Element identity
Atomic number, periodic identity and reference mass/isotope conventions are established and source-reviewed.
Atomic / electronic model
Atomic configurations and orbital teaching models are reference interpretations; heavy-element electronic structure is more complex than a classical shell picture.
Nuclear / isotope data
Half-lives, isotope identities and decay information are taken from evaluated nuclear-data/reference sources and remain isotope-specific.
Material / phase data
Ordinary bulk and phase values are shown only where defensible; solid-state transitions are kept separate from melting.
Geography / safety boundary
Occurrence, discovery and public research context may be mapped. Operational radioactive-material inventories, access routes and sensitive facility details are intentionally not mapped.
The lens describes evidence status, not confidence theatre. “Unknown” is kept unknown, and teaching schematics are not presented as direct measurements.
Uranium (U)
Explore uranium as a dense radioactive actinide: its 92-proton atomic structure, natural isotopes, long half-lives, alpha decay, solid-state phase changes, chemistry, geology, mining geography and carefully qualified nuclear-energy context.
Uranium atomic number, mass, protons, electrons and isotopes
Uranium: quick answers
How many protons, neutrons and electrons does uranium have?
Uranium’s atomic number is 92, so every uranium atom has 92 protons, and a neutral atom also has 92 electrons. Its most common natural isotope, uranium-238, has 146 neutrons (other isotopes have different neutron counts).
What is the symbol for uranium?
The chemical symbol for uranium is U.
Is uranium a solid, liquid or gas at room temperature?
Uranium is a solid at room temperature (about 25 °C).
What family (group) is uranium in?
Uranium is an actinide, in period 7 (the f-block row shown below the main table) of the periodic table.
What is the electron configuration of uranium?
The ground-state electron configuration of uranium is [Rn] 5f³ 6d¹ 7s².
From atomic number to chemistry
Read these as a chain of causes, not as isolated facts. Each step links to the concept hub if you want the underlying idea explained.
Ninety-two protons define uranium; all uranium isotopes share that proton count.
Uranium is an actinide with f-electron chemistry more complex than a simple main-group shell model.
Its actinide position connects electronic structure, multiple oxidation states and heavy-element behavior.
The isotopes share chemistry but differ in neutron count, abundance and nuclear behavior.
Spontaneous decay, half-life and neutron-induced fission are related nuclear topics but are not the same process.
Uranium is an actinide in Period 7
Uranium belongs to the actinide series, the f-block row usually displayed below the main periodic table. It follows protactinium and precedes neptunium. Uranium is naturally occurring; neptunium and the heavier transuranium elements are mainly produced through nuclear reactions.
Uranium Visual Lab
Decode the U tile, rotate a ²³⁸U educational nucleus, inspect a shell model, compare representative 7s/6d/5f probability-cloud shapes and connect uranium to geology, energy, dating and materials science.
Seven facts packed into one square
Uranium in one minute
All natural uranium isotopes are radioactive. None is stable.
²³⁸U dominates natural uranium. It accounts for about 99.2742% in the NIST reference composition.
²³⁵U is much less abundant. It is about 0.7204% naturally and is important in nuclear fission technology.
Uranium changes crystal structure before it melts. α, β and γ solid phases occur as temperature rises.
Safety has two dimensions. Uranium is radioactive and also a chemically toxic heavy metal.
92 electrons across seven principal shells
92 e⁻
The shell totals are 2-8-18-32-21-9-2. This is useful bookkeeping, but the actual ground state is better represented by the orbital configuration [Rn] 5f³ 6d¹ 7s².
Orthorhombic structure
At ambient pressure, alpha uranium is the stable low-temperature crystal phase. Its structure is orthorhombic and strongly anisotropic compared with simple close-packed metals.
Materials-science phase referenceWhy are 5f orbitals important?
Uranium is an early actinide. Its 5f, 6d and 7s orbitals are relatively close in energy, so several can matter in bonding. That helps explain uranium's richer oxidation-state chemistry compared with a simple main-group element.
Where does uranium matter?
Clickable cards connect the element to electricity generation, geochronology, geology, regulated materials use, historic glass/ceramics and scientific research.
Uranium is the primary raw material for today's fission-reactor fuel cycle
Nuclear reactors use uranium-based fuel to release energy through controlled fission. The isotope story matters because natural uranium is overwhelmingly ²³⁸U while ²³⁵U is much less abundant. This page keeps that discussion conceptual and does not provide operational enrichment or reactor-design instructions.
Radioactivity, half-life and fission: related ideas that should not be confused
Uranium is an ideal element for separating three concepts: spontaneous radioactive decay, isotope half-life and neutron-induced fission. They involve the nucleus, but they are not interchangeable terms.
Radioactive decay is spontaneous
The naturally occurring uranium isotopes are unstable. Their most common decay mode is alpha emission, in which the nucleus transforms into a different nuclide.
Half-life is a statistical timescale
A half-life is the time required for half the atoms in a very large population to decay on average. It does not predict when one particular atom will decay.
Fission is a different nuclear process
Uranium-235 can undergo fission after neutron absorption under suitable conditions. That process is distinct from the slow spontaneous alpha decay that gives uranium its natural radioactivity.
Decay chains create daughter nuclides
Long uranium decay series proceed through multiple radioactive daughters before ending at stable lead isotopes. Those parent-daughter systems are important in geochronology.
Why “alpha radiation does not penetrate skin well” is not the whole safety story
Alpha particles have very limited external penetration, so intact skin blocks them effectively. But uranium inside the body can expose sensitive tissue, and uranium is also a chemically toxic heavy metal with the kidneys as an important target organ. Real uranium materials should be handled only under applicable professional, institutional and regulatory controls.
NRC / EPA / ATSDRUranium physical, atomic, chemical and nuclear reference data
The table uses the same typography and interaction pattern as Magnesium, Carbon and Gold. Where reputable compilations disagree, the disagreement is shown rather than silently choosing a number.
| Atomic number | 92 | NIST |
|---|---|---|
| Standard atomic weight | 238.02891(3) | NIST isotopic-composition table |
| Electron configuration | [Rn] 5f³ 6d¹ 7s² | NIST ground state |
| First ionization energy | 6.19405 eV | NIST |
| Atomic radius, non-bonded | 2.41 Å | RSC compilation; radius definitions vary |
| Covalent radius | 1.83 Å | RSC compilation |
| Electronegativity | Reference-dependent | PubChem lists 1.38 (Pauling); RSC lists 1.7. Treat as a compiled scale value, not a precision measurement. |
| Appearance | Silvery metal | RSC / PubChem |
|---|---|---|
| State at 20 °C | Solid | RSC |
| Density | ≈19.1 g/cm³ | RSC reference |
| Low-temperature crystal phase | α-U, orthorhombic | Materials-science references |
| Magnetic description | Weak / complex | Do not reduce uranium to a simple classroom ferro/para label without conditions. |
| α → β solid transition | ≈942 K | IAEA thermophysical-data reference; values can vary slightly with source/sample |
|---|---|---|
| β → γ solid transition | ≈1049 K | IAEA thermophysical-data reference |
| Melting point | 1408 K (1135 °C) | RSC |
| Boiling point | 4404 K (4131 °C) | RSC |
| Specific heat capacity | 116 J kg⁻¹ K⁻¹ | RSC advanced data |
| Common oxidation states | +3, +4, +5, +6 | RSC |
|---|---|---|
| Important high oxidation-state motif | Uranyl, UO₂²⁺ | Common U(VI) aqueous/coordination motif |
| Air behavior | Oxidizes / tarnishes | Pure metal is reactive compared with noble metals |
| Finely divided metal | Can be pyrophoric | Professional safety controls required; not a classroom handling material |
| Stable isotopes | 0 | All uranium isotopes are radioactive |
|---|---|---|
| ²³⁸U abundance | 0.992742(10) | NIST isotope composition |
| ²³⁵U abundance | 0.007204(6) | NIST isotope composition |
| ²³⁴U abundance | 0.000054(5) | NIST isotope composition |
| ²³⁸U half-life | ≈4.468 billion years | NIST/DOE educational references |
| ²³⁵U half-life | ≈704 million years | NIST/DOE educational references |
| ²³⁴U half-life | ≈245,500 years | DOE/ICRP reference |
| Primary natural radiation | Alpha decay | Natural uranium isotopes primarily undergo alpha decay |
|---|---|---|
| External alpha penetration | Very low | Skin/paper can stop alpha particles; this does not make internal exposure harmless |
| Chemical toxicity | Important | ATSDR identifies kidney toxicity as a key concern for uranium exposure |
| Handling | Regulated / professional | This educational page is not a handling or exposure-control manual |
Uranium compared with thorium and plutonium
These neighboring actinides illustrate why f-block trends are more complicated than simple main-group patterns. Electron configurations, oxidation states, radioactivity and 5f-electron participation all evolve across the series.
| Family | Actinide |
|---|---|
| Configuration | [Rn] 6d² 7s² |
| Family | Actinide |
|---|---|
| Configuration | [Rn] 5f³ 6d 7s² |
| Family | Actinide |
|---|---|
| Configuration | [Rn] 5f⁶ 7s² (reference shorthand) |
Uranium state at temperature: α, β and γ solids before melting
Unlike Carbon, uranium does not skip the liquid state on this approximately-standard-pressure path. But it still needs a special note because the solid itself changes crystal structure twice before melting.
≈942 Kβ→γ
≈1049 Kmelt
1408 Kboil
4404 K6000 K
At 293 K, uranium is solid α-U. Heating at approximately standard pressure crosses β-U and γ-U solid crystal phases before the metal melts.
The slider is an educational one-pressure path. Pressure, impurities and sample history can shift detailed phase boundaries.Where on Earth is uranium mined and where are major resources?
The map separates current mine production, identified resources and natural occurrence. Production figures are year-dependent economic data; natural uranium itself occurs far more widely than a handful of producing countries.
From a new mineral element to radioactivity and nuclear science
Klaproth identifies a new element
Martin Heinrich Klaproth studied pitchblende and identified a new substance, naming uranium after the recently discovered planet Uranus.
Péligot isolates uranium metal
Eugène-Melchior Péligot produced metallic uranium, showing that Klaproth's earlier material had been a compound rather than pure metal.
Becquerel discovers radioactivity
Henri Becquerel found that uranium compounds emitted penetrating radiation capable of fogging photographic plates without prior light exposure.
Fission changes uranium's role
Research into the atomic nucleus established uranium as central to nuclear physics, power generation, geochronology and the broader actinide sciences.
How uranium moves from ore to useful material: high-level only
Uranium mining and nuclear-fuel production are heavily engineered and regulated industrial activities. The steps below are intentionally conceptual and omit operational parameters, recipes and design details.
Locate and characterize an ore body
Geologists identify uranium-bearing deposits and determine mineralogy, grade, hydrogeology and environmental context.
Mine or recover uranium-bearing material
Different deposits use different mining or recovery approaches under site-specific licensing and environmental controls.
Concentrate and purify uranium compounds
Industrial processing separates uranium from much larger quantities of rock or solution and produces controlled chemical feed material.
Manufacture regulated fuel or other approved products
Further isotope and fuel-cycle steps depend on reactor technology and national regulation. Element Lookup keeps those engineering details outside this introductory chemistry page.
What is uranium used for?
Nuclear electricity
Uranium-based fuels supply fission reactors that generate heat for electricity production and other energy applications.
Geochronology
Uranium-lead isotope systems provide some of the most important clocks for dating ancient minerals and reconstructing Earth history.
Dense regulated materials
Depleted uranium has specialized high-density uses such as counterweights and radiation shielding under applicable controls. This page does not cover weapon applications.
Historic glass and ceramics
Uranium compounds were historically used to color some glass and ceramic glazes. Modern use is restricted and regulated in many contexts.
Geological exploration
Uranium minerals, decay products and geochemical behavior are important in ore geology, environmental science and planetary studies.
Actinide research
Uranium chemistry helps scientists understand 5f bonding, relativistic electronic structure, radioactive materials and the behavior of heavier actinides.
Oxidation states and the uranyl ion
Uranium supports several common oxidation states, especially +3 through +6. In oxygen-rich chemistry, uranium(VI) often appears in the linear uranyl unit UO₂²⁺.
Uranium(IV)
U(IV) is important in minerals and reduced environments. Uranium dioxide, UO₂, is a familiar U(IV) compound and a major nuclear-fuel ceramic.
Uranium(VI) / uranyl
The uranyl ion is a central U(VI) motif in aqueous and coordination chemistry. Its chemistry strongly influences uranium mobility in oxidizing environments.
Multiple oxidation states
Uranium's 5f, 6d and 7s electrons are close enough in energy that several oxidation states can be chemically accessible, especially +3, +4, +5 and +6.
Uranium-238, uranium-235 and uranium-234
Natural uranium is a mixture of three long-lived radioactive isotopes. Their abundances are tiny or dominant depending on isotope, while their half-lives span hundreds of thousands to billions of years.
92 protons · 146 neutrons · half-life ≈4.468 billion years
Uranium-238 makes up nearly all natural uranium. It primarily undergoes alpha decay and begins a long decay series that ultimately reaches stable lead-206. Its long half-life is one reason substantial primordial uranium remains on Earth today.
Five-question Uranium check
What is uranium’s atomic number?
Which isotope dominates natural uranium?
What happens before uranium melts as it is heated?
What is uranium’s ground-state configuration?
Which statement is correct?
Uranium questions: quick answers first, then the full explanation
Each answer separates atomic chemistry, radioactivity and nuclear-energy concepts so one idea is not used as a shortcut for another.
What is uranium’s atomic number, and how many protons and electrons does it have?
Short answer: Uranium’s atomic number is 92, so every uranium nucleus has 92 protons. A neutral uranium atom has 92 electrons.
Atomic number is defined by proton count. Change the number of neutrons and you get another uranium isotope; change the number of protons and the element is no longer uranium.
For example, ²³⁸U has 92 protons and 146 neutrons, while ²³⁵U has the same 92 protons but 143 neutrons.
What is uranium’s electron configuration?
Short answer: [Rn] 5f³ 6d¹ 7s².
The radon core contains 86 electrons. The remaining six are distributed as 5f³ 6d¹ 7s² in the NIST ground-state configuration.
Because the 5f, 6d and 7s levels are relatively close in energy, uranium chemistry cannot be understood by treating only the two outermost 7s electrons as chemically relevant.
Why is uranium radioactive?
Short answer: Every naturally occurring uranium isotope is nuclear-unstable, so uranium nuclei spontaneously transform through radioactive decay even though their half-lives can be extremely long.
Natural uranium is dominated by ²³⁸U with smaller amounts of ²³⁵U and ²³⁴U. These isotopes are all radioactive, with alpha decay important in their decay chains. Their long half-lives explain why primordial uranium remains on Earth billions of years after the planet formed.
Radioactive decay is not the same process as reactor fission. Decay occurs spontaneously according to nuclear quantum probabilities; neutron-induced fission is a different nuclear reaction.
Key point: Uranium’s radioactivity is a property of its unstable nuclei, not of its electron chemistry.
Why is uranium-235 important if it is less than 1% of natural uranium?
Short answer: ²³⁵U has nuclear properties that make it important for controlled fission energy systems even though its natural abundance is only about 0.7204%.
Natural abundance tells you how common an isotope is; it does not tell you how that nucleus responds to neutrons. Uranium-235 is fissile, meaning neutron absorption can lead to fission and additional neutrons under suitable controlled conditions.
This page intentionally keeps that explanation conceptual rather than providing enrichment, criticality or reactor-design procedures.
Does uranium go straight from one solid to liquid?
Short answer: Not structurally. Uranium remains physically solid, but it passes through α, β and γ crystal phases before melting near 1408 K at approximately standard pressure.
This is why the temperature slider has more than the usual Solid/Liquid/Gas labels. A change of crystal structure is a solid-solid phase transition: the physical state stays solid while the atomic arrangement changes.
The approximate boundaries used here are about 942 K for α→β and 1049 K for β→γ, followed by melting at 1408 K.
Why is uranium dangerous?
Short answer: Uranium hazard has both chemical and radiological dimensions, and the importance of each depends on isotope mixture, chemical form, amount and exposure route.
Alpha particles have very short range outside the body, so external exposure to an intact uranium source is very different from inhaling or ingesting uranium-containing material. For natural and depleted uranium, ATSDR identifies chemical toxicity—especially to the kidneys—as an important concern, while radiological dose becomes more significant for internal exposure and for mixtures with greater specific activity.
A scientifically useful safety statement therefore avoids treating all “uranium” exposures as identical. Metal, soluble uranyl compounds, insoluble dusts and different isotope mixtures can have different exposure behavior.
Key point: Risk depends on chemical form, isotope/activity and route of exposure; “radioactive” alone is not a complete hazard assessment.
Where is uranium mined today?
Short answer: In 2024, Kazakhstan was the largest uranium producer, followed by Canada and Namibia.
World Nuclear Association reports 2024 production of about 23,270 tU for Kazakhstan, 14,309 tU for Canada and 7,333 tU for Namibia, with Australia and Uzbekistan also among the leading producers.
Production changes from year to year, so the map labels these as 2024 figures rather than treating them as permanent element properties.
How can uranium help date rocks?
Short answer: Long-lived uranium isotopes decay through known chains to lead isotopes, creating parent-daughter clocks used in geochronology.
Minerals that incorporate uranium but initially exclude much lead can preserve especially useful isotopic records. By measuring uranium and radiogenic lead isotope relationships and accounting for decay constants, geochronologists can estimate crystallization ages.
Real U-Pb geochronology is more sophisticated than a single classroom half-life equation because minerals can gain or lose parent/daughter isotopes and often require concordance tests and calibrated analytical methods.
Scientific sources for Uranium
Atomic, isotopic, thermal, safety and geographic statements are tied to scientific, government or government-sponsored sources. Year-dependent production data are labeled with the source year.
- NIST — Uranium atomic weight, ground-state electron configuration and ionization energy
- NIST — Uranium isotopic compositions and atomic masses
- Royal Society of Chemistry — Uranium physical properties, oxidation states, isotopes, history and uses
- IAEA — Thermophysical Properties of Materials for Nuclear Engineering; uranium α/β/γ phase ranges and transition-temperature references
- PubChem — Structured Uranium property and isotope cross-checks
- OECD NEA / IAEA — Uranium 2024: Resources, Production and Demand
- World Nuclear Association — Uranium mine production by country, updated January 2026
- U.S. NRC — Radiation basics and alpha-particle penetration
- U.S. EPA — Uranium radionuclide basics and exposure context
- ATSDR — Toxicological Profile for Uranium
Questions to ask next about Uranium
A good element lesson should lead to the next useful question, not end after a list of facts.
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