Periodic Table


Atomic number: 93
Atomic weight: 237
Symbol: Np
Group number: (actinide)
Electronic configuration: [Rn].5f4.6d1.7s2


Neptunium is a radioactive rare earth metal and has at least 3 allotropic forms. It is named for the planet Neptune. Np-237 is a by-product from nuclear reactors.

General information

Discoveror: Edwin M. McMillan and P. H. Abelson
Date discovered: 1940
Discovered at: USA
Meaning of name: Named after "the planet Neptune"

Physical data

Standard state: solid at 298 K
Colour: silvery metallic
Density of solid at ambient temperature/kg m-3: 20450
Molar volume/cm3: 11.59

Radii /pm

Atomic: no data
Covalent (single bond):
Pauling radius for the ion [Np]-: no data

Valence shell orbital radius maxima (Rmax)
orbital s p d f
radius223.1no data118.950.8


Both values are quoted on the Pauling scale.

Pauling: 1.36
Allred Rochow: 1.22

Crystal Structure

structure: orthorhombic

The following CrystalMaker image represents the solid state structure. For most elements, the most stable allotrope is illustrated. Try WebElements version 2 for interactive virtual reality and CHIME images.


Temperatures (/K)

melting point: 910
boiling point: 4273

Enthalpies /kJ mol-1

fusion: 9.46
vaporization: 336

single bond enthalpies:
Np-F Np-Cl Np-Br Np-I Np-Np
no data no data no data 326 no data

Ionization enthalpies /kJ mol-1

Number Enthalpy


This section gives some data for naturally occurring isotopes.
Nominal mass Accurate mass % natural abundance

Further Information

Copyright 1997 Mark Winter
Department of Chemistry at the University of Sheffield, Sheffield S3 7HF, England.

The current version of this document is at http://www.shef.ac.uk/~chem/web-elements-I/Np.html