Uranium (U)
Uranium (U) is an actinide with atomic number 92 and a standard atomic weight of 238.03. At room temperature it is a solid that melts at 1,135 °C and boils at 4,131 °C.
Uranium key facts
Uranium at a glance
Identity
| Symbol | U |
|---|---|
| Atomic number (Z) | 92 |
| Atomic weight | 238.03 ± 0.01 (CIAAW 2024) |
| Category | Actinide |
| Group | — (f-block: actinide series) |
| Period | 7 |
| Block | f-block |
| CAS number | 7440-61-1 |
Atomic properties
| Electron configuration | [Rn] 5f3 6d1 7s2 Full: 1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p6 4d10 4f14 5s2 5p6 5d10 5f3 6s2 6p6 6d1 7s2 |
|---|---|
| Electrons per shell | 2, 8, 18, 32, 21, 9, 2 |
| Oxidation states | +6 Less common: -1, +1, +2, +3, +4, +5 |
| Electronegativity (Pauling) | 1.38 |
| First ionisation energy | 6.194 eV (597.6 kJ/mol) 2nd: 11.6 eV · 3rd: 19.8 eV |
| Atomic radius (empirical) | 175 pm |
| Covalent radius | 170 pm |
| Van der Waals radius | 241 pm |
Physical properties
| State at 25 °C | Solid |
|---|---|
| Melting point | 1,135 °C · 2,075 °F · 1,408.15 K |
| Boiling point | 4,131 °C · 7,467.8 °F · 4,404.15 K |
| Density | 19.1 g/cm³ |
| Specific heat capacity | 0.116 J/(g·K) |
| Thermal conductivity | 27.5 W/(m·K) |
| Crystal structure | Orthorhombic |
Isotopes & abundance
| Natural occurrence | Primordial |
|---|---|
| Radioactive | Yes |
| Natural isotopes (abundance) | 238U 99.2742% (radioactive), 235U 0.7204% (radioactive), 234U 0.0054% (radioactive) |
| Known isotopes | 29 |
| Abundance in Earth's crust | 2.7 mg/kg |
| Abundance in seawater | 0.0032 mg/L |
Atomic weights: CIAAW Abridged Standard Atomic Weights 2024. Other properties: standard reference compilations, checked October 2026; predicted values are marked.
Uranium electron configuration
Uranium has 92 electrons. Written in short form, its ground-state configuration is [Rn] 5f3 6d1 7s2 — the electrons of the noble gas radon, plus 5f3 6d1 7s2.Written out in full: 1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p6 4d10 4f14 5s2 5p6 5d10 5f3 6s2 6p6 6d1 7s2.
An exception to the filling rule. The simple aufbau (Madelung) order predicts 5f4, 6d0, but uranium actually has 5f3, 6d1. Half-filled and filled subshells, and the close energies of the outer orbitals, make this arrangement more stable.
Discovery and name
1789 — Martin Heinrich Klaproth (metal isolated by Eugène-Melchior Péligot, 1841) · Germany.
The planet Uranus, discovered in 1781.
Known for
Uranium-235 fuels nuclear power stations; depleted uranium is used for armour and counterweights. Its decay heat helps warm Earth's interior.
Isotopes of uranium
Naturally occurring uranium is a mix of 3 isotopes. Each has 92 protons; they differ in the number of neutrons:
| Isotope | Protons | Neutrons | Natural abundance | Stability |
|---|---|---|---|---|
| 238U | 92 | 146 | 99.2742% | radioactive · half-life 4.463 billion years |
| 235U | 92 | 143 | 0.7204% | radioactive · half-life 704 million years |
| 234U | 92 | 142 | 0.0054% | radioactive · half-life 245.5 thousand years |
29 isotopes of uranium are known in our data set (half-lives as published in nuclear data tables).
Uranium 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 |
|---|---|---|---|---|---|---|
| Actinium | 89 | [227] | 1,050 °C | 3,200 °C | 10 g/cm³ | 1.1 |
| 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 |