In 1940, a new element was discovered by bombarding uranium oxide with neutrons in the Berkeley cyclotron. It was the first element to be synthesized in the series of heavy elements known as the transuranium elements. When considering an appropriate name for this element, it was noted that this new element followed uranium on the periodic table. Then noting that uranium was named for the planet Uranus, it was decided to name the new element for the planet that comes after Uranus in our solar system, Neptune.

Originally, scientists thought that neptunium was a purely artificial element. Early samples of neptunium was prepared by bombarding uranium-238 with high-energy neutrons to create uranium-239. One of the decay products yielded was the neptunium isotope. Later on, researchers found minute amounts of neptunium in naturally occurring deposits of uranium ore. There is far more artificially produced neptunium than there is naturally occurring neptunium.

Today, elemental neptunium is produced in kilogam quantities as a byproduct of breeder reactors where uranium-235 or uranium-238 is converted to plutonium and other impurities including neptunium-237. Shown above is a sample of neptunium dioxide.