Neptunium
Neptunium was the first element ever made heavier than uranium, proving that the periodic table didn't stop there. It's a dense, silvery, highly radioactive metal that essentially doesn't occur naturally in meaningful amounts — almost all of it is a byproduct of nuclear reactors rather than a mined resource.
- Group · Period
- — · 7
- At room temp
- solid
- Melts at
- 917 K
- Density
- 20.25 g/cm³
- Discovered
- 1940
Uses
Neptunium doesn’t have an established commercial or industrial use. It’s overwhelmingly a byproduct of running nuclear reactors, where uranium-238 in the fuel absorbs neutrons and eventually decays into neptunium-237, and it’s mostly of interest today as a component of nuclear waste that researchers study for long-term storage and reprocessing strategies. A small number of specialized neutron-detection setups have made use of neptunium’s fission properties, but this remains a narrow research application rather than anything approaching everyday use.
History
Neptunium was synthesized in 1940 by Edwin McMillan and Philip Abelson at the University of California, Berkeley, who bombarded uranium-238 with neutrons and observed it decay into a new element. It was the first direct evidence that elements heavier than uranium could exist, overturning the long-held assumption that uranium sat at the end of the periodic table. That breakthrough opened the door to the entire transuranic and transactinide series that followed over the next several decades.
Fun facts
- Neptunium was the first synthetic transuranium element ever made, produced in 1940 — just a year before plutonium followed it.
- It continues the solar-system naming pattern started with uranium: Uranus, then Neptune, then (with the next element) Pluto.
- Its longest-lived isotope, neptunium-237, has a half-life of about 2.1 million years and slowly builds up in spent nuclear fuel.
Frequently asked questions
Does neptunium have any real-world use?
Barely. It has no commercial or industrial role — it's mostly a byproduct that accumulates in used nuclear fuel and a subject of research into nuclear waste management. A handful of specialized neutron-detection instruments have used neptunium-237, but this is a niche research application, not a widespread use.
Where does neptunium come from if it isn't mined?
It forms inside nuclear reactors when uranium-238 absorbs neutrons and undergoes a chain of nuclear reactions. Trace natural amounts exist in uranium ores for the same reason, but in quantities far too small to be a practical source.
Why is it named neptunium?
It follows directly from uranium's name: uranium was named after Uranus, and since neptunium sits one element beyond it, it was named after the next planet out, Neptune.
Compounds
2 notable compounds containing Np
- NpO2oxide
Neptunium dioxide
A brownish-green solid sharing the fluorite structure of uranium dioxide, and the most stable and best-characterized neptunium compound known. It forms readily when neptunium metal or its salts are heated in air.
Used for: Standard chemical form for storing and studying neptunium
- NpF6halide
Neptunium hexafluoride
A volatile, reddish-brown crystalline solid, structurally analogous to uranium and plutonium hexafluoride, that sublimes at moderate temperatures. Its volatility has made it a useful probe compound in laboratory studies of actinide separation chemistry.
Isotopes
25 known isotopes
Swipe to see all columns →
| Isotope | Natural abundance | Half-life | Decay mode | Mass excess | Binding energy / nucleon | Decay width |
|---|---|---|---|---|---|---|
| Np-235 | — | 396.1 d | Electron capture (99.9974%), Alpha decay (0.0026%) | 41,043.04 keV | 7,587.06 keV | 1.33 × 10-23 eV |
| Np-236 | — | 1.55E+5 Y | Electron capture (87.8%), Beta-minus decay (12%), Alpha decay (0.16%) | 43,378.09 keV | 7,579.21 keV | 9.33 × 10-29 eV |
| Np-237 | — | 2.144E+6 Y | Alpha decay (100%), Spontaneous fission (2e-10%) | 44,871.6 keV | 7,574.99 keV | 6.74 × 10-30 eV |
Show all 25 isotopes
| Np-219 | — | 0.15 ms | Alpha decay (100%), Proton emission | 29,436.95 keV | 7,604.67 keV | 3.04 × 10-12 eV |
| Np-220 | — | 25 us | Alpha decay (100%) | 30,475.02 keV | 7,602.08 keV | 1.82 × 10-11 eV |
| Np-222 | — | 0.38 us | Alpha decay (100%) | 31,274.64 keV | 7,602.7 keV | 1.20 × 10-9 eV |
| Np-223 | — | 2.2 us | Alpha decay (100%) | 30,658.58 keV | 7,607.57 keV | 2.07 × 10-10 eV |
| Np-224 | — | 38 us | Alpha decay (100%) | 32,032.25 keV | 7,603.5 keV | 1.20 × 10-11 eV |
| Np-225 | — | 3.6 ms | Alpha decay (100%), Electron capture / beta-plus decay | 31,618.1 keV | 7,607.42 keV | 1.27 × 10-13 eV |
| Np-226 | — | 35 ms | Alpha decay (100%) | 32,816.88 keV | 7,604.17 keV | 1.30 × 10-14 eV |
| Np-227 | — | 0.51 s | Alpha decay (100%) | 32,579.02 keV | 7,607.28 keV | 8.95 × 10-16 eV |
| Np-228 | — | 61.4 s | Electron capture (60%), Alpha decay (40%) | 33,825 keV | 7,604 keV | 7.43 × 10-18 eV |
| Np-229 | — | 4.0 m | Alpha decay (68%), Electron capture (32%) | 33,801.38 keV | 7,605.99 keV | 1.90 × 10-18 eV |
| Np-230 | — | 4.6 m | Electron capture / beta-plus decay (97%), Alpha decay (3%) | 35,236.62 keV | 7,601.78 keV | 1.65 × 10-18 eV |
| Np-231 | — | 48.8 m | Electron capture / beta-plus decay (98%), Alpha decay (2%) | 35,623.69 keV | 7,602.13 keV | 1.56 × 10-19 eV |
| Np-232 | — | 14.7 m | Electron capture / beta-plus decay (100%), Alpha decay (0.0002%) | 37,359 keV | 7,597 keV | 5.17 × 10-19 eV |
| Np-233 | — | 36.2 m | Electron capture (100%), Alpha decay (0.0007%) | 37,948.53 keV | 7,596.18 keV | 2.10 × 10-19 eV |
| Np-234 | — | 4.4 d | Electron capture / beta-plus decay (100%) | 39,954.81 keV | 7,589.64 keV | 1.20 × 10-21 eV |
| Np-238 | — | 2.099 d | Beta-minus decay (100%) | 47,454.6 keV | 7,566.22 keV | 2.52 × 10-21 eV |
| Np-239 | — | 2.356 d | Beta-minus decay (100%) | 49,311.01 keV | 7,560.57 keV | 2.24 × 10-21 eV |
| Np-240 | — | 61.9 m | Beta-minus decay (100%) | 52,316.23 keV | 7,550.17 keV | 1.23 × 10-19 eV |
| Np-241 | — | 13.9 m | Beta-minus decay (100%) | 54,315.12 keV | 7,544.04 keV | 5.47 × 10-19 eV |
| Np-242 | — | 2.2 m | Beta-minus decay (100%) | 57,416.88 keV | 7,533.4 keV | 3.46 × 10-18 eV |
| Np-243 | — | 1.85 m | Beta-minus decay (100%) | 59,806 keV | 7,526 keV | 4.11 × 10-18 eV |
| Np-244 | — | 2.29 m | Beta-minus decay (100%) | 63,240 keV | 7,514 keV | 3.32 × 10-18 eV |