Actinium Oxidation States
Reference for the oxidation states of Actinium (Ac, element 89) — which states exist, why +3 dominates, and key Ac³⁺ compounds.
Actinium (Ac) is a silvery-white metal and the first member of the actinide series. All of its isotopes are radioactive. In chemistry, actinium behaves uniformly: it forms only +3 compounds, making it one of the simplest elements in terms of oxidation state diversity.
Oxidation States
| State | Status | Notes |
|---|---|---|
| +3 | Only confirmed state | Ac³⁺; forms all known actinium compounds; colorless in solution |
| +2 | Not observed | No experimental evidence under normal conditions |
| +1 | Not observed | No known compounds |
| +4 | Not observed | No known compounds; unlike heavier actinides (U, Np, Pu) |
Why Only +3?
Actinium's ground-state configuration is [Rn] 6d¹ 7s². It has three electrons outside the radon core: one in the 6d subshell and two in the 7s subshell.
Removing all three valence electrons gives Ac³⁺ with configuration [Rn] — a closed noble-gas core. This is the most stable ionic form.
Unlike heavier actinides (uranium, neptunium, plutonium), actinium has no chemically accessible 5f electrons to donate. This limits it to the +3 state — the same reason its lanthanide homolog La also stops at +3.
Common Ac³⁺ Compounds
White solid; forms when actinium metal is oxidized in air. Adopts the La₂O₃ crystal structure.
White solid; isostructural with LaF₃. Prepared by reaction of Ac³⁺ with hydrofluoric acid.
White crystalline solid; UCl₃-type structure. Used in some radiochemical studies.
White solid; similar structure to AcCl₃. All halide salts confirm the universal +3 state.
All known actinium compounds contain Ac in the +3 state. Ac³⁺ is colorless in aqueous solution.
Comparison to Lanthanum (La)
| Property | Lanthanum (La) | Actinium (Ac) |
|---|---|---|
| Atomic number | 57 | 89 |
| Period | 6 | 7 |
| Series | Lanthanide (start) | Actinide (start) |
| Electron config | [Xe] 5d¹ 6s² | [Rn] 6d¹ 7s² |
| Oxidation states | +3 only | +3 only |
| Radioactive? | No (stable isotopes exist) | Yes (all isotopes) |
Both La and Ac sit at the head of their f-block series and hold exclusively a +3 oxidation state, unlike the rest of their respective series where multiple states are common.
Radioactivity
Every isotope of actinium is radioactive. The most studied is ²²⁷Ac, which has a half-life of 21.77 years and decays primarily by beta emission (β⁻) to ²²⁷Th, with a small branch to ²²³Fr via alpha decay. Because of its radioactivity, actinium is found only in trace quantities in nature (in uranium and thorium ores) and must be produced in nuclear reactors for research and medical applications. Its radioactive nature does not alter its oxidation chemistry — Ac³⁺ is the stable ionic form regardless of which isotope is present.
Summary
Reference for the oxidation states of Actinium (Ac, element 89) — which states exist, why +3 dominates, and key Ac³⁺ compounds.
How it works
- Locate the oxidation states table to see that +3 is the only confirmed state for actinium.
- Read the electron configuration section to understand why Ac readily loses exactly three electrons.
- Review the common compounds list (Ac₂O₃, AcF₃, AcCl₃, AcBr₃) to see +3 in real chemistry.
- Check the lanthanum comparison to place actinium in a broader periodic-table context.
Use cases
- Checking the oxidation state of actinium for a chemistry problem or exam.
- Understanding why actinium does not form +2 or +4 compounds unlike some other actinides.
- Writing balanced equations for actinium compounds such as Ac₂O₃ or AcF₃.
- Comparing actinium to lanthanum as d-block period-6/7 homologs that both fix at +3.