Neptunium (Np)

Neptunium is a chemical element with the atomic number 93 in the periodic table. Despite being labeled as a synthetically produced substance, there are minuscule amounts of this transuranium element in nature. Neptunium occurs in an amount of 0.0005 parts per trillion by weight in Earth’s crust. 

Being a member of the actinides family of the periodic table, this moderately radioactive element has two valence electrons that help this chemical form numerous reactions with the other elements.

Chemical and Physical Properties of Neptunium

Atomic number93
Atomic weight (mass)(237) g.mol-1
Group (number)Actinides
Period7 (f-block)
ColorSilvery metal
Physical stateSolid at 20°C
Half-lifeFrom 0.15(+0.72-0.07) milliseconds to 2.14 million years
Electronegativity according to PaulingUnknown
Density20.2 at room temperature
Melting point644°C, 1191°F, 917 K
Boiling point3902°C, 7056°F, 4175 K
Van der Waals radiusUnknown
Ionic radiusUnknown
Most characteristic isotope237Np
Electronic shell[Rn] 5f46d17s2
The energy of the first ionizationUnknown
The energy of the second ionizationUnknown
Discovery dateIn 1940 by Edwin McMillan and Philip Abelson

With the periodic table symbol Np, atomic number 93, atomic mass of (237) g.mol-1, and electron configuration [Rn] 5f46d17s2, neptunium is ductile, silvery, radioactive metal. It reaches its boiling point at 3902°C, 7056°F, 4175 K, while the melting point is achieved at 644°C, 1191°F, 917 K. 

This transuranium element is classified between uranium and plutonium in the actinides family of the periodic table. Neptunium has the highest density among the members of the actinides, and exists in three allotropes:

  • It occurs in an orthorhombic structure at normal temperatures,
  • In a tetragonal structure above 280oC, and
  • In a cubic structure above 577oC.

How Was Neptunium Discovered?

The story on the discovery of element 93 begins in the spring of 1940. Intrigued by Otto Hahn and Fritz Strassmann’s announcement that uranium atoms could be split, the two American physicists Edwin McMillan (1907 – 1991) and his colleague Philip H. Abelson (1907 – 1991) redirected their current work onto this issue. 

These scientists almost immediately attempted bombarding 238U with neutrons to produce 239U in a cyclotron at the Berkeley Radiation Laboratory of the University of California. This uranium isotope then underwent beta decay to 239Np with a half-life of only 2.3 days. 

Neptunium-239 is the first isotope of neptunium that has ever been synthetically produced. The unusual beta-rays emitted by the isotope were a clear indication of a new chemical element. By this, McMillan and Abelson not only succeeded in confirming Hahn’s and Strasmann’s theory, but they also managed to produce a new element by using sophisticated chemical separation techniques. 

How Did Neptunium Get Its Name?

The element 93 got its name after the planet Neptune. This signaled that the practice started by the German chemist Martin Klaproth of naming the elements after planets was to be continued. Namely, Klaproth first named the element uranium after the planet Uranus. 

When neptunium was discovered, the naming of the element followed the same logic since Neptune is the next planet beyond Uranus in our solar system. This naming concept continued to be applied by naming plutonium after the planet Pluto (an element that follows neptunium in the actinide series).

Where Can You Find Neptunium?

The element 98, or neptunium, occurs in extremely small amounts in nature (in rocks, soil, and water). As a result of the beta decay of uranium, trace amounts of neptunium-237 and neptunium-239 isotopes can be found in uranium ores within Earth’s crust. 

Namely, uranium undergoes a natural nuclear reaction that results in small amounts of isotopes Np-237 to Np-240. Also, neptunium can be obtained as a by-product of nuclear reactors and plutonium production, extracted from the spent uranium fuel rods.

Neptunium is also found in spent nuclear fuel, but the process of its separation from the other isotopes of plutonium is relatively complex. 

Neptunium in Everyday Life

This radioactive transuranium element has no specific commercial application and has but a few uses. Neptunium-237 is first used to create plutonium-238 and then applied in special energy generators that can power satellites, high-energy neutrons detecting devices,  spacecraft, and lighthouses. Neptunium isotopes are also used in scientific experiments. 

Also, neptunium can be accumulated in ionizing smoke detectors, which are one of the most common household items nowadays. Namely, in order to detect smoke, americium-241 emits radiation and decays into neptunium-237. But, this chemical process is safe and has no hazardous effects upon the health or the environment. 

How Dangerous Is Neptunium?

Being a radioactive element, neptunium is a highly toxic and dangerous substance. Human exposure to neptunium may occur via the following hazardous sources:

  • Fallout from nuclear weapons; 
  • Industrial processing of 237Np produced in fission reactors;
  • Effluent cooling water from fission reactors.

Upon exposure to high amounts of neptunium, this substance typically accumulates in the gastrointestinal organs and the bones. In addition, the bones and the stomach are the areas where the neptunium’s radioactivity triggers tumorous growths and cancerous forms. 

Environmental Effects of Neptunium

So far, neptunium has not been reported to have hazardous effects on the environment.  When it occurs as a by-product of the decay of americium isotopes in smoke detectors, neptunium is regarded as an extremely long-lasting waste, with a half-life of at least 2 million years.

Isotopes of Neptunium

There are 29 radioisotopes of neptunium, with atomic mass ranging from neptunium-219 to neptunium-244. As in all other synthetically produced elements, this chemical does not have any stable isotopes.

With a half-life of 2.14 million years, neptunium-237 is the longest living form of the transuranic element 93. Neptunium-237 is produced in kilogram quantities from the radioactive waste generated by the power reactors as a by-product of the alpha decay of 241Am. Americium-241 is further produced by neutron irradiation of uranium-238. 

Furthermore, neptunium-237 decays into protactinium-233 through alpha decay. After decaying to protactinium then to uranium, this neptunium form eventually decays to produce bismuth-209 and thallium-205.


[n 1]

ZNIsotopic mass (Da)[4]

[n 2][n 3]



[n 4]



[n 5]

Spin and


[n 6][n 7]



Excitation energy[n 7]



93126219.03162(9)0.15(+0.72-0.07) msα215Pa(9/2−) 



93127220.03254(21)#25(+14-7) µsα216Pa1-# 



93129 380(+260-110) nsα218Pa1-# 



93130223.03285(21)#2.15(+100-52) µsα219Pa9/2− 



93131224.03422(21)#38(+26-11) µsα (83%)220m1Pa1-# 
α (17%)220m2Pa


93132225.03391(8)6(5) msα221Pa9/2−# 


93133226.03515(10)#35(10) msα222Pa  


93134227.03496(8)510(60) msα (99.95%)223Pa5/2−# 
β+ (.05%)227U


93135228.03618(21)#61.4(14) sβ+ (59%)228U  
α (41%)224Pa
β+, SF (.012%)(various)


93136229.03626(9)4.0(2) minα (51%)225Pa5/2+# 
β+ (49%)229U


93137230.03783(6)4.6(3) minβ+ (97%)230U  
α (3%)226Pa


93138231.03825(5)48.8(2) minβ+ (98%)231U(5/2)(+#) 
α (2%)227Pa


93139232.04011(11)#14.7(3) minβ+ (99.99%)232U(4+) 
α (.003%)228Pa


93140233.04074(5)36.2(1) minβ+ (99.99%)233U(5/2+) 
α (.001%)229Pa


93141234.042895(9)4.4(1) dβ+234U(0+) 


93142235.0440633(21)396.1(12) dEC235U5/2+ 
α (.0026%)231Pa


[n 8]

93143236.04657(5)1.54(6)×105 yEC (87.3%)236U(6−) 
β (12.5%)236Pu
α (.16%)232Pa


[n 8][n 9]

93144237.0481734(20)2.144(7)×106 yα233Pa5/2+Trace[n 10]
SF (2×10−10%)(various)
CD (4×10−12%)207Tl




93145238.0509464(20)2.117(2) dβ238Pu2+ 


93146239.0529390(22)2.356(3) dβ239Pu5/2+Trace[n 10]


93147240.056162(16)61.9(2) minβ240Pu(5+)Trace[n 11]


93148241.05825(8)13.9(2) minβ241Pu(5/2+) 


93149242.06164(21)2.2(2) minβ242Pu(1+) 


93150243.06428(3)#1.85(15) minβ243Pu(5/2−) 


93151244.06785(32)#2.29(16) minβ244Pu(7−) 

Source: Wikipedia

List of Neptunium Compounds 

This member of the actinides family of elements is extremely reactive, especially when exposed to oxygen, steam, or acids. On the other hand, neptunium does not show any reactivity when attacked by the elements of the alkali group.

When it becomes a part of a compound, neptunium can adopt many oxidation states. The most frequently occurring oxidation states of this chemical element are +3, +4, +5, +6, and +7.

Thus, neptunium produces different colors:

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) 

Some of the most common compounds of this radioactive element include:

  • Neptunium(III) chloride
  • Neptunium(III) fluoride
  • Neptunium(IV) fluoride
  • Neptunium(IV) oxide
  • Neptunium(V) fluoride
  • Neptunium(VI) fluoride
  • Neptunocene

5 Interesting Facts and Explanations

  1. The chemical elements that are classified after uranium in Mendeleev’s periodic system are labeled as transuranium elements.
  2. Neptunium is the first transuranium element of the actinide series that had been discovered. In addition, all 15 actinide elements are radioactive and have very large atomic radii.
  3. Despite the fact that traces of neptunium can be found in nature, this chemical element is considered a synthetically produced substance. 
  4. In 1951, Edwin Mattison McMillan shared the Nobel Prize in Chemistry with Glenn Seaborg for the discovery of neptunium. In 1938, Enrico Fermi received the Nobel Prize in Physics as the scientist credited for the discovery of the (essentially) same element. Namely, he succeeded in creating elements 93 and 94 by the bombardment of uranium with neutrons but did not create any transuranium element. After this unfortunate mix-up of the Nobel Committee, they have decided to wait at least 10 years before naming a new laureate of discovery in the fields of Chemistry and Physics. 
  5. Shortly after McMillan discovered the first isotope of neptunium, Glenn Seaborg and his colleagues continued their work by following McMillan’s techniques and succeeded in isolating the element 94, plutonium.