Uranium
Atomic Data
| Atomic Number | 92 |
| Symbol | U |
| Atomic Weight | 238.03 u |
| Density (STP) | 19.1 g/cm³ |
| Melting Point | 1132.15 °C (1405.3 K) |
| Boiling Point | 4130.85 °C (4404 K) |
| Electronegativity | 1.38 (Pauling) |
| Electron Config. | 1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p6 4d10 4f14 5s2 5p6 5d10 5f3 6s2 6p6 6d1 7s2 |
| Oxidation States | +3, +4, +5, +6 |
| Phase at STP | Solid |
| Category | Actinoid |
| Period / Group | 7 / None |
| CAS Number | 7440-61-1 |
Electron Configuration
[Rn] 5f3 6d1 7s2
| Shell | n | Subshell | Electrons | Cumulative |
|---|---|---|---|---|
| K | 1 | 1s | 2 | 2 |
| L | 2 | 2s | 2 | 4 |
| L | 2 | 2p | 6 | 10 |
| M | 3 | 3s | 2 | 12 |
| M | 3 | 3p | 6 | 18 |
| M | 3 | 3d | 10 | 28 |
| N | 4 | 4s | 2 | 30 |
| N | 4 | 4p | 6 | 36 |
| N | 4 | 4d | 10 | 46 |
| N | 4 | 4f | 14 | 60 |
| O | 5 | 5s | 2 | 62 |
| O | 5 | 5p | 6 | 68 |
| O | 5 | 5d | 10 | 78 |
| O | 5 | 5f | 3 | 81 |
| P | 6 | 6s | 2 | 83 |
| P | 6 | 6p | 6 | 89 |
| P | 6 | 6d | 1 | 90 |
| Q | 7 | 7s | 2 | 92 |
| Total | 92 | 92 | ||
Isotopes of Uranium
Uranium has three naturally occurring stable isotopes. The most abundant is ²³⁸U, comprising 99.2742% of all naturally occurring Uranium.
| Isotope | Symbol | Protons | Neutrons | Abundance | Stability |
|---|---|---|---|---|---|
| Uranium-234 | ²³⁴U | 92 | 142 | 0.0054 | Stable |
| Uranium-235 | ²³⁵U | 92 | 143 | 0.7204 | Stable |
| Uranium-238 | ²³⁸U | 92 | 146 | 99.2742 | Stable |
Abundance & Occurrence
Uranium is present in Earth's crust at approximately 2.7 ppm by mass and at approximately 0.009 ppm by mass throughout the universe.
Earth's Crust (ppm by mass)
Universe (ppm by mass)
Discovery & History
Read more about the discovery of the periodic table of elements →
Safety & Handling
- Radioactivity: Natural uranium (99.3% U-238, t½ = 4.5 × 109 years) is a weak alpha emitter; the primary radiological concern from inhalation of uranium dust is long-term alpha dose to the lung and internal organ irradiation.
- Chemical kidney toxicity: Uranium is a heavy metal nephrotoxin; soluble uranium compounds damage the proximal tubule of the kidneys: chemical toxicity may be the more immediate hazard for soluble forms at occupationally relevant exposures.
- Depleted uranium (DU) dust: DU penetrators and armour that fragment or burn generate respirable uranium oxide particles; military and civilian populations near DU-contaminated sites require monitoring and protective measures.
- Criticality risk: enriched uranium: Highly enriched uranium (HEU) can achieve a critical mass; strict mass and geometry controls are mandatory in all uranium enrichment and fuel fabrication facilities.
- Fire hazard: Uranium metal powder and fine turnings are pyrophoric; metal fires produce uranium oxide smoke and require Class D extinguishing agents: never use water.
Uranium in the Real World
Real-World Uses
- Nuclear reactor fuel: Uranium-235 (0.72% of natural uranium) undergoes fission when struck by thermal neutrons, releasing energy that generates ~10% of the world's electricity in approximately 440 nuclear power plants; enriched UO₂ pellets are the dominant fuel form.
- Nuclear weapons: Highly enriched uranium-235 (>90% U-235) is one of the two fissile materials used in nuclear weapons; uranium was used in the Little Boy bomb dropped on Hiroshima in 1945.
- Armour-piercing ammunition (depleted uranium): Depleted uranium (DU, mostly U-238) has very high density (19.1 g/cm³) and self-sharpening properties under impact; DU alloy rods are used as kinetic energy penetrators in anti-tank ammunition and DU armour plates reinforce tank hulls.
- Radioisotope dating: Uranium-lead (U-Pb) dating, uranium-thorium (U-Th) dating, and uranium-helium (U-He) thermochronology are used to determine the absolute ages of rocks, minerals, meteorites, and archaeological materials from thousands to billions of years.
- Uranium glass (historical and decorative): Small amounts of uranium oxide give glass a characteristic yellow-green fluorescence under UV light; uranium glass (Vaseline glass) produced before 1940 is collected as a historical curiosity, though it is mildly radioactive.
Downloadable Resources
Free periodic table reference sheets for classrooms, study sessions, and laboratory use.
Frequently Asked Questions
What is uranium used for?
Uranium's primary use is as nuclear fuel in power reactors. Uranium-235 (0.7% of natural uranium) is the fissile isotope that sustains a chain reaction. Enriched uranium (3–5% U-235) fuels light-water reactors that generate about 10% of world electricity. Depleted uranium (mostly U-238), a byproduct of enrichment, is used in armour-piercing projectiles and as radiation shielding due to its very high density (19.1 g/cm3). Uranium was historically used as a yellow-orange ceramic glaze.
Is uranium dangerous?
Uranium poses both radiological and chemical hazards. Naturally occurring uranium is only mildly radioactive (it decays very slowly), so the radiation hazard from brief external exposure is low. However, uranium dust inhaled or ingested accumulates in the kidneys and bone and can cause kidney damage: the chemical toxicity is considered more immediately significant than the radiological hazard for most exposure scenarios. Enriched or reprocessed uranium, and uranium in decay-product-rich ores, presents greater radiation risks.
How was uranium discovered?
Uranium was discovered in 1789 by German chemist Martin Heinrich Klaproth, who isolated a black powder from the mineral pitchblende and named it after the recently discovered planet Uranus. It was later found to be uranium dioxide rather than pure uranium. The metal itself was first isolated in 1841 by French chemist Eugène-Melchior Péligot by reducing uranium tetrachloride with potassium. Uranium's radioactivity was discovered by Henri Becquerel in 1896.
What is enrichment and why is it needed for nuclear fuel?
Natural uranium contains only 0.72% fissile uranium-235, with the remainder being non-fissile uranium-238. Most commercial light-water reactors require uranium enriched to 3–5% U-235 to sustain a chain reaction. Enrichment is achieved by converting uranium to the gas uranium hexafluoride (UF6) and passing it through thousands of stages of centrifuges or (historically) gas diffusion barriers, which gradually separate the slightly lighter UF6 containing U-235 from the heavier U-238 form.