Uranium
Uranium is a chemical element with symbol U and atomic number 92. It is a silvery-white metal in the actinide series of the periodic table. A uranium atom has 92 protons and 92 electrons, of which 6 are valence electrons.
Identity
- Atomic mass
- 238.029 u
- Category
- Actinides
- Group
- 3
- Period
- 7
- Block
- f
- Oxidation States
- +3, +4, +5, +6
- Phase (25 °C)
- Solid
Electronic Structure
- Electron configuration
- 1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p6 5s2 4d10 5p6 6s2 4f14 5d10 6p6 7s2 5f3 6d1
- Short configuration
- [Rn] 5f3 6d1 7s2
- Electrons per shell
- 2, 8, 18, 32, 21, 9, 2
- Electronegativity (Pauling)
- 1.38
- Electron affinity
- 50.9 kJ/mol
- 1st ionization energy
- 598 kJ/mol
- Ionization energies
- 598 · 1420 kJ/mol
Physical properties
- Melting point
- 1132.2 °C (1405 K)
- Boiling point
- 4130.9 °C (4404 K)
- Density
- 19.1 g/cm³
- Molar heat capacity
- 27.7 J/(mol·K)
Chemical behavior of uranium
Uranium has six outer electrons ([Rn] 5f³ 6d¹ 7s²) and is the actinide with the most varied chemistry: it shows +3, +4, +5, and +6 states, with +6 (the uranyl ion UO₂²⁺) the most stable in solution. Its electronegativity is low (1.38) and it is dense, malleable, and pyrophoric as fine powder. As a metal it oxidizes in air; as U-235 it is the only naturally fissile isotope.
Reactivity
U + O₂ → UO₂
In air it coats itself with dark dioxide; when heated it burns to U₃O₈. Fine powder can ignite spontaneously, so it is machined under coolant.
U + 2 H₂O → UO₂ + 2 H₂
Water barely attacks it in the cold, but hot water and steam react with it, releasing hydrogen. Peroxides and carbonates dissolve it, forming uranyl.
U + 4 HF → UF₄ + 2 H₂
With hydrofluoric acid it gives the green tetrafluoride, an intermediate toward gaseous UF₆ that feeds enrichment centrifuges.
UO₂ + 4 HF → UF₄ + 2 H₂O
The dioxide converts to the tetrafluoride and then to volatile UF₆: the fluoride route from yellowcake to enriched fuel.
Notable compounds
| Compound | Formula | Use or importance |
|---|---|---|
| Uranium dioxide | UO₂ | Nuclear fuel pellets |
| Uranium hexafluoride | UF₆ | Centrifuge enrichment |
| Yellowcake | U₃O₈ | Commercial mining concentrate |
| Uranyl nitrate | UO₂(NO₃)₂ | Reprocessing and laboratories |
| MOX fuel | UO₂-PuO₂ | Plutonium recycling in reactors |
Electron shell distribution
View in the atom builderIsotopes
Natural uranium combines uranium-238 (²³⁸U, ~99.27%, 4.47 billion years), uranium-235 (²³⁵U, ~0.72%, 704 million years), and traces of uranium-234. U-235 is the only fissile one and fuels reactors and weapons; U-238 is fertile and breeds plutonium-239. Artificial U-233 is bred from thorium in advanced cycles.
Occurrence and production
It occurs at about 2.7 ppm in granites, phosphorites, and sandstones, as uraninite (pitchblende), carnotite, and autunite. The largest deposits are in Kazakhstan, Canada, Australia, and Namibia. It is obtained by acid or alkaline leaching into yellowcake, and the metal by reduction with calcium or magnesium.
History and origin of the name
Martin Heinrich Klaproth identified it in 1789 in pitchblende and named it uranium, after Uranus, recently discovered. Péligot isolated the metal in 1841. Becquerel discovered in 1896 that it fogged photographic plates: radioactivity. Hahn and Strassmann fissioned U-235 in 1938, and the Manhattan Project took it to the bomb and the Chicago reactor.
Applications and relevance of uranium
Uses of the element
As enriched U-235 (3–5%) it forms the UO₂ pellets that heat every light-water reactor; as depleted U-238 it shields, ballasts aircraft, and drills through armor thanks to its extreme density.
Uses of its compounds
Gaseous UF₆ allows isotope separation in centrifuges; sintered UO₂ withstands 2,800 °C inside the cladding. MOX recycles plutonium, and uranyl marks datings and tracers.
Health, safety and environment
Biological role
It has no biological function and is nephrotoxic: soluble uranyl damages the renal tubule and accumulates in bone. Inhaling insoluble dust irradiates the lung with alpha particles for years.
Safety
It is handled with dust control, ventilation, and alpha monitors in mining, enrichment, and fabrication. UF₆ is also corrosive and releases HF with moisture. It does not exist in consumer products.
Environment
Mining leaves tailings ponds and waste rock with radium and radon that must be contained; spent fuel is cooled in pools and containers. In return, each kilogram fissioned avoids 2,500 tonnes of CO₂ from coal.
Frequently asked questions about Uranium
What is uranium used for in nuclear power plants?
Uranium is the main fuel of most nuclear power plants. The isotope uranium-235 is enriched to suitable concentrations and, when it fissions under control, releases heat. That heat produces steam, which drives turbines and generates electricity without direct carbon dioxide emissions during operation. The reaction is regulated by materials that absorb neutrons, such as control rods.
Is uranium dangerous?
Uranium is a toxic and radioactive metal. Its external risk is usually limited because it emits mainly alpha radiation, which is poorly penetrating, but inhaling or ingesting dust or soluble compounds can damage the kidneys and increase radiological risks. The danger depends on the chemical form, solubility, amount, and route of exposure. For this reason its mining, enrichment, and industrial handling are subject to strict controls.
Why is uranium-235 so important?
Uranium-235 is the only natural isotope that can sustain a nuclear fission chain reaction. When it absorbs a neutron, its nucleus splits and releases energy, more neutrons, and fission products. Those neutrons can trigger new fissions, producing a controlled reaction in a reactor or an explosion if it is not regulated. This property explains its central role in nuclear energy and military applications.
Can uranium from spent fuel be recycled?
Yes. Spent fuel still contains uranium and plutonium that can be recovered through chemical reprocessing, such as the PUREX process. The recovered uranium can be reused, and plutonium can be combined with uranium to make MOX fuel. However, reprocessing is complex, expensive, and subject to strict regulation because of its connection with fissile materials and high-activity radioactive waste.
What is depleted uranium and what is it used for?
Depleted uranium is a byproduct of uranium enrichment: it consists mainly of uranium-238 and contains less uranium-235 than natural uranium. Its high density makes it useful as a counterweight in aircraft, ballast in ships, shielding, and armor-piercing ammunition. It can also be used in MOX fuel when mixed with plutonium. Its external radioactivity is low, but it can be chemically toxic if dust is inhaled or ingested.
Where does uranium come from?
Uranium is extracted from minerals such as uraninite, also known as pitchblende. After mining, the ore is processed to obtain "yellowcake", a uranium concentrate that is then converted and enriched to produce nuclear fuel. Enrichment increases the proportion of uranium-235 through centrifugation or other methods. Uranium also occurs in traces in rocks, soil, and water, although not in directly usable forms.
What is the difference between natural, enriched, and depleted uranium?
Natural uranium contains approximately 99% uranium-238 and less than 1% uranium-235. Enriched uranium has a higher proportion of uranium-235 and is used as reactor fuel. Depleted uranium is the residue of enrichment, with even less uranium-235 and lower radioactivity. All three share the same basic chemical identity, but differ in isotopic composition, radioactivity, and applications.
Main sources consulted: PubChem, Royal Society of Chemistry, NIST, IUPAC. Last revision: November 2026.