It was 1940, and two physicists at Berkeley did something no one had ever done before: they created an element heavier than uranium. They bombarded uranium with neutrons in a cyclotron and got element 93. They named it neptunium, after Neptune — the planet that follows Uranus — just as this element follows uranium in the periodic table.
Neptunium is a silvery radioactive metal, nearly twice as dense as lead at 20.25 g/cm³. It barely exists in nature, yet nuclear reactors produce over a ton of it every year as a byproduct in spent fuel. Its most stable isotope, Np-237, has a half-life of 2.14 million years.
In pure form, neptunium is a shiny silvery metal that quickly tarnishes in air. It has three distinct crystal structures (allotropic forms) — more than most metals. It melts at 644 °C and boils at a scorching 3,902 °C. Chemically, neptunium is remarkably versatile: it can exist in five different oxidation states, from +3 to +7. No stable isotope of neptunium exists. Every single one is radioactive.
Neptunium is extremely dangerous. Np-237 is an alpha emitter with a half-life of 2.14 million years. If inhaled or ingested, it accumulates in bones and liver, causing cancer through prolonged internal radiation. Its critical mass is about 60 kg, requiring strict storage controls. All work with neptunium must be done inside sealed gloveboxes with radiation shielding and continuous monitoring.
Neptunium was the first transuranic element ever made. Edwin McMillan won the Nobel Prize in Chemistry in 1951 for this breakthrough.
The name is a cosmic analogy. Uranus → Neptune: planet Neptune follows Uranus, and element neptunium follows uranium in the periodic table.
Np-237 decays incredibly slowly — its half-life is 2.14 million years. It will outlast any civilization.
Nuclear reactors around the world produce over 1,000 kg of neptunium every year. It accumulates in spent fuel as a byproduct.
Neptunium holds a record among actinides: it has three crystal forms (α, β, γ). Most metals have just one or two.
Neptunium is turned into plutonium-238 for space batteries (RTGs). These batteries power Voyager, which is flying beyond the solar system right now.
| Isotope | Mass (u) | Abundance | Half-life | Decay |
|---|---|---|---|---|
236Np☢ | 236.046570 | synthetic | 1.54×10⁵ years | EC/β− |
237Np☢ | 237.048173 | synthetic | 2.144×10⁶ years | α |
Cyclotron bombardment of uranium