Neptunium (Np)
Neptunium (symbol Np) is the chemical element with atomic number 93: an actinide, solid at 0 °C, radioactive, with an atomic mass of 237 u. Discovered in 1940 (Edwin McMillan and Philip Abelson).
Neptunium is the first “transuranium” element — heavier than uranium. It was first made in 1940 at Berkeley and named after Neptune, the planet beyond Uranus. It forms in significant amounts in the fuel of nuclear power plants. Its most stable isotope, neptunium-237, has a half-life of 2.1 million years.
Key facts
- Atomic number
- 93
- Symbol
- Np
- French name
- Neptunium
- Atomic mass
- 237 u
- Molar mass
- [237] g/mol
- Family
- Actinides
- Position
- period 7, f-block
- State at 0 °C
- Solid
- Melting point
- 638.9 °C
- Boiling point
- 4,173.9 °C
- Density
- 20.45 g/cm³
- Electronegativity (Pauling)
- 1.36
- First ionization energy
- 605 kJ/mol
- Atomic radius
- 175 pm
- Electron configuration
- [Rn] 5f⁴ 6d¹ 7s²
- Indicative price
- 660,000 $/kg (order of magnitude)
- Radioactive
- yes
- Discovery
- 1940, Edwin McMillan and Philip Abelson
Origin of the name
From the planet Neptune, Uranus's neighbor.
What does neptunium look like?
A dense (about 20 g/cm³), silvery and fairly reactive metal that tarnishes in air. It takes on several crystal structures depending on temperature.
Where is neptunium found?
Tiny traces form naturally in uranium ores when uranium captures a neutron. Most of the neptunium on Earth, however, was made by humans, in nuclear reactors and during atomic weapons tests.
Natural sources
- Traces in uraninite
How is neptunium produced?
It is recovered when spent nuclear fuel is reprocessed, since the uranium captured neutrons during its time in the reactor. The world's reactors produce several tonnes of it every year, a small part of which is separated.
Top producers
- United States
- Russia
- France (reprocessing)
What is neptunium used for?
- Raw material for making the plutonium-238 that powers space generators
- High-energy neutron detectors
- Research on nuclear waste and its transmutation
In everyday life
- None
Role in living things
No biological role. Once absorbed, it builds up in the bones and liver.
Hazards of neptunium
Radioactive and radiotoxic, especially when inhaled. Neptunium-237 is one of the worrying components of long-lived nuclear waste, because it moves fairly easily through the environment.
Cosmic origin
- Human-made: Does not exist (or barely) in nature: synthesized in a reactor or accelerator.
History of neptunium
Enrico Fermi thought he had created element 93 in 1934 (“ausonium”), but he had actually triggered fission without realizing it. In 1940, Edwin McMillan and Philip Abelson at Berkeley bombarded uranium with neutrons and identified neptunium-239. McMillan would share the 1951 Nobel Prize in Chemistry with Glenn Seaborg.
Did you know?
- It was named after Neptune because it follows uranium just as the planet Neptune follows Uranus; plutonium would follow the same logic with Pluto.
- A sphere of about 60 kg of neptunium-237 could in theory reach critical mass, which is why it is monitored under nonproliferation rules.
- In 1939, France's Yvette Cauchois and Romania's Horia Hulubei believed they had found it in nature and called it “sequanium,” after the Seine.
Isotopes of neptunium
No stable isotopes; neptunium-237, the longest-lived, is a long-lasting waste product of nuclear reactors. Tiny traces of it also form in uranium ores.
- neptunium-236: radioactive, half-life 153,000 years
- neptunium-237: radioactive, half-life 2.14 × 10⁶ years. Irradiated in reactors to make the plutonium-238 that powers space probes.
- neptunium-239: radioactive, half-life 2.36 days. The element's first synthesized isotope, in 1940; it turns into plutonium-239.
Related elements
Same family (actinides): Actinium (Ac), Thorium (Th), Protactinium (Pa), Uranium (U), Plutonium (Pu), Americium (Am), Curium (Cm), Berkelium (Bk), Californium (Cf), Einsteinium (Es), Fermium (Fm), Mendelevium (Md), Nobelium (No), Lawrencium (Lr).