Atomic number 92

Uranium

U
+ phase
− phase

Shells Orbitals 5f 6d 7s

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92 electrons in 7 shells: 2, 8, 18, 32, 21, 9, 2.

Shells are a teaching model: an electron does not travel on a ring at a fixed distance.

Category: Actinide
Period: 7
Block: f
Standard atomic weight: 238.03(1) Da
Electron configuration: [Rn] 5f³ 6d¹ 7s²
Phase at 25 °C: solid
Melting point: 1133 °C
Boiling point: 4131 °C
Density: 18.95 g/cm³
Electronegativity (Pauling): 1.38
Discovered: 1789, Martin H. Klaproth

Neighbours in the table

Pa Np

Every uranium atom dug from the ground belongs to an unstable isotope, which Los Alamos gives as the reason it is weakly radioactive. Uranium-238 is 99.2742 per cent of a natural sample on CIAAW’s figures. Its half-life, the time for half a sample to decay, is 4.468 billion years for the IAEA and 4.463 billion for the Atomic Mass Data Center. Uranium-235, 0.7204 per cent, has one of 704 million years on both, 700 million in EPA’s rounding. Such slow decay, ATSDR notes in PubChem, is why natural uranium has not all decayed away.

Out to shell seven, uranium’s 92 electrons run 2, 8, 18, 32, 21, 9 and 2, or [Rn] 5f³ 6d¹ 7s². Uranium is an inner transition element, a metal whose last electron enters an f orbital. OpenStax notes that the simple filling order fails for the actinides, whose subshells lie very close in energy. Los Alamos calls +4 and +6 its important oxidation states, bookkeeping charges that assume ionic bonds, and in solution the yellow UO₂²⁺ ion, uranium at +6, is the most stable.

Los Alamos describes the metal as malleable, hammerable without shattering, and ductile, about 70 per cent denser than lead yet slightly less dense than gold. It calls uranium a poor electrical conductor, although metallic solids in general conduct well, according to OpenStax.

Where it is found

Uranium never occurs in nature as a metal, only in minerals, as ATSDR’s summary in PubChem puts it. EPA finds it in virtually all soil, rock and water. PubChem’s element record gives 2.7 milligrams in a kilogram of crust and 3.2 × 10⁻³ milligrams in a litre of seawater. Los Alamos ranks it above mercury, silver and cadmium in abundance, about level with molybdenum and arsenic. In 1972 Francis Perrin found that ore at the Oklo mine in Gabon had once hosted natural fission reactors, running in prehistoric times, Los Alamos records.

The ores include pitchblende, uraninite, carnotite and autunite; Jefferson Lab writes uraninite as UO₂ and autunite as Ca(UO₂)₂(PO₄)₂·10H₂O. Phosphate rocks, lignite and monazite sands carry it as well. US ores hold 0.05 to 0.3 per cent of the oxide U₃O₈, according to OpenStax. EPA describes two ways the US mining industry wins it: mining the rock, or dissolving the uranium underground with strong chemicals and pumping the solution up. Refining passes through yellow cake, uranium(VI) oxides in Los Alamos’s caption and sodium diuranate or U₃O₈ in Jefferson Lab’s text.

Why it matters

Los Alamos traces uranium’s use to 79 AD, as a yellow colouring in ceramic glazes. Martin H. Klaproth concluded in 1789 that pitchblende held a new element and named it after Uranus, the planet found in 1781. Los Alamos calls Uranus the Greek sky god; CIAAW’s note gives the Roman “Father Heaven”. Klaproth’s product was an oxide, uranium dioxide in Jefferson Lab’s account. Péligot freed the metal in 1841 with potassium, from the tetrachloride per Los Alamos, from the dioxide per Jefferson Lab.

In 1896 Antoine H. Becquerel found that uranium gave off invisible rays: radioactivity. In 1939 Meitner, Hahn and Strassman bombarded uranium-235 with slow neutrons and found barium and krypton among the products. Los Alamos calls uranium-235 the only natural fissile isotope, one slow neutrons can split. Each split frees more neutrons, which once slowed can split others. Reactor fuel rods hold thimble-sized pellets, usually UO₂, and OpenStax reckons a kilogram of uranium-235 releases about 2.5 million times the energy of a kilogram of coal.

Uranium-238 is fertile rather than fissile: after capturing a neutron it becomes plutonium-239 through two beta decays, and OpenStax notes that this plutonium also fissions in a running reactor. Depleted uranium, poorer in uranium-235 than the natural metal, weights aircraft control surfaces. Los Alamos adds gyro compasses, shielding and X-ray targets.

Properties

Properties of uranium Standard atomic weights from IUPAC/CIAAW (abridged, 2024); other values from PubChem's periodic table, kelvin converted to °C.
Property	Value	Unit	Source
Atomic number	92
Standard atomic weight	238.03(1) Da	Da	IUPAC/CIAAW 2024
Melting point	1133 °C	°C	Los Alamos: 1133 °C
Boiling point	4131 °C	°C	Los Alamos: 4131 °C
Density	18.95 g/cm³	g/cm³
Electronegativity (Pauling)	1.38
Electron configuration	1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹⁰ 4p⁶ 5s² 4d¹⁰ 5p⁶ 6s² 4f¹⁴ 5d¹⁰ 6p⁶ 7s² 5f³ 6d¹
Phase at 25 °C	solid

Safety

Uranium is toxic both as a chemical and as a source of ionising radiation, Los Alamos says. Skin stops its alpha particles, yet EPA calls swallowed uranium a chemical hazard, linking large swallowed amounts to bone or liver cancer and heavy inhalation to lung cancer. Haz-Map, in PubChem, names the kidney as the critical organ and rates internal toxicity very high; the EU classification carries the signal word Danger, fatal if swallowed or inhaled. The PubChem Laboratory Chemical Safety Summary and the NIOSH card, which covers the metal as a potential occupational carcinogen, are the sheets to read.

Questions

Is uranium dangerous to touch?

Alpha particles are normally halted by the outer layer of skin cells, OpenStax notes, so outside the body they are rarely dangerous, though quite hazardous if swallowed or breathed in. EPA says the small uranium samples kept in school labs are not harmful when handled properly. The NIOSH card still lists skin contact as a route of workplace exposure and dermatitis among its symptoms; the PubChem safety summary covers handling.

Why is uranium radioactive?

None of its natural isotopes, uranium-238, -235 and -234, has a stable nucleus, which Los Alamos gives as the reason. Uranium-238 almost always emits an alpha particle, two protons and two neutrons, and becomes thorium-234. In 0.0000545 per cent of decays on the IAEA's evaluation, or 0.0000544 per cent on the Atomic Mass Data Center's, the nucleus splits by itself (spontaneous fission); the Data Center also lists double beta decay, far rarer still.

How is uranium used to date rocks?

The two long-lived isotopes end their decay chains at different lead isotopes, uranium-238 at lead-206 and uranium-235 at lead-207, Los Alamos says. Once a mineral becomes a closed system, exchanging no matter with its surroundings, it gathers lead at a known pace, and the ratio of lead to uranium records its age. Zircons from the Jack Hills in Australia have been dated this way at almost 4.4 billion years, according to OpenStax.

Is uranium glass radioactive?

Yes. Jefferson Lab's text in PubChem's record says the uranium in vaseline glass and glazes is radioactive and that such pieces deserve careful handling. The practice is old: Los Alamos describes yellow glass holding 1 per cent uranium oxide from a Roman villa near Naples. Uranium trioxide, an orange powder, went into Fiestaware plates, a glaze Los Alamos says was later discontinued for health reasons.