![Neptunium element card: symbol Np, atomic number 93, atomic mass [237], electron configuration [Rn] 5f⁴ 6d¹ 7s², oxidation states +5, Actinide, f-block, period 7, solid at room temperature.](https://stage.chemistry-dictionary.com/wp-content/uploads/2026/09/neptunium-chemical-element.webp)
A transuranic element has an atomic number higher than 92. It sits just past uranium on the periodic table. Active nuclear reactors produce most neptunium today.
It forms as a radioactive byproduct while the reactor runs. This happens when uranium fuel rods absorb extra neutrons. Trace amounts of neptunium also exist naturally in deep uranium ores.
These natural traces form through rare changes caused by background neutrons. This process is called transmutation. Transmutation means one chemical element changes into a totally different element.
The most stable type of neptunium is neptunium-237. It has a very long half-life of over two million years. A half-life is the time it takes for half of a radioactive sample to break down.
Neptunium-237 breaks down by shooting out energetic alpha particles. Neptunium has a very complex chemistry. It can take on many different oxidation states, from +3 to +7.
An oxidation state tells us how many electrons an atom gains, loses, or shares. Neptunium easily forms many complex chemical compounds. These include oxides, halides, and ions mixed in water.
The pure metal takes three different solid crystal shapes. We call these different shapes allotropes. The metal changes between these shapes as it gets hotter.
Neptunium-237 creates a big problem for storing nuclear waste safely. It stays dangerous for a long time and moves easily through dirt and rock.