Neptunium
actinideProperties
| Property | Value |
|---|---|
| Atomic Mass | 237 amu |
| Category | actinide |
| Period | 7 |
| Electron Configuration | [Rn] 5f4 6d1 7s2 |
| Electronegativity | 1.36 (Pauling) |
| Oxidation States | 7, 6, 5, 4, 3 |
| Melting Point | 917 K (643.9 °C) |
| Boiling Point | 4175 K (3901.8 °C) |
| Density | 20.45 g/cm³ |
| Discovered By | Edwin McMillan, Philip Abelson (1940) |
About Neptunium
Neptunium was the first transuranium — McMillan and Abelson made it in 1940 at Berkeley by bombarding ²³⁸U with neutrons and watching the beta-decay product behave like a new element rather than the rare-earth chemistry everyone expected. That misidentification is what eventually forced Seaborg's redrawing of the periodic table to include the actinide series. Np shows the full +3 through +7 oxidation state range, but +5 (the dioxoneptunyl cation NpO₂⁺) dominates aqueous chemistry, which is exactly why it's a headache for spent-fuel reprocessing: NpO₂⁺ is a singly-charged linear cation that doesn't extract cleanly into TBP/kerosene like the doubly-charged actinyls. ²³⁷Np has a 2.14 million year half-life and accumulates in reactor fuel as a decay product of ²⁴¹Am and through (n,2n) reactions on ²³⁸U; the tonnes of it sitting in spent fuel are also the feedstock for ²³⁸Pu production via neutron capture, which is what powers RTGs on Voyager, Cassini, and the Mars Science Laboratory.
Fun Fact
Neptunium was the first element humans created beyond what nature provides — its discovery in 1940 opened the door to an entirely new frontier of synthetic elements that now extends to element 118.
Common Uses
- Target material for ²³⁸Pu production via neutron capture
- Active component in some neutron detectors (high fission cross-section)
- Reference standard for actinide separation chemistry research
- Marker isotope for tracking spent-fuel reprocessing streams
- Ongoing studies for partitioning and transmutation of nuclear waste