Uranium Facts – Atomic Number 92 Element Symbol U

Uranium Facts
Uranium is a radioactive metal with atomic number 92 and element symbol U.

Uranium is a radioactive element with atomic number 92 and element symbol U. This gray metal is used in ammunition, armor, nuclear weapons, and nuclear power plants. Here is a collection of interesting uranium facts, including the element’s discovery, uses, sources, and toxicity.

Uranium Element Facts

Electron Levels of a Uranium Atom
Uranium atom electron configuration

Name: Uranium

Atomic Number: 92

Element Symbol: U

Atomic Weight: 238.02891(3)

Appearance: Silvery-gray metal

Period: Period 7

Block: f-block

Element Family: Actinide

Electron Configuration: [Rn] 5f3 6d1 7s2

Electrons per Shell: 2, 8, 18, 32, 21, 9, 2

Discovery: Martin Klaproth (1789)

Name Origin: Planet Uranus

History Of Discovery

Uranium is a silvery-white metal. Photo is a billet of highly enriched uranium recovered from scrap processed at the Y-12 Facility in Oak Ridge, TN. U.S. Department of Energy
Uranium is a silvery-white metal. Photo is a billet of highly enriched uranium recovered from scrap processed at the Y-12 Facility in Oak Ridge, TN. U.S. Department of Energy

Prior to uranium’s discovery as an element, its oxide was used by the Romans at least as early as 79 CE as a yellow ceramic pigment. In the Middle Ages, pitchblende was used as a glass colorant.

German chemist Martin Heinrich Klaproth discovered uranium in 1789. Klaproth obtained a yellow compound by dissolving pitchblende in nitric acid and neutralizing the solution with sodium hydroxide. He believed the compound was the oxide of a new element, which he named for the planet Uranus (which is named, in turn, for the Greek god of the sky).

In 1841, French chemist Eugène-Melchior Péligot became the first person to isolate uranium. He obtained the metal by heating uranium tetrachloride with potassium.

Uranium Isotopes

All uranium isotopes are radioactive. The natural element consists of three isotopes: uranium-238 (99.28%), uranium-235 (0.71%), and uranium-234 (0.0054%). These isotopes emit alpha particles and have a small probability of spontaneous fission. Five other isotopes occur naturally in trace amounts from radioactive decay and neutron capture. Uranium-238 is the most stable isotope, with a half-life of about 4.468×109 years (approximately the age of the Earth).

“Enriched” uranium is uranium that is processed so that it consists of 3% to 5% uranium-235. Increasing the amount of this isotope makes makes uranium more fissionable. Uranium is enriched using a gas centrifuge. Here, uranium hexafluoride (UF6) gas in a centrifuge separates by diffusion through a silver-zinc membrane. Since 238UF6 is slightly heavier than 235UF6, it diffuses more slowly, so the gas that passes through the membrane becomes slightly richer in the desired isotope. Other methods of uranium enrichment include liquid thermal diffusion and atomic vapor laser isotope separation (AVLIS).

Biological Role and Toxicity

Uranium serves no known biological function in animals. The element is toxic to people, as well as radioactive. The radiological effects are mainly local (usually to the lungs) because alpha particles act over a short range and cannot penetrate skin. Toxicity from the element affects the reproductive system, brain, heart, liver, and kidneys. Additionally, some uranium decay products are toxic and radioactive. The metal may be handled safely so long as care is taken that it is not inhaled or ingested. Uranium compounds are toxic.

While uranium is harmful to most organisms, some bacteria convert the element’s oxidation state from U(VI) to U(IV) to obtain energy for their growth. The species include Shewanella putrefaciensGeobacter metallireducens, and Burkholderia fungorum. The bacterium Citrobacter and the lichen Trapelia involuta can absorb high concentrations of uranium.

Sources of Uranium

Uranium (and all natural elements with atomic numbers greater than iron) forms from rapid neutron capture (r-process) in supernovae and neutron star mergers. Most of the Earth’s uranium is primordial (present when the planet formed), although a small amount is produced by radioactive decay of plutonium and curium.

Uranium is the highest-atomic number element found in appreciable quantities in the Earth’s crust. It is the 51st most-abundant element and occurs in water, soil, and rock at concentrations ranging from 2 to 4 ppm. It is around 40 times more abundant than silver.

The element occurs in many minerals, including uraninite, carnotite, uranophane, autunite, coffinite, and tobernite. Uranium mining occurs in Kazakhstan, Canada, Australia, Niger, Namibia, and Russia.

Uranium glass exhibits a characteristic green fluorescence under black or ultraviolet light. Nerdtalker, Creative Commons License
Uranium glass or vaseline glass exhibits a characteristic green fluorescence under black or ultraviolet light. Nerdtalker, Creative Commons License

Uranium Uses

Uranium has many uses:

  • Uranium is used to make pottery glazes, yellow glass, and vaseline glass. At one time, it was used in Fiestaware.
  • It’s used as a mordant in wool and silk dyeing.
  • Uranium nitrate is photographic toner.
  • It’s used to make transuranium elements.
  • Uranium-238 is used to make plutonium-239, which is another fissionable element.
  • It’s used to make nuclear weapons.
  • Uranium is used as fuel in nuclear power plants.
  • Depleted uranium (uranium low in uranium-235) is used to make penetrating ammunition, armor, and shielding material for nuclear materials.
  • Uranyl acetate and uranyl formate are used to stain specimens for transmission electron microscopy.
  • Because of its long half-life, uranium-238 is used to estimate the age of igneous rocks.
  • Uranium is used as an x-ray target to produce high-energy x-radiation.

Uranium Compounds

Uranium has multiple oxidation states and forms many compounds, including oxides, water-soluble salts (which form colorful solutions), carbonate, nitrides, carbides, and hydrides. Yellowcake is the name given to a mixture of uranium oxides. Uranium always occurs in compounds and not as a native pure metal.

Physical Data

Density (room temperature): 19.1 g/cm3

Melting Point: 1405.3 K ​(1132.2 °C, ​2070 °F)

Boiling Point: 4404 K ​(4131 °C, ​7468 °F)

State at 20ºC: solid

Heat of Fusion: 9.14 kJ/mol

Heat of Vaporization: 417.1 kJ/mol

Molar Heat Capacity: 27.665 J/(mol·K)

Thermal Expansion: 13.9 µm/(m·K) (at 25 °C)

Thermal Conductivity: 27.5 W/(m·K)

Crystal Structure: Orthorhombic

Magnetic Ordering: Paramagnetic

Atomic Data

Atomic Radius: empirical: 156 pm

Covalent Radius: 196±7 pm

Van der Waals Radius: 186 pm

Electronegativity: Pauling scale: 1.38

1st Ionization Energy: 597.6 kJ/mol

2nd Ionization Energy: 1420 kJ/mol

Oxidation States: +1, +2, +3, +4, +5, +6 (amphoteric oxide)

Interesting Uranium Facts

  • Henri Becquerel discovered radioactivity in 1896 when he noticed a sample of uranium exposed photographic plates stored nearby.
  • Uranium is pyrophoric. Uranium metal granules spontaneously ignites in air at room temperature.
  • Like most metals, uranium is malleable and ductile. Unlike many metals, it’s a poor electrical conductor.
  • Freshly-prepared uranium metal is a silver color, but it quickly oxidizes in air and darkens.
  • Complete fission of one kilogram of uranium-235 theoretically produces up to 20 terajoules of energy, which is as much energy as 1.5 million kilograms of coal.
  • Uranium metal has three allotropes (crystal forms). The alpha allotrope is orthorhombic and stable up to 668 °C.  The tetragonal beta allotrope occurs at temperatures between 668 °C and 775 °C. From 775 °C up to the element’s melting point, the gamma form occurs. The gamma allotrope has a body-centered cubic structure and is the most malleable and ductile form of the metal.
  • Due to uranium’s effect on the reproductive system, uranium miners are more likely to have female children.


  • Emsley, John (2001). “Uranium”. Nature’s Building Blocks: An A to Z Guide to the Elements. Oxford: Oxford University Press. pp. 476–482. ISBN 978-0-19-850340-8.
  • Morss, L.R.; Edelstein, N.M.; Fuger, J., eds. (2006). The Chemistry of the Actinide and Transactinide Elements (3rd ed.). Netherlands: Springer. ISBN 978-9048131464. doi:10.1007/1-4020-3598-5_5
  • Seaborg, Glenn T. (1968). “Uranium”. The Encyclopedia of the Chemical Elements. Skokie, Illinois: Reinhold Book Corporation. pp. 773–786. LCCCN 68-29938.