Uranium is a dense, silvery-gray metal known for its radioactivity and heavy mass. It is element 92 in the periodic table (symbol U), belonging to the actinide series (period 7, f-block). Uranium has no stable isotopes; its natural mix is about 99.3% uranium-238 and 0.7% uranium-235 (with a trace of U-234). This heavy metal has a high atomic weight (≈238.03), a very high density (~19.05 g/cm³ at 20 °C), and is a solid under standard conditions. Common oxidation states are +6, +5, +4, and +3, with +6 (the uranyl form UO₂²⁺) being especially characteristic in compounds. Uranium is best known as the principal fuel for nuclear reactors and weapons, but it has applications ranging from colored glass to counterweights. In room air it slowly tarnishes, developing dark oxides on its surface.

Atomic Structure and Electron Configuration

Uranium
Atomic number	92
Symbol	U
Group	Actinides
Discovery	1789 (M. H. Klaproth)
Electron configuration	[Rn] 5f3 6d1 7s2
Melting point	1132 °C
Period	7
Main isotopes	234U, 235U, 238U
Phase STP	Solid
Block	f
Oxidation states	+4, +5, +6
Wikidata	Q1098

With atomic number 92, a neutral uranium atom has 92 protons and 92 electrons. Its electron configuration is [Rn] 5f³6d¹7s², meaning that beyond the radon core it has three electrons in 5f orbitals, one in 6d, and two in 7s. These six outer electrons (commonly called valence electrons) give uranium complex chemistry and allow multiple oxidation states. Uranium’s large atomic number and f-electrons make it one of the heavier elements. Its atomic radius is large – the covalent radius is about 196 pm – reflecting its many electron shells. Despite its size, uranium’s 5f electrons are fairly tightly bound (the 5f orbitals contract slightly, an effect known as actinide contraction), so its electronegativity on the Pauling scale is only around 1.38. The first ionization energy (energy to remove one electron) is about 6.19 eV, comparatively low for a metal, which helps explain its metallic bonding and reactivity. In periodic trends, uranium follows the actinide pattern: it is less electronegative and has a larger radius than the preceding lanthanides, but as the heaviest actinide its radii begin to contract relative to earlier actinides.

Isotopes and Nuclear Properties

Uranium’s isotopes are all radioactive. The three isotopes present in nature are U-238 (mass 238.05, ~99.27% of natural uranium), U-235 (235.04, ~0.72%), and U-234 (234.04, ~0.0055%). Uranium-238 has a half-life of about 4.47 × 10^9 years, nearly the age of the Earth, while U-235’s half-life is about 7.04 × 10^8 years, and U-234’s is ~2.45 × 10^5 years. U-238 and U-235 both decay by alpha emission: U-238 decays through a series of alpha and beta decays to lead-206 (the “uranium series”), and U-235 decays to lead-207 (the “actinium series”). A brief version of these chains:

U-238 chain (uranium series): U-238 → Th-234 (α decay) → Pa-234 (β) → U-234 → … → Ra-226 → Rn-222 → … → Pb-206 (stable). This chain includes radon-222, a gas that contributes to natural background radiation.
U-235 chain (actinium series): U-235 → Th-231 → Pa-231 → … → Bi-211 → Tl-207 → Pb-207 (stable).

Because of these decays, uranium and its daughters (radium, radon, polonium, lead) are important in geology and medicine. For example, the varying proportions of uranium and lead isotopes in minerals allow uranium–lead dating, a key method for determining geological ages.

Uranium isotopes also have important nuclear properties. Uranium-235 is fissile: when struck by a thermal neutron, it can split (fission) to release energy and more neutrons. U-238 is fertile: it is not readily fissioned by thermal neutrons but can absorb a neutron to become U-239 (which beta-decays to Np-239 and then to plutonium-239), yielding another fissile element (Pu-239). These properties underpin the nuclear fuel cycle. Nuclear spins are 7/2 for U-235 and 0 for U-238 (both parity 0+), though these nuclear properties mainly matter in specialized applications (e.g. nuclear magnetic resonance studies or enrichment processes). No isotope of uranium is stable; even very short-lived isotopes (made artificially) and long-lived ones (U-234, U-235, U-238) are ultimately radioactive. The daughter products of uranium decay (especially radon gas) are significant for environmental radiation exposure.

Allotropes and Typical Compounds

Allotropes: Uranium metal has three solid allotropes (crystal forms), depending on temperature. At room temperature up to 940 K (~667 °C), the alpha phase (α-U) is orthorhombic. From 940 K to about 1048 K (~775 °C) it converts to beta-U, which is tetragonal. Above 1048 K and up to its melting point (~1405 K) it becomes gamma-U, which is body-centered cubic (bcc). The α-phase is relatively brittle at lower temperatures, whereas the β and γ phases (especially γ) are more ductile and malleable. The metastable forms revert on cooling.

Typical Compounds: Uranium forms many compounds in multiple oxidation states, but it is especially noted for its +6 state as the uranyl ion (UO₂²⁺). The linear uranyl ion (O=U=O) is very stable and appears in most uranium(VI) compounds. Important oxygen-containing compounds include:

UO₂ (uranium dioxide): A black, refractory solid; it is the primary fuel material in most nuclear reactors (fuel pellets of UO₂). UO₂ is stable and has a fluorite structure; it much less soluble in water than higher oxides.
U₃O₈ (triuranium octoxide): A yellow or green powder (“yellowcake”) used as an intermediate concentrate in uranium processing. It contains mixed oxidation states (approx U(IV) and U(VI)).
UO₃ (uranium trioxide): Able to exist in several forms (yellow/orange polymorphs); it is more soluble, and can form hydrates.

In lower oxidation states (+4, +3), uranium forms compounds akin to lanthanides: UO₂ (IV), U(OH)₄, UCl₄ (tetrahalides), U(N₃)₃, etc. Uranium also forms inorganic salts in higher oxidation states: for example, UO₂(NO₃)₂ (uranyl nitrate), UO₂(SO₄)₂, and carbonates like Na₂UO₂(CO₃)₃ (with the uranyl unit).

Halides: Especially notable is uranium hexafluoride UF₆, a volatile, pale yellow solid used for isotope separation (enrichment). At room temp UF₆ is a solid; above 56 °C it sublimes to a gas. It reacts violently with water. Uranium tetrafluoride UF₄ (“green salt”) is an important intermediate in fuel processing. Other halides include UCl₄, UCl₃, bromides, and iodides; these mostly exist as U(III) or U(IV) salts (higher halides U(V) and U(VI) halides are rare and unstable).

Hydrides and Carbides: Uranium reacts with hydrogen to form uranium hydride (UH₃, a pyrophoric black powder) if heated under H₂, and with carbon to form uranium carbide such as UC, UC₂. These compounds have very high melting points and have been investigated as nuclear fuel for high-temperature reactors. Uranium nitride (UN) and uranium carbide (UC) have also been studied as compact high-density fuels.

Organometallics: A few organo-uranium compounds exist (with uranium-carbon bonds), but they are mainly of academic interest. In summary, uranium typically forms high-oxidation-state ionic/covalent bonds, with its chemistry dominated by oxygen coordination (uranyl complexes and oxides).

Physical Properties

Uranium metal is lustrous and silvery-white when freshly cut, though it tarnishes in air to a dull gray or black as oxides form. It is one of the densest naturally occurring elements (density ~19.05 g/cm³ at 20 °C, about 70% denser than iron). It is slightly less dense than osmium or platinum, but denser than lead or gold. The metal is moderately hard (Mohs hardness ~6) and has a melting point of about 1405 K (1132 °C) and a boiling point near 4404 K (4131 °C).

Electrically, uranium is a conductive metal: its electrical conductivity is much lower than copper but still metallic (resistivity ~3.5×10^–7 Ω·m at 20 °C). Its thermal conductivity is relatively low for a metal (~27 W/m·K), so it does not transfer heat as well as copper or aluminum. Magnetically, uranium is paramagnetic (it has some unpaired electrons, but it does not exhibit strong ferromagnetism). Uranium’s heat capacity is about 0.12 J/g·K, and it expands upon heating like most metals, with an unusual small range of anisotropic thermal expansion due to its complex crystal structures.

At room conditions, uranium has the orthorhombic α-phase structure. As mentioned, heating transforms it through β (tetragonal) to γ (body-centered cubic) before melting. The α-phase is relatively brittle at room temperature (with slip surfaces making it not very malleable), while β and γ become progressively more ductile.

Uranium metal oxidizes on exposure to air or water, generally forming a thin layer of UO₂/U₃O₈ on the surface (the oxide prevents rapid further oxidation to some extent). Finely divided uranium (powder or filings) is pyrophoric – it can ignite in air. Normal uranium is stable in dry air but in moist air or at elevated temperature will corrode.

Spectroscopically, uranium compounds exhibit characteristic emission spectra (especially the uranyl ion, which gives greenish or yellow fluorescence under UV light) and has some electron transitions in the UV-visible range. The metal itself does not have notable visible emission lines for casual mention, but its presence in glass (as uranium glass) causes strong fluorescence under UV.

Chemical Reactivity and Trends

Uranium is chemically reactive in ways typical of a strong electropositive metal, especially in high oxidation states. It readily oxidizes; for example, it will tarnish in air, reacting with oxygen to form UO₂, and can burn in air at ~570 °C to form U₃O₈. It reacts with halogens at elevated temperatures, forming uranium halides (e.g. U + Cl₂ → UCl₄). The metal also reacts with hot water or steam (more slowly than alkali metals), producing UO₂ and hydrogen gas: \[3\,U + 4\,H2O → 3\,UO2 + 4\,H2.\] In nitric acid (a strong oxidizer), uranium dissolves vigorously, producing the uranyl ion (yellow-green UO₂²⁺) in solution (e.g. with nitrate anions). It dissolves more slowly in dilute hydrochloric or sulfuric acids (forming U(IV) or UO₂²⁺ species depending on conditions and oxidation). In basic or carbonate solutions, uranium(VI) forms highly soluble carbonate complexes (e.g. UO₂(CO₃)₃⁴⁻), which makes hexavalent uranium mobile in alkaline groundwater.

Chemically, uranium behaves somewhat like an electropositive actinide: its +3 oxidation state (U³⁺) is a strong reducing agent (similar to lanthanide +3 chemistry), capable of reducing water or hydrogen peroxide. In +4 (as UO₂ or UCl₄), uranium is still somewhat reducing (e.g. UO₂ can reduce peroxides to oxygen). In the +6 state (as uranyl UO₂²⁺), it is a powerful oxidizer. U(V) species (UO₂⁺) exist only fleetingly; two UO₂²⁺ units can share an electron to form a U(IV)-U(V)-U(IV) dimer, but true pentavalent uranium compounds are rare.

Because of these properties, uranium often forms stable complexes with oxygen and nitrogen ligands. For example, uranium(IV) can form octahedral complexes like [U(NH₃)8]^(4+), while uranium(VI) forms the linear uranyl ion that can coordinate 4-6 additional ligands (e.g. [UO₂(CO₃)₃]⁴⁻). Uranium also forms coordination compounds with large ligands like nitrates and phosphates. Complexation with fluoride or chloride is strong in low oxidation states (UF₃, UCl₄), whereas oxide and nitrate complexes dominate in high states.

In redox terms, uranium sits high on the electrochemical series among the metals, meaning it is readily oxidized (for example, more easily than iron or copper metals, but not as reactive as alkali metals). It will displace many metals from their salts. Unlike the most reactive metals (which are found at the bottom of the reactivity series), uranium requires oxidizing conditions to reach its highest state (+6).

Uranium’s chemical reactivity has practical implications: for instance, when exposed to moisture and oxidizers, it can corrode (rust) to form stable oxides. The corrosion products of uranium (like UO₂) help passivate the metal to some degree. Uranium metal can also absorb hydrogen to form UH₃, a brittle hydride. It will react with molten alkali (e.g. U in molten sodium) to form alloys or intermetallics.

Occurrence and Production

Cosmic and Terrestrial Abundance: Uranium is a primordial element formed by rapid neutron-capture processes in supernovae and neutron-star mergers. It has existed since before the formation of the solar system (~4.5 billion years). On Earth, uranium is relatively abundant by crustal standards: its average concentration in the continental crust is about 2–3 parts per million by weight, similar to elements like tin or lead. It is not uniformly distributed but tends to concentrate in certain minerals and geological settings. The oceans also contain dissolved uranium (~3 µg per liter of seawater), though this is a vastly dilute resource.

Major Ores: Uranium is mined from several types of deposits. The most important ore mineral is uraninite (principally UO₂), also called pitchblende, which has been historically mined in the Czech Republic, Germany, Canada, and elsewhere. Other minerals include carnotite (a uranium-vanadium oxide from Colorado), autunite (a calcium uranyl phosphate from France/USA), and coffinite (a uranium silicate). Sedimentary sandstones, coal and phosphates also sometimes have elevated uranium. Of note, about 1.7 billion years ago, natural nuclear fission reactors occurred in the Oklo region of Gabon; higher U-235 content then allowed moderated chain reactions for thousands of years until the isotope was depleted.

Mining and Extraction: Uranium is produced in many parts of the world. Major producing countries include Kazakhstan (the world’s largest producer at about 40% of global output), Canada, Australia, Namibia, Niger, Russia, Uzbekistan, China, and the United States. Kazakhstan alone produces on the order of 20,000–25,000 metric tonnes of uranium per year. Globally, annual primary production is in the range of 50,000–60,000 tonnes.

Extraction methods include open-pit or underground mining for hard-rock deposits, and in-situ leaching (injecting acid or alkaline solutions into the ground) for roll-front sandstone deposits, which is common in Kazakhstan and parts of the U.S. Ores are crushed and chemically processed: usually ground ore is leached with acid or alkaline solutions that dissolve uranium into solution. The uranium is then precipitated, filtered, and dried into a stable oxide concentrate called “yellowcake,” typically U₃O₈ powder. This yellowcake is the crude product shipped from mines.

Refining, Enrichment, and Depletion: The yellowcake (U₃O₈) is converted into uranium dioxide (UO₂) or uranium hexafluoride (UF₆) for use in reactors. UF₆, a gas at moderate temperatures, is the form used for isotope enrichment. Natural uranium has ~0.7% U-235; most nuclear reactors require enrichment to 3–5% U-235. Enrichment is done by gas centrifuges (modern method) or the older gaseous diffusion, separating UF₆ isotopically. The process yields enriched uranium (higher U-235 fraction) and “depleted uranium” (DU), which is mostly U-238 (about 0.2-0.3% U-235).

Depleted uranium is still mildly radioactive and chemically identical to natural U but is much less useful for fission power. DU finds secondary uses due to its high density.

Production Facilities and Major Players: Several large mining and fuel companies handle uranium. Some countries have large reserves (Australia ~30% of world’s uranium resources, Kazakhstan ~14%, Canada, Russia). In the nuclear fuel cycle, conversion facilities turn yellowcake into UF₆, and enrichment plants prepare reactor fuel. Some nations (e.g. France, Russia, China) produce fuel assemblies after conversion. The global production of uranium is complemented by civilian stockpiles and downblending of weapons-grade uranium (the “Megatons to Megawatts” program converted weapons U-235 into reactor fuel).

Applications and Technology

Uranium’s predominant use is in nuclear power generation. Enriched uranium fuel (as UO₂ pellets in fuel rods) is used in reactors worldwide to harness energy from nuclear fission. Both thermal reactors (light-water reactors, heavy-water reactors) primarily burn U-235 while breeding Pu-239 from U-238. Uranium’s high fission energy density means a small amount of material can release vast energy; roughly 1 kg of U-235 yields ~8×10^13 joules (millions of times more than burning a kilogram of coal).

Relatedly, uranium (in enriched form) was famously used in the first atomic bombs of World War II (e.g. the “Little Boy” bomb had about 64 kg of highly enriched U-235). U-238 itself is not directly used in bombs, but it can be converted to fissile Pu-239 in breeder reactors or in nuclear weapons to “boost” yield.

Depleted Uranium (DU): The uranium left after enrichment (mostly U-238) is called depleted uranium. DU has a higher density than steel and is pyrophoric. It has been used in military applications: for example, in armor-piercing ammunition (tanks and aircraft missiles) and in tank armor as a protective layer. When these projectiles strike a hard target, the DU can self-sharpen and burn, penetrating armor effectively. DU is also used as a radiation shielding material (e.g. in containers for radioactive sources) because of its density.

Other Industrial Uses: Uranium had some historical uses in glass and ceramics; uranium salts (particularly uranium oxide) imparted bright yellow and green colors to glass (called uranium glass or Vaseline glass) and glazes. Those uses largely ended due to radioactivity concerns, but antique uranium glassware is known for its fluorescent glow under UV light. Uranium oxide has also been used as a pigment (some yellow glazes).

In the past, uranium tetrafluoride (UF₄) was used in ceramic glazes. Chemically, uranium compounds have been explored as catalysts (for example, in certain organic oxidation reactions), but these are niche. Uranium was once used in X-ray tubes as a target material because of its high atomic number, but tungsten and copper are more common now.

Nuclear Technology: Uranium is key in breeder reactors and the fuel cycle. In fast breeder reactors, U-238 is deliberately converted to Pu-239 to create more fuel. Thorium fuel cycles (using U-233 bred from thorium) rely on uranium separators and understanding their chemistry. Uranium is also used in research (e.g. depleted U metal rods are used as a stable form of giant atomic mass for experiments).

Tracers and Misc: Radioactive isotopes of uranium have been used as tracers in geology and hydrology (though chemical toxicity and radioactivity limit this). The longer-lived isotopes (U-234, U-238) even trace environmental processes (e.g. measuring U-series in groundwater).

Biology, Environment, and Safety

Uranium has no known essential biological function and is considered a toxic heavy metal. It is both chemically toxic (like other heavy metals) and radioactive (although its relatively weak radioactivity makes chemical toxicity the main biological issue).

Health Effects: Ingestion or inhalation of uranium compounds can cause kidney and bone damage. When soluble, uranium compounds can be absorbed and deposit in the kidneys and bones; insoluble oxides tend to be excreted more quickly but can cause lung damage if inhaled as dust. Radiation from uranium is mostly alpha particles (emitted by isotopes like U-238 and U-234). Alpha radiation cannot penetrate skin but is dangerous if inhaled or ingested, as it can cause cellular damage internally. U-238 also emits a small amount of penetrating gamma radiation. Another hazard is radon gas (Rn-222) produced in the decay of U-238 in soils or homes, which can lead to lung cancer on chronic exposure.

Occupational exposure limits are regulated (e.g. by the U.S. Nuclear Regulatory Commission or OSHA). For example, uranium dust exposure in the workplace is tightly controlled (effective dose limit ~50 mSv/year for radiation workers, plus chemical exposure limits). Depleted uranium (DU) has similar chemical toxicity but about half the radioactivity of natural uranium; its main hazard is heavy-metal. In short, uranium requires careful handling with protective equipment to avoid inhalation or ingestion, and shielding/waste protocols for its radioactivity.

Environmental Impact: Uranium mining and milling produce tailings that contain radium and residual uranium, which can leach into water and release radon. Spent nuclear fuel (largely U and newly formed Pu) is a long-term waste hazard requiring isolation. In water, uranium mobility depends on oxidation state: U(VI) (as UO₂²⁺) is quite soluble, especially in carbonate water, so it can contaminate groundwater; U(IV) (as UO₂) is insoluble. Thus reducing conditions tend to immobilize uranium (so-called in-situ remediation can precipitate U as UO₂).

Biologically, plants and microorganisms do not require uranium, but they can inadvertently concentrate it from soil. Some plants near uranium mines can take up uranium into tissues; animals can then ingest it through contaminated vegetation. The U.S. Environmental Protection Agency and other agencies set limits on uranium in drinking water (e.g. EPA limit ~30 µg/L) based on combined chemical and radiological risk. In the oceans, marine organisms incorporate uranium only in trace amounts (much less than calcium).

Safety and Handling: uranium metal and compounds must be stored to prevent inhalation/ingestion. Protective measures include respirators or glove boxes when handling fine powders or UF₆ (since UF₆ reacts with moisture to produce corrosive HF and uranyl fluoride). Drinking water limits for uranium are governed by its solubility and intake dose. Uranium-contaminated sites require remediation (like removal of tailings).

In summary, uranium’s durability and radioactivity mean its environmental presence requires long-term management. However, recognition of its hazards and tight regulation (along with technologies like encapsulation of waste) mitigate many risks.

History and Etymology

Uranium was discovered in 1789 by the German chemist Martin Heinrich Klaproth. Working with pitchblende (a uranium oxide mineral from Joachimsthal, Bohemia), Klaproth isolated a yellow compound and identified a new metal, which he named “uranium” in honor of the recently discovered planet Uranus (the planet had been discovered in 1781). Klaproth’s uranium compound was actually UO₂; the metal itself was later isolated (but difficult due to uranium’s reactivity).

In the late 19th century, uranium became famous for radioactivity. In 1896, Henri Becquerel discovered that uranium salts could fog photographic plates (proving spontaneous radioactivity). This led Pierre and Marie Curie to further investigate radioactive elements; they discovered polonium and radium in uranium ores. By 1905, Bertram Boltwood showed that uranium decayed to helium and lead, establishing U-Pb dating for geology. Ernest Rutherford proposed using uranium decay as a clock to age the Earth (leading to the first estimates of Earth’s age in the early 1900s). The word “uranium” comes from the Greek Ouranos (“sky” or “Heaven”), via the name of the planet.

In the 20th century, uranium’s most profound roles emerged. In 1934, Enrico Fermi produced radioactive elements by neutron-irradiating uranium. In 1938, Otto Hahn and Fritz Strassmann (with Lise Meitner and Otto Frisch) discovered nuclear fission of uranium (splitting the atom) by bombarding U-235 with neutrons. This discovery led to the first nuclear chain reaction (Chicago Pile-1, 1942, by Fermi) and then to the development of nuclear weapons. The Manhattan Project (1942–1945) massively expanded uranium production and forged the first atomic bombs (Hiroshima, 1945 used uranium-235).

Post-World War II, uranium mining and nuclear power grew. The Cold War produced a large stockpile of military uranium and plutonium; the first nuclear reactors for electricity (e.g. Obninsk USSR 1954, Calder Hall UK 1956) used enriched U fuel. Depleted uranium, a byproduct of enrichment, found applications such as tank armor by the 1970s. Environmental and safety concerns over nuclear technology (from incidents like Three Mile Island, Chernobyl, Fukushima) have since influenced uranium policy and handling. Uranium’s role in climate (the heat from radioactive decay helps drive Earth’s interior processes) and societal energy needs continues to be significant.

Throughout this history, uranium’s name and symbols have been tied to its mythos: the discoverer Klaproth’s naming after the sky fits given uranium’s celestial origins and its storied journey from planet to element.

Property	Value
Symbol	U
Atomic number (Z)	92
Category	Actinide (radioactive metal, f-block, period 7)
Atomic weight	238.02891 (standard atomic weight, no stable isotopes)
Electron configuration	[Rn] 5f³ 6d¹ 7s²
Common oxidation states	+6, +5, +4, +3 (with +6 as UO₂²⁺ uranyl)
Atomic/covalent radius	~156 pm (metallic); covalent ~196 pm
Electronegativity	1.38 (Pauling scale)
First ionization energy	6.194 eV
Density (20 °C)	19.05 g·cm⁻³ (≈1.905×10^4 kg·m⁻³)
Melting point	1405 K (1132 °C)
Boiling point	4404 K (4131 °C)
Crystal structure	Orthorhombic (α-phase at room temperature; tetragonal β and bcc γ phases at higher T)
Magnetic ordering	Paramagnetic (no magnetic ordering at ambient conditions)
Thermal conductivity	~27 W/(m·K)
Heat capacity (20 °C)	~0.12 J/(g·K)
Appearance	Silvery-gray metal; tarnishes in air to gray/black oxide
Natural isotopes	⁻²³⁸U (99.27%, t₁/₂≈4.47×10⁹ y), ²³⁵U (0.72%, t₁/₂≈7.04×10⁸ y), ²³⁴U (0.0055%, t₁/₂≈2.45×10⁵ y)
Major decay products	Mostly Pb-206 (from U-238 series) and Pb-207 (from U-235 series)
Notable uses	Nuclear reactor fuel; nuclear weapons; depleted uranium (armor, ammo, shielding); uranium glass
Discovery	1789 by Klaproth (from pitchblende); named for planet Uranus (Greek Ouranos, “sky”),
Key compounds	UO₂ (fuel), U₃O₈ (ore concentrate), UF₆ (enrichment), UO₃ (oxide), UCl₄, UH₃, UO₂²⁺ (uranyl)