Uranium · element 92
The heaviest element found in bulk in nature, and the one that changed the twentieth century
Key facts
Uranium is a dense, silvery-grey metal that most people know for a single reason: it can be split. When a uranium-235 nucleus captures a neutron it breaks in two, releases a burst of energy and throws out more neutrons that can split further nuclei. That chain reaction runs every nuclear power station on Earth and was the basis of the first atomic weapons.
Yet uranium was known for a century and a half before anyone knew about fission. It was used to colour glass and ceramics, its radioactivity was the first ever discovered, and its slow decay is the clock geologists use to measure the age of the planet.
Where it is found
Uranium is more common than many people expect. The Earth's crust holds around two to three parts per million, which makes it roughly as abundant as tin and far more common than silver or gold. It is spread thinly through granite, sandstone and seawater, and it concentrates in ore minerals such as uraninite, also called pitchblende, and carnotite.
Natural uranium is almost all uranium-238, with about 0.7 per cent uranium-235 and a trace of uranium-234. Kazakhstan, Canada, Australia and Namibia are among the largest producers today. Much modern mining uses in situ leaching, where a solution is pumped through the ore underground and the dissolved uranium is brought to the surface. Mined uranium is processed into a yellow oxide concentrate known as yellowcake before further refining.
How it was discovered
In 1789 the German chemist Martin Heinrich Klaproth treated pitchblende from the silver mines of Joachimsthal with acid and isolated a yellow compound. He believed he had found a new element and named it after Uranus, the planet William Herschel had discovered eight years earlier. What Klaproth actually had was an oxide. The pure metal was first produced in 1841 by Eugene-Melchior Peligot, who reduced uranium tetrachloride with potassium.
The element's radioactivity was uncovered in 1896 by Henri Becquerel, who noticed that uranium salts fogged photographic plates through black paper without any light. In December 1938 Otto Hahn and Fritz Strassmann bombarded uranium with neutrons and found barium in the products. Lise Meitner and Otto Frisch explained the result as the nucleus splitting, and gave it the name fission.
What we use it for
Nuclear power is by far the biggest use. Most reactors need uranium enriched to between 3 and 5 per cent uranium-235, achieved by spinning uranium hexafluoride gas in centrifuges. A single fuel pellet the size of a fingertip releases roughly as much energy as a tonne of coal. Uranium fuel provides around a tenth of the world's electricity.
The leftover material from enrichment, called depleted uranium, is extremely dense and is used for radiation shielding, aircraft counterweights and armour-piercing projectiles. Historically uranium oxide gave glass a yellow-green tint that glows under ultraviolet light, and it coloured ceramic glazes. Uranium-238's steady decay to lead lets geologists date rocks that are billions of years old, and the ratio of uranium to lead in meteorites gives the age of the solar system.
Reading the numbers
Uranium melts at about 1,130 °C and boils near 4,130 °C, similar to copper for melting but far higher for boiling. Its density of 19.1 grams per cubic centimetre is close to that of gold and tungsten, and about two and a half times that of iron. A litre of uranium would weigh 19 kilograms. The electron configuration [Rn] 5f3 6d1 7s2 places it firmly in the actinide series, with three electrons in the 5f shell. Those f electrons are not tightly bound, so uranium shows oxidation states from +3 to +6, and the +6 state, as the uranyl ion, is the one found in most minerals and in yellowcake. Its Pauling electronegativity of 1.38 marks it as a reactive metal, and finely divided uranium will catch fire in air.
Safety and the environment
Natural uranium is only weakly radioactive because of its very long half-lives, so the main hazard from handling it is chemical: as a heavy metal it damages the kidneys. The bigger radiological concern is radon gas, a decay product that seeps out of uranium-bearing rock and can build up in buildings. Mining leaves large volumes of tailings that must be capped to stop dust and radon escaping. Spent reactor fuel contains fission products and plutonium and stays hazardous for thousands of years, which is why deep geological disposal is planned in several countries.
Did you know?
- Uranium-238 has a half-life of about 4.5 billion years, almost exactly the age of the Earth, so roughly half the uranium the planet started with is still here.
- Uranium glass, popular from the 1830s to the 1940s, fluoresces bright green under ultraviolet light, and collectors still hunt for it at car boot sales.
- Seawater contains about three micrograms of uranium per litre, and the oceans hold far more uranium in total than all known land deposits.
- At Oklo in Gabon, uranium ore was once rich enough in uranium-235 to run a natural fission reactor underground, around two billion years ago.
- Uranium was named after the planet Uranus, and the next two elements, neptunium and plutonium, continued the planetary theme.
More in the actinides
The nearest neighbours of uranium in the family, by atomic number.
See all 15 actinides on one page, or browse all 118 elements.
Compare Uranium with another element
Each comparison has a side-by-side data table and a plain-English summary of the differences.
Frequently asked questions
Why is uranium radioactive?
Every element beyond lead has nuclei with too many protons for the nuclear force to hold together indefinitely. Uranium nuclei occasionally emit alpha particles to shed mass, but they do so very slowly, which is why uranium has survived since the Earth formed.
Is uranium dangerous to touch?
A lump of natural uranium metal is only mildly radioactive, and skin blocks alpha particles. The real risks are breathing in dust or swallowing it, because the metal is toxic to the kidneys and alpha radiation inside the body damages cells.
What is enriched uranium?
Natural uranium is only 0.7 per cent uranium-235, the isotope that fissions easily. Enrichment raises that share by separating isotopes in centrifuges. Reactor fuel is enriched to 3 to 5 per cent, while weapons need around 90 per cent.
How much energy is in uranium?
Fissioning one kilogram of uranium-235 releases about as much energy as burning a few thousand tonnes of coal. That is why a reactor can run for years on a few tonnes of fuel.
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Written by Anthony. Updated September 2026. Numerical data from Periodic-Table-JSON (CC BY-SA), cross-checked against the Uranium reference article. Spotted an error? Tell us.