Neptunium (Np)
“The first element forged beyond uranium — made in a Berkeley cyclotron in 1940 and named, like its neighbour, for the next planet out: Neptune.”
How its electrons are arranged
Computed from the aufbau principle, with known real-world exceptions applied automatically.
The notes
Characteristics
- Electron configuration [Rn] 5f⁴ 6d¹ 7s² — period 7, f-block actinide; it shows oxidation states from +3 up to +7, and in water chemistry the calm +5 neptunyl ion Np(V) holds the throne.
- Its workhorse isotope Np-237, half-life 2.14 million years, is a long-lived alpha emitter — the very reason neptunium heads the list of million-year hazards in buried nuclear waste.
- Nature keeps only fingerprints: inside uranium ores the stray neutron from rare spontaneous fissions occasionally builds a neptunium atom, so the element is natural — but only just.
- Fissionable by fast neutrons and the decay-parent of americium: Am-241 of household smoke detectors slowly crumbles into Np-237, linking the two elements through decay alone.
A silvery, ductile metal that swiftly wraps itself in an oxide film and settles into a steely sheen. It is a dense actinide nearly as heavy as uranium, melts near 644 °C, and reshuffles into three different crystal forms as the temperature changes.
Where you meet it
✦ Uses
- Growing plutonium-238 for space batteries: NASA’s restarted Oak Ridge production irradiates neptunium-237 targets, and the Pu-238 born from them drives the RTGs aboard rovers and probes like Curiosity and Perseverance.
- Neutron detection: fission chambers plated with Np-237 answer only to fast neutrons, so reactor physicists deploy them to chart a reactor’s neutron energy spectrum.
- A pure research object: it anchors the study of the 5f electron series and of how actinides creep through rock, clay and groundwater — the data on which nuclear-waste repositories are designed.
- An unwanted weapon fact: in 1962 the United States detonated an underground device partly built from neptunium-237 to prove it could fuel a bomb, which is why the isotope is now guarded like weapons material.
◍ Everyday examples
- News stories about nuclear-powered spacecraft — Voyager and its heirs — are really Pu-238 stories, and Pu-238 begins life as a neptunium target, so the element rides along on every such mission.
- Debates over burying spent reactor fuel for a million years keep naming neptunium-237, because it outlives nearly everything else in the waste.
- The frontier arrow in textbook periodic tables — “elements beyond uranium” — begins exactly at neptunium, element 93.
History & name
In the spring of 1940 at the University of California, Berkeley, Edwin McMillan noticed that neutron-bombarded uranium kept producing a beta emitter unlike anything known. With the young physical chemist Philip Abelson he pinned the culprit down by that May: uranium-239 had beta-decayed into element 93 — the first transuranium element humanity had ever made. Following the naming custom begun with uranium, they called it neptunium after Neptune, the next planet beyond Uranus, and published openly — the metal’s military significance was not yet suspected. For opening the door to the synthetic-element era, McMillan shared the 1951 Nobel Prize in Chemistry.