Neptunium Facts

Chemical & Physical Properties

This is a photo of a 6-kg sphere of neptunium-237.
Neptunium is a silvery radioactive heavy metal. This is a photo of a 6-kg sphere of neptunium-237. Los Alamos National Laboratory

Neptunium Basic Facts


Atomic Number: 93

Symbol: Np

Atomic Weight: 237.0482

Discovery: E.M. McMillan and P.H. Abelson 1940 (United States)

Electron Configuration: [Rn] 5f4 6d1 7s2

Word Origin: Named after the planet Neptune.

Isotopes: 20 isotopes of Neptunium are known. The most stable of these is neptunium-237, with a half-life of 2.14 million years Properties: Neptunium has a melting point of 913.2 K, boiling point of 4175 K, heat of fusion of 5.190 kJ/mol, sp.

gr. 20.25 at 20°C; valence +3, +4, +5, or +6. Neptunium is a silvery, ductile, radioactive metal. Three allotropes are known. At room temperature it exists primarily in an orthorhombic crystalline state.

Uses: Neptunium-237 is used in neutron-detection equipment. Sources McMillan and Abelson produced neptunium-239 (half-life 2.3 days) by bombarding uranium with neutrons from a cyclotron at the U. of California at Berkeley. Neptunium is also found in very small quantities associated with uranium ores.

Element Classification: Radioactive Rare Earth Element (Actinide Series)

Density (g/cc): 20.25

Neptunium Physical Data

Melting Point (K): 913

Boiling Point (K): 4175

Appearance: silvery metal

Atomic Radius (pm): 130

Atomic Volume (cc/mol): 21.1

Ionic Radius: 95 (+4e) 110 (+3e)

Fusion Heat (kJ/mol): (9.6)

Evaporation Heat (kJ/mol): 336

Pauling Negativity Number: 1.36

Oxidation States: 6, 5, 4, 3

Lattice Structure: Orthorhombic

Lattice Constant (Å): 4.720

References: Los Alamos National Laboratory (2001), Crescent Chemical Company (2001), Lange's Handbook of Chemistry (1952), CRC Handbook of Chemistry & Physics (18th Ed.)

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