Protactinium Oxidation States
Reference for the oxidation states of Protactinium (Pa, element 91) — the dominant +5 and significant +4 states, key compounds, and an interactive explorer.
Protactinium (Pa) is a dense, silvery-gray actinide metal with atomic number 91. It is among the rarest naturally occurring elements, found only in trace amounts in uranium ores. Unlike the simpler actinides before it, Pa displays multiple oxidation states — most prominently +5 and +4 — because its 5f electrons are energetically available for bonding.
Oxidation States
| State | Status | Ion / Form | Notes |
|---|---|---|---|
| +5 | Most stable | PaO₂⁺ | Dominant in aqueous solution; forms dioxo "protactinyl" cation; colorless |
| +4 | Well-characterized | Pa⁴⁺ | Stable under reducing conditions; tetravalent aquo ion; oxidized to +5 by air |
| +3 | Rare | Pa³⁺ | Reported in some solid-state systems; not stable in aqueous solution |
| +2 | Not observed | — | No confirmed compounds |
Oxidation State Explorer
Why Multiple Oxidation States?
Protactinium's ground-state configuration is [Rn] 5f² 6d¹ 7s². It has five electrons outside the radon core: two in 5f, one in 6d, and two in 7s.
In early actinides (Pa, U, Np, Pu), the 5f orbitals are energetically close to the 6d and 7s levels. Electrons in 5f can participate in bonding, allowing Pa to reach +5 by losing all five outer electrons.
Removing four electrons (one 5f, one 6d, two 7s) gives Pa⁴⁺ with configuration [Rn] 5f¹. This 5f¹ ion is stable under reducing conditions, giving Pa chemistry in both +4 and +5.
Thorium (Z=90, [Rn] 6d² 7s²) lacks accessible 5f electrons for a fifth ionization step, capping it at +4. Actinium (Z=89, [Rn] 6d¹ 7s²) has no 5f electrons at all and is limited to +3. Protactinium bridges them, being the first element where 5f involvement in oxidation chemistry is clearly demonstrated.
Key Protactinium Compounds
White solid; the most common Pa oxide; formed on combustion of Pa metal in air.
Volatile white solid; reacts with fluorine at elevated temperatures; isostructural with NbF₅ and TaF₅.
Yellow solid; one of the most studied Pa compounds; used in synthetic protactinium chemistry.
Black solid with fluorite structure; isostructural with ThO₂ and UO₂; formed under reducing conditions.
Brown solid; isostructural with UF₄ and ThF₄; confirmed the +4 state in solid-state Pa chemistry.
Green solid; synthesized from Pa metal and chlorine; stable when kept away from oxidizing agents.
Early Actinide Oxidation State Comparison
| Element | Z | Config | States | Most stable |
|---|---|---|---|---|
| Actinium (Ac) | 89 | [Rn] 6d¹ 7s² | +3 | +3 |
| Thorium (Th) | 90 | [Rn] 6d² 7s² | +2, +3, +4 | +4 |
| Protactinium (Pa) | 91 | [Rn] 5f² 6d¹ 7s² | +3, +4, +5 | +5 |
| Uranium (U) | 92 | [Rn] 5f³ 6d¹ 7s² | +3, +4, +5, +6 | +6 |
The maximum oxidation state increases across the early actinide series as 5f electrons become progressively available for bonding.
Radioactivity and Natural Occurrence
All isotopes of protactinium are radioactive. The longest-lived and most important is ²³¹Pa, with a half-life of 32,760 years. It occurs naturally in uranium ores as a decay product of ²³⁵U. A second notable isotope, ²³³Pa (half-life 27 days), is an intermediate in the thorium-232 fuel cycle: ²³²Th captures a neutron to form ²³³Th, which beta-decays to ²³³Pa, then to fissile ²³³U. Because of its extreme rarity in nature (about 1 part per billion in uranium ore), most protactinium used in research is isolated from spent nuclear fuel. Its radioactivity has no effect on its oxidation chemistry; +5 remains the dominant state for all isotopes.
Summary
Reference for the oxidation states of Protactinium (Pa, element 91) — the dominant +5 and significant +4 states, key compounds, and an interactive explorer.
How it works
- Review the oxidation states table to see which states are confirmed and their relative stability.
- Read the electron configuration section to understand how Pa accesses +4 and +5 by using 5f and 6d electrons.
- Use the interactive oxidation state explorer to examine each state, its ion form, and representative compounds.
- Check the compounds reference for specific Pa⁵⁺ and Pa⁴⁺ examples encountered in radiochemistry.
- Compare Pa to its actinide neighbors (Th and U) to see how oxidation state diversity evolves across the series.
Use cases
- Looking up the most stable oxidation state of protactinium for a chemistry exam or problem set.
- Understanding why Pa favors +5 while thorium stops at +4 and actinium at +3.
- Writing balanced redox equations involving Pa⁵⁺/Pa⁴⁺ couples in aqueous solution.
- Studying 5f-electron participation in actinide chemistry across the early members of the series.
- Identifying which protactinium compounds correspond to each oxidation state.
- Comparing early actinide oxidation state trends for a periodic table assignment.