Neptunium · element 93
The first element made beyond uranium, and the quiet gateway to plutonium
Key facts
In 1940 the periodic table ended at uranium. Then two physicists in California showed that a uranium nucleus could swallow a neutron and turn into something new. That something was neptunium, the first of the transuranium elements, named after the planet beyond Uranus. Its arrival proved that the table had no natural end, only a practical one set by how long nuclei can hold together.
Neptunium is a dense, silvery metal that tarnishes in air. Though it was born in a laboratory, it is now made by the tonne inside nuclear reactors as an unavoidable by-product, and it is one of the isotopes that waste planners worry about most.
Where it is found
Traces of neptunium do occur naturally. Stray neutrons in uranium ore are occasionally captured by uranium-238 atoms, which decay to neptunium-239 and then to plutonium. The quantities are unimaginably small, and for practical purposes neptunium is a man-made element.
The real source is nuclear fuel. Inside a reactor, uranium-235 sometimes captures neutrons without splitting, and after two captures and a beta decay it becomes neptunium-237. Uranium-238 can also lose a neutron in a fast collision and end up as the same isotope. A large reactor produces several kilograms of neptunium-237 a year, and worldwide stocks in spent fuel run to tens of tonnes. It can be separated chemically when fuel is reprocessed.
How it was discovered
Edwin McMillan and Philip Abelson at the University of California, Berkeley, found neptunium in 1940. McMillan had been bombarding uranium with neutrons from the 60-inch cyclotron and studying the fission fragments, which recoil a long way from the target. He noticed a radioactive product that barely moved at all, suggesting a heavy nucleus rather than a light fragment. Abelson joined him and showed by chemistry that it was not uranium or any known element.
They had made neptunium-239, which lives for a little over two days. Following the planetary naming set by uranium, they called it neptunium. Earlier claims for element 93 in the 1930s had turned out to be fission products that were misinterpreted, and neptunium was the first transuranium element confirmed beyond doubt.
What we use it for
Neptunium-237 has one important job: it is the raw material for plutonium-238. Targets of neptunium-237 are placed in a reactor, absorb a neutron and decay to plutonium-238, the isotope that powers radioisotope generators on deep-space probes and Mars rovers. Without neptunium there would be no fresh plutonium-238 supply.
Neptunium-237 is also used in neutron detection equipment because it fissions readily when struck by fast neutrons, giving a direct measure of high-energy neutron flux. Beyond that, its uses are limited to research into actinide chemistry, where its unusual range of oxidation states makes it a useful test case for theories about the 5f electrons.
Reading the numbers
Neptunium melts at a modest 639 °C, lower than aluminium at 660 °C, yet it boils at around 4,170 °C. That enormous gap between melting and boiling is one of the widest of any element. Its density of about 20.4 grams per cubic centimetre is higher than uranium and gold, making it one of the densest actinides and around two and a half times as dense as iron. The configuration [Rn] 5f4 6d1 7s2 continues the filling of the 5f shell. In neptunium these electrons remain mobile enough to take part in bonding, so the metal shows oxidation states from +3 to +7, each with a different colour in solution. The +5 state is the most stable in water, which is unusual among actinides and matters for how it moves through the environment.
Safety and the environment
Neptunium-237 has a half-life of about 2.1 million years. That is long enough that after several thousand years, once shorter-lived isotopes have decayed, it becomes one of the main contributors to the radiotoxicity of spent nuclear fuel. Its +5 ion dissolves in water and is not strongly held by clay or rock, so it could migrate from a waste repository faster than plutonium. It is an alpha emitter that settles in bone and liver, and it is handled only in shielded facilities. Because fast neutrons can fission it, stocks of separated neptunium are also tracked under international safeguards.
Did you know?
- Neptunium-239, the isotope found in 1940, lasts just over two days before turning into plutonium-239; it is the route by which all reactor plutonium is made.
- The mistaken belief in the 1930s that element 93 had already been found helped delay the discovery of fission, because chemists were looking for heavy elements instead of split nuclei.
- Neptunium has been detected in tiny quantities in uranium ore, formed by natural neutron capture.
- Neptunium is denser than uranium: a cube of neptunium 10 centimetres on each side would weigh about 20 kilograms.
- Every space probe carrying a plutonium-238 power source depends on neptunium-237 targets irradiated in a reactor.
More in the actinides
The nearest neighbours of neptunium in the family, by atomic number.
See all 15 actinides on one page, or browse all 118 elements.
Frequently asked questions
Is neptunium found in nature?
Only in trace quantities within uranium ores, where it is formed by neutron capture. For practical purposes it is a synthetic element made in nuclear reactors.
What is neptunium used for?
Its main use is as a target material for making plutonium-238, the heat source in spacecraft power generators. It also appears in some detectors for fast neutrons.
Is neptunium dangerous?
Yes. It emits alpha particles and accumulates in bone if taken into the body. Its very long half-life and mobility in water make it a long-term concern in nuclear waste.
Why is it called neptunium?
It follows uranium in the periodic table just as Neptune follows Uranus in the solar system. Plutonium, the next element, was named after Pluto to complete the sequence.
Read next
Guess the mystery element in six tries. A new one every day, free, no sign-up.
Written by Anthony. Updated September 2026. Numerical data from Periodic-Table-JSON (CC BY-SA), cross-checked against the Neptunium reference article. Spotted an error? Tell us.