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What is Neptunium

Neptunium (pronounced as nep-TOO-nee-em) belongs to the family of actinide elements and is denoted by the chemical symbol Np [1].

Neptunium Isotopes

It has 20 isotopes out of which neptunium-237 is the most stable one with a half-life of 2.14 million years that undergoes alpha decay to form protactinium-233 [3].

Where is Neptunium Found on Earth

The element is known to be present in small quantities associated with uranium ore deposits [1]. Its natural occurrence is due to beta decay of uranium into neptunium-237 and neptunium-239 [3].



Origin of its Name: It has been named after the planet Neptune [1].

Who Discovered Neptunium: Edwin McMillan and Philip Abelson were the discoverers of the element [1].

When and Where was it Discovered

In 1934, Enrico Fermi in Italy claimed to be successful in producing elements 93 and 94 by bombarding uranium with neutrons. However, Ida Tacke-Noddack challenged his analysis by pointing out that he actually discovered nuclear fission of uranium. In 1938, similar claims were made by Horia Hulubei and Yvette Cauchoi, but they were rejected, as the new element was not believed at that time to be present in nature.

Finally, in 1940, McMillan and Abelson at Berkeley, California were finally able to confirm the presence of the new element. While bombarding slow neutrons with uranium, beta-rays were produced with unusual properties, suggesting the presence of a new isotope, later confirmed to be the element 93 [1].

Neptunium Element

Classification and Position of Neptunium on the Periodic Table [1]

Group Number Unknown
Group Name Actinides
Period 7
Block f

Properties of Neptunium [1, 2, 3]

General Properties

Atomic Mass 223

Physical Properties

Color Silver-like
Luster Unknown
Odor Unknown
Malleability Unknown
Melting point/freezing point 1191°F (644°C)
Boiling Point 7056°F (3902°C)
Density 20.2 g/cm3
State of matter at room temperature (solid/liquid/gas) Solid
Hardness (Mohs) Unknown
Electrical conductivity 0.8 x 10Sm-1
Thermal (heat) conductivity 6.3 W m-1 K-1

Chemical Properties

Flammability Unknown
Oxidation states 6 5 4 3

Atomic Properties of the Element [1, 2, 3]

Atomic number 93
Valence electrons 7
Electron configuration (noble gas configuration) [Rn] 5f46d17s2
Atomic structure
– Number of electrons 93
– Number of neutrons 144
– Number of protons 93
Energy levels
– First energy level 2
– Second energy level 8
– Third energy level 18
– Fourth energy level 32
– Fifth energy level 23
– Sixth energy level 8
– Seventh energy level 2
Radius of atom
 – Atomic radius 2.39 Å
– Covalent radius 1.80 Å
Electronegativity 1.3
Ionization energy


1st 2nd 3rd 4th 5th 6th 7th

Neptunium Bohr Model

Neptunium Uses

Neptunium-237 is used in neutron detectors. When bombarded with neutrons, it produces plutonium-238 which in turn in is used in spacecraft generators and terrestrial navigation beacons [1, 3].

Interesting Facts

  • Neptunium exists in three different physical forms or allotropes; orthorhombic at room temperature, tetragonal above 280°C, and cubic above 577°C [3, 4].
  • It is obtained as a byproduct from nuclear reactors as well as spent nuclear fuel rods [1, 4].

Pictures of Neptunium

Toxicity and Dangerous Effects of Neptunium

Although there is not much information about its hazardous effects, some animal studies have shown its accidental ingestion could cause damage to the liver, kidneys, and brain [5].

Neptunium Cost

It is not available outside of laboratory production.



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