Uranium Oxidation States
Reference for the oxidation states of Uranium (U, element 92) — which states exist, why +6 dominates in air, key compounds like UO₂ and UF₆, and an interactive oxidation state explorer.
Uranium (U) is a dense, silvery actinide metal and the heaviest naturally occurring element in significant quantities. Unlike simpler actinides that fix at +3, uranium's partially filled 5f orbitals allow it to access +3, +4, +5, and +6 oxidation states — making it one of the most chemically versatile heavy elements.
Oxidation States
| State | Status | Key Example | Notes |
|---|---|---|---|
| +6 | Most common (air) | UF₆, UO₃, UO₂²⁺ | Stable in air; uranyl ion dominates in oxidizing water |
| +4 | Common (reducing) | UO₂, UCl₄, UF₄ | Nuclear fuel form; stable under low-oxygen conditions |
| +5 | Rare / unstable | UF₅, UCl₅ | Disproportionates to U(IV) + U(VI) in solution |
| +3 | Rare / reducing | UCl₃, UI₃ | Strong reducing agent; oxidized readily in air or water |
Interactive Oxidation State Explorer
Why Does Uranium Have Multiple Oxidation States?
Uranium's ground-state configuration is [Rn] 5f³ 6d¹ 7s². It has six electrons beyond the radon core across three subshells (5f, 6d, 7s).
In actinides up to about curium, the 5f orbitals are close enough in energy to 6d and 7s to participate in bonding. Uranium can donate 3, 4, 5, or 6 electrons depending on the ligand environment, giving four oxidation states. Its lighter neighbor thorium (Th) is mostly +4 only; its heavier neighbors extend even further.
Most lanthanides (La through Lu) fix at +3 because their 4f electrons are too contracted to participate in bonding. Uranium's 5f electrons are spatially more extended and accessible, which is the fundamental reason actinides show richer oxidation state chemistry than lanthanides.
Key Uranium Compounds by Oxidation State
Colorless volatile solid (sublimes 56 °C). Used in uranium enrichment centrifuges. All six F are equivalent; U is in the highest +6 state.
Linear O=U=O dication; dominant U(VI) species in water. Highly soluble and mobile in oxidizing groundwater. Forms stable carbonate complexes.
Black ceramic solid. Primary form of nuclear fuel pellets in light-water reactors. Fluorite crystal structure. Melting point ~2865 °C.
Dark green crystalline solid. Intermediate in uranium metal production. Reacts with water to give UO₂²⁺ in solution under oxidizing conditions.
Light-yellow solid. Prepared by reduction of UF₆. Disproportionates in solution to U(IV) + U(VI). One of the few accessible U(V) compounds.
Red-brown solid. Highly air-sensitive; oxidizes immediately to U(IV) in air or water. UCl₃ is a strong reductant used in experimental actinide chemistry.
Comparison to Neighboring Actinides
| Element | Z | Config (beyond Rn) | Oxidation States | Most Stable |
|---|---|---|---|---|
| Actinium (Ac) | 89 | 6d¹ 7s² | +3 only | +3 |
| Thorium (Th) | 90 | 6d² 7s² | +4 (mainly) | +4 |
| Protactinium (Pa) | 91 | 5f² 6d¹ 7s² | +4, +5 | +5 |
| Uranium (U) | 92 | 5f³ 6d¹ 7s² | +3, +4, +5, +6 | +6 (air) / +4 (reducing) |
| Neptunium (Np) | 93 | 5f⁴ 6d¹ 7s² | +3, +4, +5, +6, +7 | +5 |
As atomic number increases across early actinides, the 5f orbitals become progressively more involved in bonding, expanding the accessible oxidation state range before contracting again in heavier actinides.
Environmental and Nuclear Significance
The interplay between U(IV) and U(VI) is central to environmental geochemistry. In oxidizing surface water, uranium exists primarily as the soluble uranyl ion UO₂²⁺ (+6), making it mobile and a contamination concern near mining sites. In reducing conditions (deep groundwater, sediments rich in organic matter), microbial activity can reduce U(VI) to insoluble UO₂ (+4), immobilizing uranium in place — a natural attenuation mechanism studied for site remediation.
In the nuclear fuel cycle, natural uranium (predominantly ²³⁸U with 0.72% ²³⁵U) is converted to UF₆ (+6) for isotopic enrichment, then converted back to UO₂ (+4) ceramic pellets for reactor fuel. Spent fuel contains a complex mix of oxidation states as fission products accumulate.
Summary
Reference for the oxidation states of Uranium (U, element 92) — which states exist, why +6 dominates in air, key compounds like UO₂ and UF₆, and an interactive oxidation state explorer.
How it works
- Review the oxidation states table to see which states (+3 through +6) are confirmed for uranium.
- Read the electron configuration section to understand how 5f electrons enable multiple oxidation states.
- Use the interactive explorer to select an oxidation state and see the corresponding compounds and properties.
- Check the compounds section for UO₂, UF₆, UCl₄, and the uranyl ion UO₂²⁺.
- Compare uranium to its actinide neighbors (Pa, Np) to place its chemistry in context.
Use cases
- Identifying the oxidation state of uranium in a given compound for a chemistry problem or exam.
- Understanding nuclear fuel chemistry and why UO₂ uses the +4 state.
- Writing balanced redox equations involving uranium species in aqueous solution.
- Explaining why uranium hexafluoride (UF₆) is used in uranium enrichment centrifuges.
- Comparing uranium oxidation chemistry to other heavy actinides like neptunium and plutonium.
- Studying environmental uranium speciation between U(IV) and U(VI) in groundwater.