Uranium – facts and properties
- Symbol
- U
- Atomic number
- 92
- Atomic mass
- 238.029 u
- Period
- 7
- Category
- actinide
- Electron configuration
- [Rn] 5f³ 6d¹ 7s²
- Electronegativity
- 1.38
- Atomic radius
- 175 pm
- Valence electrons
- 6
- State at standard conditions
- solid
- Melting point
- 1135 °C
- Boiling point
- 4131 °C
- Density
- 18.95 g/cm³
- Discovered
- 1789
Uranium is a silvery-white, very dense and weakly radioactive metal with atomic number 92. Martin Heinrich Klaproth discovered it in 1789 in pitchblende and named it after Uranus, the planet found a few years earlier. In 1896 Henri Becquerel came across radioactivity in uranium salts, and in 1938 Otto Hahn and Fritz Strassmann discovered nuclear fission. Today uranium fuels nuclear power stations, tints collectors' glass and serves geologists as a clock.
Position in the periodic table
Occurrence and extraction
Uranium is not rare: in the Earth's crust it is about as common as tin, but it is mostly spread thinly through rocks, soils and seawater. The main ore is uraninite, a uranium oxide that forms the black, pitch-like lumps called pitchblende. Mining is concentrated in Kazakhstan, Canada, Namibia and Australia.
Natural uranium is a mixture of isotopes, and all of them are radioactive. More than 99 percent is uranium-238, with a half-life of roughly four and a half billion years, as old as the Earth. Less than one percent is uranium-235, at around 700 million years.
In 1789 Klaproth examined pitchblende from the Ore Mountains and took the substance he obtained for a new metal. In fact he held a uranium oxide. He named the element after Uranus, discovered in 1781.
Properties and reactions
Uranium is in period 7 and is the fourth actinoid, in the actinoid row of the table between protactinium on its left and neptunium on its right. Its electron configuration is [Rn] 5f³ 6d¹ 7s², its electronegativity is 1.38 and its atomic radius is 175 pm. At 18.95 g/cm³ uranium is much denser than lead. Uranium melts at 1135 °C and boils at 4131 °C. Its atomic mass is given as 238.029 u, an average set by the abundant isotope uranium-238.
The silvery metal quickly darkens in air as an oxide layer forms. As a fine powder it can catch fire by itself. In compounds uranium mostly shows the oxidation states plus four and plus six. The hexavalent uranyl salts are yellow to yellow-green.
In 1896 Becquerel placed uranium salts on a photographic plate wrapped against light and found it blackened. The uranium itself had given off an invisible radiation.
As a heavy metal, uranium is chemically poisonous and damages above all the kidneys. As an alpha emitter it is only weakly radioactive because of its long half-lives, and skin already shields the alpha radiation from outside. It becomes hazardous mainly when dust or soluble compounds enter the body.
Nuclear power, uranium glass and dating
In Berlin in 1938 Hahn and Strassmann bombarded uranium with neutrons and afterwards detected the much lighter element barium. Lise Meitner and Otto Frisch soon explained that the uranium nucleus breaks into two parts and releases a great deal of energy. When a uranium-235 nucleus splits, new neutrons appear, and they can split further nuclei. This sequence is called a chain reaction. It was also put to military use: in August 1945 the USA used two atomic bombs on Hiroshima and Nagasaki, and the first contained uranium.
In nuclear power stations, by contrast, the chain reaction runs under control. Natural uranium contains little uranium-235, so its share is raised, which is called enrichment. The fuel is pressed into uranium dioxide pellets. The heat released makes steam that drives turbines. Around a tenth of the world's electricity comes from nuclear energy. In Germany the last plants were shut down in April 2023. Spent fuel has to be stored safely over very long periods.
Enrichment leaves depleted uranium behind, which holds less uranium-235 than natural uranium. It is very dense and has been used as counterweight and radiation shielding, also in ammunition, which is disputed.
As early as the nineteenth century glass was coloured yellow to yellow-green with uranium compounds. This uranium glass glows a strong green under ultraviolet light, because the uranyl ion absorbs the light and gives it out again as green light.
Uranium is also a clock for rocks. Uranium-238 decays through a long series to stable lead-206, and uranium-235 to lead-207. The mineral zircon takes up some uranium when it crystallises, but practically no lead. The ratio of the two elements shows how old a crystal is. Some zircons are more than four billion years old.
Quick question
What is the atomic number of uranium (U)?
FAQ
Is uranium dangerous?
Uranium is a poisonous heavy metal, above all for the kidneys, and it is radioactive. Its radiation is weak because of the long half-lives, and skin shields its alpha rays. The main danger is dust that is inhaled or swallowed.
What is uranium used for?
Mainly as fuel for nuclear power stations, where the splitting of uranium-235 provides heat and so electricity. Rocks are also dated with it.
Why does uranium glass glow green?
The uranyl ion in the glass absorbs ultraviolet light and gives the energy out again as green light. This is called fluorescence. Under a UV lamp it glows conspicuously.