Neptunium (Np)
Neptunium (Np) is an actinide with atomic number 93; its most stable known isotope has mass number 237. At room temperature it is a solid that melts at 644 °C.
Neptunium key facts
Neptunium at a glance
Identity
| Symbol | Np |
|---|---|
| Atomic number (Z) | 93 |
| Atomic weight | [237] mass number of the most stable isotope |
| Category | Actinide |
| Group | — (f-block: actinide series) |
| Period | 7 |
| Block | f-block |
| CAS number | 7439-99-8 |
Atomic properties
| Electron configuration | [Rn] 5f4 6d1 7s2 Full: 1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p6 4d10 4f14 5s2 5p6 5d10 5f4 6s2 6p6 6d1 7s2 |
|---|---|
| Electrons per shell | 2, 8, 18, 32, 22, 9, 2 |
| Oxidation states | +5 Less common: +2, +3, +4, +6, +7 |
| Electronegativity (Pauling) | 1.36 |
| First ionisation energy | 6.266 eV (604.5 kJ/mol) 2nd: 11.5 eV · 3rd: 19.7 eV |
| Atomic radius (empirical) | 175 pm |
| Covalent radius | 171 pm |
| Van der Waals radius | 239 pm |
Physical properties
| State at 25 °C | Solid |
|---|---|
| Melting point | 644 °C · 1,191.2 °F · 917.15 K |
| Density | 20.2 g/cm³ |
| Crystal structure | Orthorhombic |
Isotopes & abundance
| Natural occurrence | Synthetic (traces in uranium ores) |
|---|---|
| Radioactive | Yes |
| Most stable isotope | 237Np — half-life 2.144 million years |
| Known isotopes | 27 |
Atomic weights: CIAAW Abridged Standard Atomic Weights 2024. Other properties: standard reference compilations, checked October 2026; predicted values are marked.
Neptunium electron configuration
Neptunium has 93 electrons. Written in short form, its ground-state configuration is [Rn] 5f4 6d1 7s2 — the electrons of the noble gas radon, plus 5f4 6d1 7s2.Written out in full: 1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p6 4d10 4f14 5s2 5p6 5d10 5f4 6s2 6p6 6d1 7s2.
An exception to the filling rule. The simple aufbau (Madelung) order predicts 5f5, 6d0, but neptunium actually has 5f4, 6d1. Half-filled and filled subshells, and the close energies of the outer orbitals, make this arrangement more stable.
Discovery and name
1940 — Edwin McMillan and Philip Abelson · United States (Berkeley).
The planet Neptune.
Known for
The first transuranium element. Neptunium-237 is a by-product of reactors and is used to make plutonium-238.
Isotopes of neptunium
Neptunium has no stable isotopes and does not occur naturally in significant amounts. Its longest-lived known isotope is:
| Isotope | Protons | Neutrons | Half-life |
|---|---|---|---|
| 237Np | 93 | 144 | 2.144 million years |
27 isotopes of neptunium are known in our data set (half-lives as published in nuclear data tables).
Neptunium and its neighbours in the actinide series
Neighbouring elements in the same f-block series:
| Element | Z | Atomic weight | Melting point | Boiling point | Density | Electronegativity |
|---|---|---|---|---|---|---|
| Thorium | 90 | 232.04 | 1,750 °C | 4,785 °C | 11.7 g/cm³ | 1.3 |
| Protactinium | 91 | 231.04 | 1,572 °C | — | 15.4 g/cm³ | 1.5 |
| Uranium | 92 | 238.03 | 1,135 °C | 4,131 °C | 19.1 g/cm³ | 1.38 |
| Neptunium | 93 | [237] | 644 °C | — | 20.2 g/cm³ | 1.36 |
| Plutonium | 94 | [244] | 640 °C | 3,228 °C | 19.7 g/cm³ | 1.28 |
| Americium | 95 | [243] | 1,176 °C | — | 12 g/cm³ | 1.3 |
| Curium | 96 | [247] | 1,345 °C | — | 13.51 g/cm³ | 1.3 |