- Neptunium (Np) is a silvery, metallic, radioactive element with atomic number 93, belonging to the actinide series.
- Its electron configuration is [Rn] 5f⁴ 6d¹ 7s², and it exhibits multiple oxidation states, most commonly +5, but also +3, +4, +6, and +7. The atomic structure contains ninety-three protons, typically one hundred and forty-four neutrons in its most common isotope, and ninety-three electrons arranged in seven shells.
- Neptunium does not occur in significant quantities in nature, but trace amounts of neptunium-237 (²³⁷Np) are formed naturally in uranium ores by neutron capture in uranium-235, followed by beta decay. The isotope ²³⁷Np, with a half-life of about 2.14 million years, is the most stable and significant isotope of the element.
- Neptunium was the first synthetic transuranium element to be discovered. It was identified in 1940 by Edwin McMillan and Philip Abelson at the University of California, Berkeley, when they bombarded uranium-238 (²³⁸U) with neutrons in a cyclotron, producing ²³⁹U, which beta decayed into ²³⁹Np. The element was named after Neptune, the planet beyond Uranus, following the naming pattern set by uranium (Uranus) and neptunium (Neptune).
- Although neptunium has no widespread commercial applications, it is important in nuclear science. Neptunium-237 can be used as a precursor for producing plutonium-238 (²³⁸Pu), which is a vital isotope for powering radioisotope thermoelectric generators (RTGs) in deep-space probes and equipment. Neptunium is also relevant in nuclear reactor physics and is a component of high-level nuclear waste, requiring long-term storage solutions due to its long half-life and radiotoxicity.
- Chemically, neptunium is highly reactive, especially in finely divided form. It tarnishes in air to form an oxide layer and reacts readily with halogens, acids, and some alkalis. Its chemistry is complex, with multiple oxidation states leading to various oxides (NpO₂, Np₂O₅), halides (NpF₃, NpCl₄), and oxycations such as neptunyl (NpO₂⁺ and NpO₂²⁺).
- Biologically, neptunium has no known role in living systems and is highly toxic due to its alpha radiation. Once inside the body, it tends to accumulate in bones and liver, posing a long-term cancer risk.
- Environmentally, neptunium is not naturally abundant, but it is produced in nuclear reactors and during nuclear weapons production. Because of its long half-life, it is a significant contributor to the long-term radiotoxicity of nuclear waste, making its management a critical issue in radioactive waste disposal.
