptunium-239 are found in nature as a result of beta decay of uranium.)

Neptunium exists in three allotropes: it has an orthorhombic structure at normal temperatures, a tetragonal structure above 280oC and a cubic structure above 577oC. (2)

Neptunium has five oxidation states (+3 to +7) producing different colors in solution:

III:   Np3+ (violet)
IV:   Np4+ (yellow green)
V:   NpO2+ in acidic solution (green) and in alkaline solution (yellow)
VI:   NpO22+ (pink red)
VII:  Np(VII) in alkaline solution (green) or possibly in acidic conditions (brownish-red). (3)

Uses of Neptunium

Neptunium is used mainly for research purposes.

When bombarded with neutrons neptunium-237 is used to produce plutonium-238 which is used for spacecraft generators and terrestrial navigation beacons.

Neptunium is also used in neutron detection equipment.

Abundance and Isotopes

Abundance earth’s crust: negligible

Abundance solar system: unknown

Cost, pure: per g

Cost, bulk: per 100g

Source: Miniscule concentrations of neptunium-237 and neptunium-239 are found naturally in uranium ores. Neptunium-237 is produced in kilogram quantities from radioactive waste from power reactors. Neptunium-238 is produced from the production of plutonium-238. (3)

Isotopes: Neptunium has 20 isotopes whose half-lives are known, with mass numbers 225 to 244. Neptunium has no stable isotopes. Its longest lived isotopes are 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.


1. I. Perlman, The Transuranium Elements and Nuclear Chemistry., Journal of Chemical Education., May 1948, p275-276.
2. David R. Lide, CRC Handbook of Chemistry and Physics., 86th edition., p4-24.
3.Karen Nilsson and Lars Carlsen, The Migration Chemistry of Neptunium. (pdf document)

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