ElementsWhat is neptunium?
Neptunium is a radioactive, silvery metal. It was the first element heavier than uranium to be made.
What is neptunium used for?
Mostly research. It is also a starting material for making plutonium-238.
Where the name comes from
From the planet Neptune, the next planet after Uranus, as neptunium follows uranium.
Key facts
- Symbol
- Np
- Atomic number
- 93
- Atomic mass
- 237.048172 u
- Family
- Actinide
- Period
- 7
- Group
- none (f-block)
- Block
- f
- State at room temperature
- Solid
- Melting point
- 643.9 °C (917 K, 1,191 °F)
- Boiling point
- 3,902 °C (4,175 K, 7,055 °F)
- Density
- 20.25 g/cm³
- Electrons per shell
- 2, 8, 18, 32, 22, 9, 2
- Electron configuration
- [Rn]7s2 5f4 6d1
- Common oxidation states
- +6, +5, +4, +3
- Electronegativity (Pauling)
- 1.36
- First ionisation energy
- 6.266 eV
- Electron affinity
- no value
- Atomic radius (van der Waals)
- 221 pm
- Discovered
- 1940
Place in the periodic table
Neptunium is element number 93. It sits in period 7 (row 7) of the periodic table, in the separate row of actinides drawn below the main table, and belongs to the f-block. Just before it comes uranium (92), and just after it comes plutonium (94).
Family: Actinides.
Physical properties
At room temperature neptunium is a solid. It melts at 643.9 °C and boils at 3,902 °C, so it stays liquid over a range of 3,258 degrees. Its density is 20.25 grams per cubic centimetre, so a one-litre block would weigh about 20.25 kilograms. By density it ranks 5th among the 96 elements with a measured value.
Inside the atom
An atom of neptunium has 93 protons in its nucleus and, when neutral, 93 electrons around it. The electrons are arranged in 7 shells: 2, 8, 18, 32, 22, 9, 2. The outermost shell holds 2 electrons. Written in full, the electron configuration is [Rn]7s2 5f4 6d1.
Its electronegativity on the Pauling scale is 1.36. This number shows how strongly an atom pulls shared electrons towards itself: fluorine, the strongest, is 3.98. Removing one electron from an atom takes 6.266 electronvolts (about 605 kilojoules per mole). One mole of neptunium weighs 237.048172 grams, so a single gram contains about 2.54 × 1021 atoms.
The ring picture is a simple model that only counts the electrons in each shell. Real atoms do not look like this.
Next steps
- Work out the weight of a compound that contains neptunium with the molar mass calculator.
- Compare it with every other element in the sortable list.
- Test yourself with the symbol quiz.
Figures on this page are from PubChem (U.S. National Library of Medicine), as downloaded on 11 November 2026. See About and data sources.