Neptunium Facts (Np or Atomic Number 93)

Neptunium Facts
Neptunium is an actinide element. All of its isotopes are radioactive.

Neptunium is a silver radioactive metal with atomic number 93 and element symbol Np. While you might think it’s an exotic element that you never encounter, it actually occurs in many smoke detectors as a decay product of americium-241. Here are interesting neptunium facts, including the its discovery, uses, sources, and health risks.

10 Interesting Neptunium Facts

  1. Neptunium is the element with atomic number 93. This means the nucleus of every neptunium atom contains 93 protons. Large atomic nuclei are inherently unstable, so all neptunium atoms are radioactive. There are at least 24 neptunium isotopes. The most stable is 237Np, with a half-life of 2.14 million years.
  2. The element gets its name for the planet Neptune. The order of the atomic numbers for uranium, neptunium, and plutonium reflects the order of the planets in the solar system.
  3. Edwin McMillan and Philip Abelson synthesized and discovered neptunium in 1940 at Berkeley, California. They bombarded a uranium target with neutrons. The beta radiation released by the treated target was the proof of the new element’s existence. Neptunium was the first synthetic transuranium element (element heavier than uranium) to be discovered.
  4. Neptunium occurs naturally, mainly in uranium ores, as a decay product of more radioactive elements and neutron capture from uranium atoms. But, most of the neptunium in the environment today comes from atomic testing. Radioactive waste from nuclear power plants is a source of large amounts of neptunium. Smaller amounts come from other radioactive waste (e.g., discarded smoke detectors).
  5. The main use for neptunium is as a precursor for making plutonium. The element finds use in physics for detecting high energy neutrons. Theoretically, neptunium could be used as nuclear reactor fuel or for nuclear weapons.
  6. Probably the most important thing to know about neptunium is that it presents a huge nuclear waste problem. Most of its isotopes have a long half-life, so containing the waste only defers the problem of its release. Scientists are working on ways of eliminating neptunium-237 (and americium-241) by transmuting it into other isotopes that decay more rapidly.
  7. Neptunium serves no biological role in any organism. Not only is it radioactive, but it’s also poisonous and pyrophoric (spontaneously burns in air). Fortunately, it is not absorbed by the digestive tract. However, if it enters the body through injection or an open wound, it concentrates in bones.
  8. Neptunium is silver, hard, and ductile. But, like the other actinides, it readily tarnishes in air.
  9. Neptunium has multiple oxidation states. The most common oxidation state is 5+. The different oxidation states produce colors in aqueous solution: Np3+ is violet; Np4+ is yellow green; Np5+ is blue green (acidic) or yellow (alkaline); Np6+ is pink; Np7+ is reddish brown (acidic) or green (alkaline).
  10. There are at least three neptunium allotropes. At normal temperatures, the element has an orthorhombic crystal structure. This changes to a tetragonal structure above 280oC and a cubic structure above 577oC.

Key Neptunium Facts

  • Name: Neptunium
  • Element Symbol: Np
  • Atomic Number: 93
  • Atomic Mass: [237]
  • Electron Configuration: [Rn] 5f4 6d17s2
  • Element Group: Actinide
  • Appearance: Solid, silver-colored metal
  • Density (g/cm3): 19.38 g/cm3
  • Melting Point: 912 K ​(693 °C, ​1182 °F)
  • Boiling Point: 4447 K ​(4174 °C, ​7545 °F (extrapolated)
  • Atomic Radius: 155 pm
  • Covalent Radius: 190±1 pm
  • Heat of Fusion (kJ/mol): 5.19
  • Heat of Vaporization (kJ/mol): 336
  • Pauling Electronegativity: 1.36
  • First Ionization Energy (kJ/mol): 604.5
  • Oxidation States: +2, +3, +4, +5, +6, +7
  • Crystal Structure: Orthorhombic


  • Emsley, John (2011). Nature’s Building Blocks: An A-Z Guide to the Elements. Oxford University Press. ISBN 978-0-19-960563-7.
  • Greenwood, Norman N.; Earnshaw, Alan (1997). Chemistry of the Elements (2nd ed.). Butterworth-Heinemann. ISBN 978-0-08-037941-8.
  • Hammond, C. R. (2004). Handbook of Chemistry and Physics (81st ed.). CRC press. ISBN 978-0-8493-0485-9.
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  • Weast, Robert (1984). CRC, Handbook of Chemistry and Physics. Boca Raton, Florida: Chemical Rubber Company Publishing. ISBN 0-8493-0464-4.