Periodic Table > Neptunium


Some elements exist in several different structural forms, called allotropes. Each allotrope has different physical properties.

For more information on the Visual Elements image see the Uses and properties section below.



A vertical column in the periodic table. Members of a group typically have similar properties and electron configurations in their outer shell.

A horizontal row in the periodic table. The atomic number of each element increases by one, reading from left to right.

Elements are organised into blocks by the orbital type in which the outer electrons are found. These blocks are named for the characteristic spectra they produce: sharp (s), principal (p), diffuse (d), and fundamental (f).

Atomic number
The number of protons in an atom.

Electron configuration
The arrangements of electrons above the last (closed shell) noble gas.

Melting point
The temperature at which the solid–liquid phase change occurs.

Boiling point
The temperature at which the liquid–gas phase change occurs.

The transition of a substance directly from the solid to the gas phase without passing through a liquid phase.

Density (g cm−3)
Density is the mass of a substance that would fill 1 cm3 at room temperature.

Relative atomic mass
The mass of an atom relative to that of carbon-12. This is approximately the sum of the number of protons and neutrons in the nucleus. Where more than one isotope exists, the value given is the abundance weighted average.

Atoms of the same element with different numbers of neutrons.

CAS number
The Chemical Abstracts Service registry number is a unique identifier of a particular chemical, designed to prevent confusion arising from different languages and naming systems.

Fact box

Group Actinides  Melting point 644°C, 1191°F, 917 K 
Period Boiling point 3902°C, 7056°F, 4175 K 
Block Density (g cm−3) 20.2 
Atomic number 93  Relative atomic mass [237]  
State at 20°C Solid  Key isotopes 237Np 
Electron configuration [Rn] 5f46d17s2  CAS number 7439-99-8 
ChemSpider ID 22375 ChemSpider is a free chemical structure database


Image explanation

Murray Robertson is the artist behind the images which make up Visual Elements. This is where the artist explains his interpretation of the element and the science behind the picture.


The description of the element in its natural form.

Biological role

The role of the element in humans, animals and plants.

Natural abundance

Where the element is most commonly found in nature, and how it is sourced commercially.

Uses and properties

Image explanation
The symbol used is a representation of the trident belonging to the Roman god Neptune.
A radioactive metal.
Neptunium is little used outside research. The isotope neptunium-237 has been used in neutron detectors.
Biological role
Neptunium has no known biological role. It is toxic due to its radioactivity.
Natural abundance
Neptunium is obtained as a by-product from nuclear reactors. It is extracted from the spent uranium fuel rods. Trace quantities occur naturally in uranium ores.
  Help text not available for this section currently


In early 1934, Enrico Fermi in Italy tried to produce elements 93 and 94 by bombarding uranium with neutrons, and claimed success. Ida Tacke-Noddack questioned Fermi’s claim, pointing out he had failed to do a complete analysis, and all that he had found were fission products of uranium. (Fermi had in fact discovered nuclear fission but not realised it.) In 1938, Horia Hulubei and Yvette Cauchois claimed to have discovered element 93, but the claim was also criticised on the grounds that element 93 did not occur naturally.

Neptunium was first made in 1940 by Edwin McMillan and Philip Abelson at Berkeley, California. It came from a uranium target that had been bombarded with slow neutrons and which then emitted unusual beta-rays indicating a new isotope. Abelson proved there was indeed a new element present.

Atomic radius, non-bonded
Half of the distance between two unbonded atoms of the same element when the electrostatic forces are balanced. These values were determined using several different methods.

Covalent radius
Half of the distance between two atoms within a single covalent bond. Values are given for typical oxidation number and coordination.

Electron affinity
The energy released when an electron is added to the neutral atom and a negative ion is formed.

Electronegativity (Pauling scale)
The tendency of an atom to attract electrons towards itself, expressed on a relative scale.

First ionisation energy
The minimum energy required to remove an electron from a neutral atom in its ground state.

Atomic data

Atomic radius, non-bonded (Å) 2.39 Covalent radius (Å) 1.80
Electron affinity (kJ mol−1) Unknown Electronegativity
(Pauling scale)
Ionisation energies
(kJ mol−1)


Common oxidation states

The oxidation state of an atom is a measure of the degree of oxidation of an atom. It is defined as being the charge that an atom would have if all bonds were ionic. Uncombined elements have an oxidation state of 0. The sum of the oxidation states within a compound or ion must equal the overall charge.


Atoms of the same element with different numbers of neutrons.

Key for isotopes

Half life
  y years
  d days
  h hours
  m minutes
  s seconds
Mode of decay
  α alpha particle emission
  β negative beta (electron) emission
  β+ positron emission
  EC orbital electron capture
  sf spontaneous fission
  ββ double beta emission
  ECEC double orbital electron capture

Oxidation states and isotopes

Common oxidation states 6, 5, 4, 3
Isotopes Isotope Atomic mass Natural abundance (%) Half life Mode of decay
  236Np 236.047 - 1.55 x 105 EC 
  237Np 237.048 - 2.14 x 106 α 
        1 x 1018 sf 


Data for this section been provided by the British Geological Survey.

Relative supply risk

An integrated supply risk index from 1 (very low risk) to 10 (very high risk). This is calculated by combining the scores for crustal abundance, reserve distribution, production concentration, substitutability, recycling rate and political stability scores.

Crustal abundance (ppm)

The number of atoms of the element per 1 million atoms of the Earth’s crust.

Recycling rate

The percentage of a commodity which is recycled. A higher recycling rate may reduce risk to supply.


The availability of suitable substitutes for a given commodity.
High = substitution not possible or very difficult.
Medium = substitution is possible but there may be an economic and/or performance impact
Low = substitution is possible with little or no economic and/or performance impact

Production concentration

The percentage of an element produced in the top producing country. The higher the value, the larger risk there is to supply.

Reserve distribution

The percentage of the world reserves located in the country with the largest reserves. The higher the value, the larger risk there is to supply.

Political stability of top producer

A percentile rank for the political stability of the top producing country, derived from World Bank governance indicators.

Political stability of top reserve holder

A percentile rank for the political stability of the country with the largest reserves, derived from World Bank governance indicators.



Specific heat capacity (J kg−1 K−1)

Specific heat capacity is the amount of energy needed to change the temperature of a kilogram of a substance by 1 K.

Young's modulus

A measure of the stiffness of a substance. It provides a measure of how difficult it is to extend a material, with a value given by the ratio of tensile strength to tensile strain.

Shear modulus

A measure of how difficult it is to deform a material. It is given by the ratio of the shear stress to the shear strain.

Bulk modulus

A measure of how difficult it is to compress a substance. It is given by the ratio of the pressure on a body to the fractional decrease in volume.

Vapour pressure

A measure of the propensity of a substance to evaporate. It is defined as the equilibrium pressure exerted by the gas produced above a substance in a closed system.

Pressure and temperature data – advanced

Specific heat capacity
(J kg−1 K−1)
Unknown Young's modulus (GPa) Unknown
Shear modulus (GPa) Unknown Bulk modulus (GPa) Unknown
Vapour pressure  
Temperature (K)
400 600 800 1000 1200 1400 1600 1800 2000 2200 2400
Pressure (Pa)
- - - - 3.31
x 10-9
x 10-6
0.000168 0.00604 0.105 1.06 7.28
  Help text not available for this section currently


Listen to Neptunium Podcast
Transcript :

Chemistry in its element: neptunium


You're listening to Chemistry in its element brought to you by Chemistry World, the magazine of the Royal Society of Chemistry.

(End promo)

Meera Senthilingam

This week, a planetary element that helped create the atomic bomb.

Brian Clegg

We're so familiar with uranium and plutonium that it's easy to miss that they are named after the seventh and ninth planets of the solar system. (At least, Pluto was the ninth planet until it was stripped of its status in 2006.) Between those planets sits Neptune, and the gap between the two elements leaves a space for their relatively unsung cousin, neptunium - element number 93 in the periodic table.

In June 1940, American physicists Edwin McMillan and Philip Abelson, working at the Berkeley Radiation Laboratory, wrote a paper describing a reaction of uranium that had been discovered when bombarding it with neutrons using a cyclotron particle accelerator. Remarkably, the openly published Berkeley paper would show the first step to overcoming one of the biggest obstacles to building an atomic bomb - a paper published when both sides in the Second World War were searching for a solution to the uranium problem.

The trouble with uranium was that the isotope uranium 235 needed to build a bomb was incredibly difficult to separate from the much less rare uranium 238. They are chemically identical. But if uranium 238 can be encouraged to absorb a slow neutron in a reactor, it becomes the unstable isotope uranium 239. This undergoes the nuclear reaction called beta decay, where a neutron turns into a proton, giving off an electron in the process (for historical reasons, the electron is called a beta particle in such circumstances).

The result of McMillan and Abelson's reaction was the production of a new element, one that had never been seen in nature. By the following year, this element was being called neptunium. But neptunium 239 is also unstable and soon generates another electron, adding a second proton to the nucleus to become plutonium. This was the material that would be used to build the world's first atomic bomb.

For our purposes, though, the important thing here is that neptunium had been called into existence. It was third time lucky for using this name for an element. In 1877 a German chemist named Hermann had found what he believed was a new element in the mineral tantalite and called it neptunium. Then in 1886, another German, Clemens Winkler, had isolated what we now call germanium and intended to call this neptunium until he discovered Hermann had used the name first. But Hermann's claim was later proved to be a mistake and the neptunium was free again, ready for McMillan and Abelson to deploy.

The real neptunium sits between uranium and plutonium in the actinides, the floating bar on the periodic table that pops out from between radium and lawrencium. A silvery, metallic substance like so many of its neighbours, its most stable form is the isotope neptunium 237 with a half life - the time it takes for half of the original amount to decay - of over 2 million years, and this is the type of neptunium most likely now to be produced as a by product from nuclear reactors. In the original reaction, though, it was neptunium 239 with a half life of just over 2 days that was formed.

Although it wasn't spotted until it had already been made in reactors, neptunium does actually exist in a natural form on the earth, when uranium undergoes the process that takes place in a reactor, capturing a neutron from another uranium atom that has split, and emitting a beta particle to transmute it to neptunium - but this only happens in the tiniest quantities. There's much more neptunium to be found in the average household.

That's because many smoke detectors use alpha particles from the element americium 241 to ionize the air in a detection chamber. The americium gradually converts to neptunium as it decays, though thanks to americium's 432 year half life, there won't be much produced in the lifetime of a detector.

In practice there is very little use for neptunium. The only significant application is in monitors for high energy neutrons, and even here it is rare. In principle, though, it could have a more deadly use. Where the neptunium 239 produced in 1940 was too unstable to use, quickly transforming into plutonium, Neptunium 237 would be just fine to make an atomic bomb.

Get enough neptunium 237 together and you've got a nuclear device. The necessary amount to go critical and produce a nuclear explosion is about 60 kilograms. This isn't an impractical quantity. Over 50 tonnes of neptunium is produced as waste from nuclear reactors each year. But neptunium has no particular advantage over plutonium or enriched uranium, so has not been deployed. Even so, because of the risk of it falling into the hands of terrorists or rogue states, neptunium waste has to be treated with the same level of security as the traditional ingredients of atomic bombs.

In the end, Neptunium has not proved to be the most useful of elements. When it turns up in a nuclear reactor, or as the end product of the decay of americium in smoke detectors, it is regarded as waste, and it's a particularly long lasting, nasty waste with its immense 2 million year half life. But at least neptunium fans can say that it has a name that trumps even New York. Because neptunium was so good they named it thrice.

Meera Senthilingam

And so good that it can produce nuclear explosions. That was Brian Clegg with the explosive and long lasting chemistry of neptunium. Now next week an element that likes to avoid the limelight for itself but helps others to get there instead.

Simon Cotton

There are lots of everyday applications for yttrium compounds. In its compounds yttrium is always present as the yttrium three plus ion, which means that it is colourless and has no unpaired electrons; therefore it does not have any interesting magnetic or spectroscopic properties of its own. The up side of this is that yttrium compounds make very good host materials for other lanthanides. The most familiar application lies in the red phosphor in cathode ray tubes, as used in traditional colour TV sets.

Meera Senthilingam

And Simon Cotton will be revealing more of the supporting roles of yttrium in next week's Chemistry in its element. Until then I'm Meera Senthilingam and thank you for listening.


Chemistry in its element is brought to you by the Royal Society of Chemistry and produced by There's more information and other episodes of Chemistry in its element on our website at

(End promo)
  Help text not available for this section currently
  Help Text


Description :
The history and background of the Nobel prizes.
Description :
This is an introduction and overview to five Commerical Skills Modules for Chemists. It includes a briefing for students, presentation video, an overview guide and presentation for tutors and market r...
Description :
This module asks teams of students to evaluate a number of potential projects, with a chemical background, and identify why they might be of potential value. This resource provides all the information...
Description :
This module asks teams of students to identify the route to market, and produce a ‘project plan’, for a number of potential projects with a chemical background. This resource contains a Tutor Guide an...
Description :
In this module students are asked to summarise and critique two out of the five chemistry-based projects, based on their previous work, and present their preferred option to a "business" audience.  Th...
Description :
This module asks teams of students to estimate the market value of a number of potential projects with a chemical background. This resource contains all the information necessary to run the Market Res...

Terms & Conditions

Images © Murray Robertson 1999-2011
Text © The Royal Society of Chemistry 1999-2011

Welcome to "A Visual Interpretation of The Table of Elements", the most striking version of the periodic table on the web. This Site has been carefully prepared for your visit, and we ask you to honour and agree to the following terms and conditions when using this Site.

Copyright of and ownership in the Images reside with Murray Robertson. The RSC has been granted the sole and exclusive right and licence to produce, publish and further license the Images.

The RSC maintains this Site for your information, education, communication, and personal entertainment. You may browse, download or print out one copy of the material displayed on the Site for your personal, non-commercial, non-public use, but you must retain all copyright and other proprietary notices contained on the materials. You may not further copy, alter, distribute or otherwise use any of the materials from this Site without the advance, written consent of the RSC. The images may not be posted on any website, shared in any disc library, image storage mechanism, network system or similar arrangement. Pornographic, defamatory, libellous, scandalous, fraudulent, immoral, infringing or otherwise unlawful use of the Images is, of course, prohibited.

If you wish to use the Images in a manner not permitted by these terms and conditions please contact the Publishing Services Department by email. If you are in any doubt, please ask.

Commercial use of the Images will be charged at a rate based on the particular use, prices on application. In such cases we would ask you to sign a Visual Elements licence agreement, tailored to the specific use you propose.

The RSC makes no representations whatsoever about the suitability of the information contained in the documents and related graphics published on this Site for any purpose. All such documents and related graphics are provided "as is" without any representation or endorsement made and warranty of any kind, whether expressed or implied, including but not limited to the implied warranties of fitness for a particular purpose, non-infringement, compatibility, security and accuracy.

In no event shall the RSC be liable for any damages including, without limitation, indirect or consequential damages, or any damages whatsoever arising from use or loss of use, data or profits, whether in action of contract, negligence or other tortious action, arising out of or in connection with the use of the material available from this Site. Nor shall the RSC be in any event liable for any damage to your computer equipment or software which may occur on account of your access to or use of the Site, or your downloading of materials, data, text, software, or images from the Site, whether caused by a virus, bug or otherwise.

We hope that you enjoy your visit to this Site. We welcome your feedback.


Visual Elements images and videos
© Murray Robertson 2011.



W. M. Haynes, ed., CRC Handbook of Chemistry and Physics, CRC Press/Taylor and Francis, Boca Raton, FL, 95th Edition, Internet Version 2015, accessed December 2014.
Tables of Physical & Chemical Constants, Kaye & Laby Online, 16th edition, 1995. Version 1.0 (2005), accessed December 2014.
J. S. Coursey, D. J. Schwab, J.J. Tsai, and R. A. Dragoset, Atomic Weights and Isotopic Compositions(version 3.0), 2010, National Institute of Standards and Technology, Gaithersburg, MD, accessed December 2014.
T. L. Cottrell, The Strengths of Chemical Bonds, Butterworth, London, 1954.


Uses and properties

John Emsley, Nature’s Building Blocks: An A-Z Guide to the Elements, Oxford University Press, New York, 2nd Edition, 2011.
Thomas Jefferson National Accelerator Facility - Office of Science Education, It’s Elemental - The Periodic Table of Elements, accessed December 2014.
Periodic Table of Videos, accessed December 2014.


Supply risk data

Derived in part from material provided by the British Geological Survey © NERC.


History text

© John Emsley 2012.



Produced by The Naked Scientists.


Periodic Table of Videos

Created by video journalist Brady Haran working with chemists at The University of Nottingham.
  • Download our free Periodic Table app for mobile phones and tablets.
  • App Store
  • Google play