Neptunium
Atomic Data
| Atomic Number | 93 |
| Symbol | Np |
| Atomic Weight | 237 u |
| Density (STP) | 20.25 g/cm³ |
| Melting Point | 638.85 °C (912 K) |
| Boiling Point | 4173.85 °C (4447 K) |
| Electronegativity | 1.36 (Pauling) |
| Electron Config. | 1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p6 4d10 4f14 5s2 5p6 5d10 5f4 6s2 6p6 6d1 7s2 |
| Oxidation States | +3, +4, +5, +6, +7 |
| Phase at STP | Solid |
| Category | Actinoid |
| Period / Group | 7 / None |
| CAS Number | 7439-99-8 |
Electron Configuration
[Rn] 5f4 6d1 7s2
| Shell | n | Subshell | Electrons | Cumulative |
|---|---|---|---|---|
| K | 1 | 1s | 2 | 2 |
| L | 2 | 2s | 2 | 4 |
| L | 2 | 2p | 6 | 10 |
| M | 3 | 3s | 2 | 12 |
| M | 3 | 3p | 6 | 18 |
| M | 3 | 3d | 10 | 28 |
| N | 4 | 4s | 2 | 30 |
| N | 4 | 4p | 6 | 36 |
| N | 4 | 4d | 10 | 46 |
| N | 4 | 4f | 14 | 60 |
| O | 5 | 5s | 2 | 62 |
| O | 5 | 5p | 6 | 68 |
| O | 5 | 5d | 10 | 78 |
| O | 5 | 5f | 4 | 82 |
| P | 6 | 6s | 2 | 84 |
| P | 6 | 6p | 6 | 90 |
| P | 6 | 6d | 1 | 91 |
| Q | 7 | 7s | 2 | 93 |
| Total | 93 | 93 | ||
Isotopes of Neptunium
Neptunium is monoisotopic: ²³⁷Np is its only naturally occurring stable isotope, accounting for 100% of all natural Neptunium.
| Isotope | Symbol | Protons | Neutrons | Abundance | Stability |
|---|---|---|---|---|---|
| Neptunium-237 | ²³⁷Np | 93 | 144 | trace | Stable |
Abundance & Occurrence
Neptunium is present in Earth's crust at approximately trace amounts by mass and at approximately trace amounts by mass throughout the universe.
Earth's Crust (ppm by mass)
Universe (ppm by mass)
Discovery & History
Read more about the discovery of the periodic table of elements →
Safety & Handling
- Radioactivity: Neptunium-237 (t½ = 2.14 million years, alpha emitter) is a long-lived radiotoxin; it is produced in spent nuclear fuel and accumulates in nuclear waste repositories as a long-term radiological concern.
- Internal dose hazard: Np-237 concentrates in bone and liver if ingested or inhaled; all handling must prevent contamination and ingestion using glove boxes, respiratory protection, and rigorous contamination monitoring.
- Criticality risk: Np-237 can sustain a nuclear chain reaction; accumulation of sufficient quantities in a compact geometry creates a criticality hazard: handled under strict mass limits in licensed facilities.
- Regulatory controls: All neptunium work requires national nuclear regulatory authority licensing, detailed accountancy of quantities, and reporting to relevant safeguards authorities.
Neptunium in the Real World
Real-World Uses
- Long-range neutron detection: Neptunium-237 (t½ = 2.1 My) fission chambers exploit Np-237's low fission threshold with fast neutrons to detect neutrons at distances too great for conventional thermal-neutron detectors; used in nuclear safeguards and arms control monitoring.
- Production of plutonium-238: Np-237 irradiated in nuclear reactors captures neutrons to form Np-238, which beta-decays to Pu-238; Pu-238 is the preferred radioisotope for radioisotope thermoelectric generators (RTGs) in deep space probes such as Voyager and New Horizons.
- Nuclear fuel cycle research: Neptunium is a long-lived minor actinide produced in nuclear reactors; its management (transmutation by fast neutrons) is studied as part of advanced nuclear fuel cycle strategies aimed at reducing the long-term radiotoxicity of high-level nuclear waste.
Downloadable Resources
Free periodic table reference sheets for classrooms, study sessions, and laboratory use.
Frequently Asked Questions
What is neptunium used for?
Neptunium has very few practical applications. Neptunium-237 is used in research nuclear reactors as a target to produce plutonium-238, which is the fuel in radioisotope thermoelectric generators (RTGs) used to power deep space probes. Np-237 can also be used in nuclear weapon design (it is fissile under fast neutron conditions). Otherwise, neptunium is primarily a subject of scientific study.
Is neptunium the first transuranium element?
Yes. Neptunium (Z=93) was the first transuranium element: the first element with an atomic number greater than that of uranium (92): to be synthesised. It was produced in 1940 at the University of California, Berkeley, by Edwin McMillan and Philip Abelson. It is named after Neptune, the planet beyond Uranus, continuing the planetary naming convention (uranium after Uranus, neptunium after Neptune, and plutonium after Pluto).
How was neptunium discovered?
Neptunium was synthesised in February 1940 by Edwin McMillan and Philip Abelson at Berkeley. They bombarded uranium-238 with neutron beams and observed that uranium-239, formed by neutron capture, decayed by beta emission to form element 93. McMillan shared the 1951 Nobel Prize in Chemistry for the discoveries of the transuranium elements (with Glenn Seaborg for the discovery of plutonium and later elements).
Does neptunium occur naturally?
Yes, but only in trace amounts. Tiny quantities of neptunium-237 (half-life 2.14 million years) and neptunium-239 (half-life 2.36 days) are produced naturally in uranium ores by spontaneous fission of uranium and neutron capture reactions driven by cosmic rays. Neptunium-237 is present in uranium ores at about 1 part per trillion relative to uranium: vanishingly small but detectable by sensitive mass spectrometry.