Curium Oxidation States
Reference for the oxidation states of Curium (Cm, element 96) — which states exist (+3 dominant, +4 known), and key compounds for each.
Curium (Cm) is a silvery actinide metal named after Marie and Pierre Curie. It was first synthesized in 1944 by bombarding 239Pu with alpha particles. In chemistry, curium is dominated by the +3 state due to the extra stability of its half-filled 5f7 shell, though a +4 state is known in select solid compounds.
Oxidation States
| State | Status | Example compounds |
|---|---|---|
| +3 | Most stable | CmCl₃, Cm₂O₃, CmF₃, Cm(OH)₃ |
| +4 | Known (solid only) | CmO₂, CmF₄ |
| +2 | Not observed | No known stable compounds |
| +5 / +6 | Not observed | No known compounds |
Why +3 Dominates
Curium's ground state is [Rn] 5f⁷ 6d¹ 7s². It has three electrons outside the 5f⁷ shell — one 6d and two 7s — that are lost first upon ionization.
Removing those three electrons gives Cm³⁺ with configuration [Rn] 5f⁷. A half-filled f subshell carries significant exchange-energy stabilization, making this ion particularly inert to further oxidation.
Gadolinium (atomic number 64) is the lanthanide with a half-filled 4f⁷ shell in Gd³⁺ and it also overwhelmingly favors +3. Curium mirrors this behavior one period later. This analogy is a well-established pattern in f-block chemistry.
Forming Cm⁴⁺ requires removing a 5f electron, breaking the half-filled shell. In solid fluoride and oxide lattices the crystal field energy can compensate, but in aqueous solution Cm⁴⁺ is a strong oxidizer that immediately oxidizes water back to Cm³⁺.
Known Curium Compounds
White/pale solid; forms when curium is oxidized at moderate temperatures. Adopts the lanthanide sesquioxide structure.
White crystalline solid; UCl₃-type structure. Prepared by reaction of Cm oxide with HCl or CCl₄.
White solid; LaF₃-type structure. Produced by treatment of aqueous Cm³⁺ with hydrofluoric acid.
Soluble salt; used in radiochemical separations. Represents the typical aqueous-phase Cm³⁺ chemistry.
Black solid with fluorite structure (like UO₂ and PuO₂). Stable in the solid state; Cm⁴⁺ is not accessible in solution.
White solid prepared by treatment of Cm³⁺ fluoride with F₂ gas under specific conditions. Fluoride's high electronegativity stabilizes the +4 state.
Comparison to Neighboring Actinides
| Element | Z | 5f config (neutral) | Common states |
|---|---|---|---|
| Americium (Am) | 95 | 5f⁷ 7s² | +3 (dominant), +2, +4, +5, +6 |
| Curium (Cm) | 96 | 5f⁷ 6d¹ 7s² | +3 (dominant), +4 |
| Berkelium (Bk) | 97 | 5f⁹ 7s² | +3 (dominant), +4 |
| Californium (Cf) | 98 | 5f¹⁰ 7s² | +3 (only confirmed) |
Curium's restricted oxidation state range (essentially +3 with limited +4) stands out among its neighbors. Americium, just before it, shows a surprisingly wide range despite also having a 5f⁷ configuration in the neutral atom — a result of different 5f orbital energies at lower Z.
Radioactivity and Practical Notes
All curium isotopes are radioactive. The most commonly used in research and applications is ²⁴⁴Cm (half-life 18.1 years, alpha emitter), which is produced in nuclear reactors and used in alpha particle X-ray spectrometers (APXS) on Mars rovers. The longest-lived isotope, ²⁴⁷Cm, has a half-life of approximately 15.6 million years. Because of intense alpha and gamma radiation, all curium chemistry is conducted in specialized radiochemical facilities. Radiation does not affect the oxidation state chemistry — Cm³⁺ remains the dominant form regardless of which isotope is present.
Summary
Reference for the oxidation states of Curium (Cm, element 96) — which states exist (+3 dominant, +4 known), and key compounds for each.
How it works
- Read the oxidation states table to see that +3 is the dominant state and +4 is known in limited compounds.
- Review the electron configuration section to understand how the half-filled 5f⁷ shell stabilizes Cm³⁺.
- Browse the compounds list to see real examples of Cm³⁺ (CmCl₃, Cm₂O₃) and Cm⁴⁺ (CmO₂, CmF₄).
- Check the actinide comparison table to place curium in the broader context of neighboring elements.
Use cases
- Checking the oxidation state of curium for a chemistry exam or nuclear chemistry problem.
- Understanding why +3 is preferred over +4 for curium despite 5f electron availability.
- Writing balanced equations for curium compounds such as CmCl₃ or CmO₂.
- Comparing curium to gadolinium as half-filled f-shell analogs across the two series.