Neptunium – facts and properties
- Symbol
- Np
- Atomic number
- 93
- Atomic mass
- 237 u
- Period
- 7
- Category
- actinide
- Electron configuration
- [Rn] 5f⁴ 6d¹ 7s²
- Electronegativity
- 1.36
- Atomic radius
- 175 pm
- Valence electrons
- 7
- State at standard conditions
- solid
- Melting point
- 644 °C
- Boiling point
- 3902 °C
- Density
- 20.25 g/cm³
- Discovered
- 1940
Neptunium is a silvery, radioactive metal with atomic number 93 and the first transuranic element, that is, the first one beyond uranium. Edwin McMillan and Philip Abelson made it artificially in Berkeley in 1940 and named it after the planet Neptune, which follows Uranus in the solar system. Only tiny traces occur in nature. Today neptunium is produced in nuclear reactors and serves as the starting material for plutonium-238.
Position in the periodic table
Occurrence and extraction
In nature neptunium occurs only in tiny traces, in uranium ores where neutrons strike uranium nuclei. All its isotopes are radioactive, and the longest-lived is neptunium-237, with a half-life of just over two million years. That is far shorter than the age of the Earth, so any original neptunium decayed long ago.
In nuclear reactors it forms as a by-product when uranium nuclei capture neutrons. In spent fuel, neptunium-237 is one of the long-lived substances that keep the waste radioactive for a very long time. It is separated from such material for research.
As early as 1934 Enrico Fermi believed he had made a new element by bombarding uranium with neutrons. It later turned out that he had produced fission products. In 1940 McMillan and Abelson irradiated uranium with neutrons from a particle accelerator. Uranium-238 absorbed a neutron, and the uranium-239 formed turned by beta decay into neptunium-239, whose half-life is just under two and a half days. The two showed by chemical tests that the new radiation came from an unknown element. The name neptunium follows on from uranium, since Neptune comes after Uranus.
Properties and reactions
Neptunium is in period 7 and is the fifth actinoid, in the actinoid row of the table between uranium on its left and plutonium on its right. The atom's electron configuration is [Rn] 5f⁴ 6d¹ 7s², with an electronegativity of 1.36 and an atomic radius of 175 pm. Neptunium has a density of 20.25 g/cm³ and melts at 644 °C. The boiling point is only extrapolated. Its atomic mass is given as 237 u, the mass number of the longest-lived isotope, neptunium-237.
The metal is dense and silvery and tarnishes in air. In compounds it shows oxidation states from plus three to plus seven. In aqueous solution plus five is the most stable, as the neptunyl ion NpO₂⁺. This ion dissolves well and travels easily in groundwater, which matters when searching for safe repositories for radioactive waste.
Neptunium-237 emits alpha particles and weak gamma rays. Because of the long half-life the radiation per gram is low, but if the element enters the body it settles in bone and liver and irradiates the tissue for a long time. Specialists therefore work in sealed glove boxes. Little is known about its chemical toxicity.
The natural decay series of thorium and uranium end in lead. The neptunium series ends instead in bismuth-209. It can no longer be found in nature, because its longest-lived member, neptunium-237, is too short-lived compared with the age of the Earth.
Research and synthesis
There is no everyday use. Neptunium matters as the starting material for plutonium-238. For this, neptunium-237 is irradiated with neutrons in a research reactor. This forms neptunium-238, which becomes plutonium-238 by beta decay after a few days. That isotope supplies heat and electricity for space probes in radioisotope batteries. In this way neptunium from reactor waste indirectly helps to power space missions.
Neptunium-237 is also an object of research. Chemists study how it behaves in solutions, rocks and soils, in order to predict whether and how fast long-lived waste could migrate out of a repository.
Quick question
What is the atomic number of neptunium (Np)?
FAQ
Why is neptunium called neptunium?
It follows uranium in the periodic table, just as the planet Neptune follows Uranus in the solar system. Uranium is named after the planet Uranus, so McMillan and Abelson chose the next planet.
Does neptunium occur in nature?
Only in tiny traces in uranium ores. Original neptunium decayed long ago, because its half-life is short compared with the age of the Earth. Meaningful amounts form in nuclear reactors.
What is neptunium used for?
Almost only for research and to make plutonium-238, which supplies heat and electricity in the radioisotope batteries of space probes.