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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 Electron shell diagram of neptunium.

Electron shell diagram of neptunium.

Greg Robson This is the location of neptunium on the periodic table.

This is the location of neptunium on the periodic table.

Todd Helmenstine
Periodic Table of the Elements


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

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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