Uranium (U)

Uranium (symbol U) is the chemical element with atomic number 92: an actinide, solid at 0 °C, radioactive, with an atomic mass of 238.03 u. Discovered in 1789 (Martin Heinrich Klaproth).

Uranium is the heaviest element found in abundance in nature. Weakly radioactive, it is famous mainly because one of its isotopes, uranium-235, can split in two (fission) and release colossal energy: one gram fissioned equals nearly three tonnes of burned coal. This property powers nuclear plants — about 65 to 70% of France's electricity — and fueled the Hiroshima bomb. It was forged in stellar explosions and neutron star mergers, long before the Sun was born.

Key facts

Atomic number: 92
Symbol: U
French name: Uranium
Atomic mass: 238.03 u
Molar mass: 238.03 g/mol
Family: Actinides
Position: period 7, f-block
State at 0 °C: Solid
Melting point: 1,132.2 °C
Boiling point: 4,130.9 °C
Density: 19.1 g/cm³
Electronegativity (Pauling): 1.38
First ionization energy: 598 kJ/mol
Atomic radius: 175 pm
Electron configuration: [Rn] 5f³ 6d¹ 7s²
Abundance in Earth's crust: 2.7 ppm (49th most abundant element)
Indicative price: 100 $/kg (order of magnitude)
Radioactive: yes
Discovery: 1789, Martin Heinrich Klaproth

Origin of the name

From the planet Uranus, discovered shortly before.

What does uranium look like?

A silvery-gray metal, very dense (19.1 g/cm³, nearly as much as gold), that tarnishes black in air. As a fine powder, it ignites spontaneously. Its compounds are often yellow or green — hence “yellowcake,” the powdered uranium concentrate.

Where is uranium found?

About 2 to 3 ppm in the Earth's crust, more than tin or silver. Seawater contains about 3 micrograms per liter, adding up to more than 4 billion tonnes in all. The human body holds a few tens of micrograms on average, taken in through food and water.

Natural sources

Uraninite (pitchblende)
Carnotite
Autunite
Coffinite
Brannerite

How is uranium produced?

It is mined in open pits or underground, or increasingly by in situ leaching: a solution is injected into the deposit to dissolve the uranium and then pumped back out. The ore yields yellowcake, which is then converted and enriched: natural uranium contains only 0.7% uranium-235, while reactors need 3 to 5% (and weapons about 90%).

Top producers

Kazakhstan
Canada
Namibia
Australia
Uzbekistan
Russia

What is uranium used for?

Fuel for nuclear power plants and nuclear-powered submarines
Nuclear weapons (highly enriched uranium)
Depleted uranium: armor and armor-piercing shells
Depleted uranium: aircraft counterweights and radiation shielding
Dating the oldest rocks (uranium–lead method)

In everyday life

The electricity in your wall outlet — mostly nuclear in France
Uranium glass (“Vaseline glass”) that fluoresces green under UV
Old orange-glazed ceramics (1930s–1940s)
The granite of your countertop or of some houses in Brittany, France

Role in living things

No biological role. It is mainly toxic chemically, like a heavy metal, attacking the kidneys. For natural or depleted uranium, this chemical toxicity is more of a concern than its weak radioactivity.

Hazards of uranium

Natural uranium is weakly radioactive and can be held briefly in the hand without significant danger. The risks come mainly from inhaling dust, from its kidney toxicity, and from radon escaping from ores and mines. Enriched uranium carries a criticality risk: gather enough of it and a chain reaction can start.

Cosmic origin

Neutron stars: Created when two neutron stars collide — the “forge” of gold.

History of uranium

In 1789, German chemist Martin Heinrich Klaproth extracted an oxide from pitchblende and named it after Uranus, a planet discovered eight years earlier. Frenchman Eugène-Melchior Péligot isolated the metal in 1841. In 1896, Henri Becquerel discovered radioactivity thanks to uranium salts left on a photographic plate. In 1938, Otto Hahn and Fritz Strassmann observed fission, explained by Lise Meitner and Otto Frisch; Fermi's first reactor went critical in Chicago in 1942.

Did you know?

Two billion years ago at Oklo, in Gabon, uranium deposits spontaneously ran as natural nuclear reactors for hundreds of thousands of years; France's atomic energy agency (CEA) discovered this in 1972.
The Hiroshima bomb contained about 64 kg of enriched uranium, but less than a kilogram actually fissioned.
Uranium and thorium decaying in the mantle supply a good share of the Earth's internal heat.
Uranium glass, hugely fashionable in the 19th century, glows a spectacular neon green under a UV lamp.

Isotopes of uranium

No stable isotopes, but two isotopes so long-lived that they have been around since the Earth formed. Only the rare uranium-235 can sustain a chain reaction.

uranium-233: radioactive, half-life 159,000 years. A fissile fuel bred from thorium.
uranium-234: 0.005% of natural uranium, radioactive, half-life 246,000 years
uranium-235: 0.72% of natural uranium, radioactive, half-life 7.04 × 10⁸ years. Fuel for nuclear power plants (enriched to 3–5%) and for the first atomic bomb used in war (Hiroshima).
uranium-236: radioactive, half-life 2.34 × 10⁷ years. Formed in reactors, it betrays the presence of previously irradiated uranium.
uranium-238: 99.274% of natural uranium, radioactive, half-life 4.47 × 10⁹ years. Used to date the oldest rocks; becomes plutonium-239 in reactors.

Common compounds

Uranium dioxide (UO₂)

Related elements

Same family (actinides): Actinium (Ac), Thorium (Th), Protactinium (Pa), Neptunium (Np), Plutonium (Pu), Americium (Am), Curium (Cm), Berkelium (Bk), Californium (Cf), Einsteinium (Es), Fermium (Fm), Mendelevium (Md), Nobelium (No), Lawrencium (Lr).