Neptunium (Np), with atomic number 93, is the first transuranic element ever synthesized. Transuranic elements are defined as those possessing an atomic number greater than 92, the atomic number of naturally occurring uranium. Its creation marked a significant turning point in nuclear chemistry. The successful synthesis and isolation of this element is attributed to the collaborative work of American scientists Edwin McMillan and Philip Abelson.
Defining the First Transuranic Element
Neptunium is an actinide metal, a group of elements typically displayed below the main body of the periodic table. Its position immediately following uranium (U) gives it the symbol Np and the atomic number 93. Although considered synthetic, trace amounts of neptunium isotopes are found in nature within uranium ores, created through natural neutron capture reactions.
As an actinide, it is a dense, silvery, and highly radioactive element. All isotopes of neptunium are unstable, undergoing radioactive decay. Neptunium-237 (\(\text{Np-237}\)) is the most stable isotope, possessing a half-life of approximately 2.14 million years.
The discovery of Neptunium-239 (\(\text{Np-239}\)) in 1940 proved that elements heavier than uranium could be artificially created. This confirmed a new chemical class and paved the way for the synthesis of many more elements. The element exhibits a variety of oxidation states in solution, ranging from +3 to +7.
The 1940 Discovery: McMillan and Abelson
The initial observation leading to the element’s discovery occurred at the University of California, Berkeley Radiation Laboratory. Edwin McMillan was investigating the products formed when uranium was bombarded with neutrons generated by the 60-inch cyclotron. He noticed an unusual radioactive product that did not behave like a fission fragment or any known element.
This product exhibited a half-life of 2.3 days and was initially identified as an isotope of uranium. McMillan suspected it was a decay product of a heavier element, Element 93, forming after neutron capture. The specific reaction involved bombarding \(\text{U-238}\) with neutrons, which created the unstable isotope \(\text{U-239}\).
The \(\text{U-239}\) atom then underwent beta decay, where a neutron transforms into a proton and a beta particle is emitted. This process increased the atomic number from 92 to 93, creating \(\text{Np-239}\). McMillan separated the decay product from the main uranium target, but the chemical identification remained inconclusive.
Philip Abelson, a physical chemist visiting Berkeley, joined the effort to confirm the element’s existence. Abelson used refined chemical separation techniques to isolate the observed beta emitter. His methods demonstrated that the radioactive substance possessed distinct chemical properties, different from both uranium and lighter fission products. This confirmed that the activity belonged to a new element.
Chemical Confirmation and Naming
Abelson’s chemical work was crucial for formally proving the element’s identity. The initial search for Element 93 assumed it would chemically resemble Rhenium, as predicted by the periodic table structure of the time. Abelson’s radiochemical analysis showed that the element did not share the predicted properties.
Instead, the behavior of the element suggested a new chemical series was beginning after uranium. This finding supported the realization that neptunium belonged to the Actinide series, a group of elements with similar electron configurations. The scientific community accepted the discovery upon the publication of the results in May 1940.
The choice of name followed the astronomical convention established by uranium. Uranium was named after the planet Uranus, which was the most recently discovered planet when it was named in 1789. The scientists named the new element neptunium after Neptune, the next planet beyond Uranus. This naming tradition was continued with the next element, plutonium, named after the dwarf planet Pluto.
Neptunium Today: Sources and Significance
Today, neptunium is produced primarily as a byproduct within nuclear reactors, extracted from spent nuclear fuel rods. It forms when uranium fuel (\(\text{U-238}\) and \(\text{U-235}\)) captures neutrons during the reactor’s operation. The long-lived isotope \(\text{Np-237}\) is the most prevalent form found in this nuclear waste, created in kilogram quantities annually in large power reactors.
The element is scientifically significant for its role as a precursor material. Bombarding \(\text{Np-237}\) with neutrons is a process used to synthesize the isotope \(\text{Pu-238}\). Plutonium-238 is a powerful heat source used in radioisotope thermoelectric generators, providing long-duration power for deep-space probes and remote terrestrial applications.
Neptunium is also a substance of interest in nuclear waste management due to its long half-life and mobility in the environment. Understanding its chemistry is important for the secure, long-term storage of nuclear waste. \(\text{Np-237}\) is used in specialized equipment for detecting high-energy neutrons in physics research.