Curium
Curium is a transuranic radioactive chemical element with symbol Cm and atomic number 96. This element of the actinide series was named after Marie and Pierre Curie – both were known for their research on radioactivity. Curium was first intentionally produced and identified in July 1944 by the group of Glenn T. Seaborg at the University of California, Berkeley.
Identity
- Atomic mass
- [247] u
- Category
- Actinides
- Group
- 3
- Period
- 7
- Block
- f
- Oxidation States
- +3, +4
- Phase (25 °C)
- Solid
- Appearance
- silvery metallic, glows purple in the dark
Electronic Structure
- Electron configuration
- 1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p6 5s2 4d10 5p6 6s2 4f14 5d10 6p6 7s2 5f7 6d1
- Short configuration
- [Rn] 5f7 6d1 7s2
- Electrons per shell
- 2, 8, 18, 32, 25, 9, 2
- Electronegativity (Pauling)
- 1.28
- Electron affinity
- 27.2 kJ/mol
- 1st ionization energy
- 581 kJ/mol
Physical properties
- Melting point
- 1339.8 °C (1613 K)
- Boiling point
- 3109.8 °C (3383 K)
- Density
- 13.51 g/cm³
- Molar heat capacity
- —
Chemical behavior of curium
Curium has eight outer electrons ([Rn] 5f⁷ 6d¹ 7s²) and acts as +3, with a stable +4 in the dioxide and fluorides. Its electronegativity is low (1.28) and it is dense, brittle, and highly radioactive: it glows violet through radioluminescence and heats its compounds. Its half-filled 5f shell makes it gadolinium's twin; Cm-244 emits the alphas that analyze Mars.
Reactivity
4 Cm + 3 O₂ → 2 Cm₂O₃
In air it rapidly oxidizes to the dark dioxide. The heat of decay keeps the sample hot and speeds up the oxidation.
2 Cm + 6 HCl → 2 CmCl₃ + 3 H₂
It dissolves in acids to give colorless curium(III) salts that glow in the dark. The chloride is the route for purifying Cm-244.
CmO₂ + 4 HF → CmF₄ + 2 H₂O
The dioxide converts to the tetrafluoride, the precursor of the metal by reduction with barium and of targets for berkelium.
²⁴⁴Cm → ²⁴⁰Pu + α
Cm-244 decays emitting 5.8 MeV alphas that excite X-rays in Martian rocks and generate heat for RTGs.
Notable compounds
| Compound | Formula | Use or importance |
|---|---|---|
| Curium-244 dioxide | ²⁴⁴CmO₂ | Rover APXS and RTGs |
| Curium tetrafluoride | CmF₄ | Metal production |
| Curium oxalate | Cm₂(C₂O₄)₃ | Radiochemical purification |
| Cm-248 target | ²⁴⁸Cm | Superheavy element synthesis |
| Curium nitrate | Cm(NO₃)₃ | Fuel separation |
Electron shell distribution
View in the atom builderIsotopes
No curium isotope is stable. Curium-244 (²⁴⁴Cm, 18.1 years) powers early APXS and RTGs. Curium-242 (²⁴²Cm, 163 days) heated the first probes. Curium-248 (²⁴⁸Cm, 340,000 years) is a target for synthesizing seaborgium and hassium.
Occurrence and production
It does not exist in nature: all curium is bred in reactors, when Pu-239 captures neutrons up to Am-243, which decays via Cm-244. It is extracted from spent fuel by advanced nuclear reprocessing: each tonne contains grams. Only Oak Ridge and Dimitrovgrad isolate it at scale.
History and origin of the name
Seaborg, James, and Ghiorso synthesized it in 1944 by bombarding plutonium with alphas in Berkeley and named it curium, after Pierre and Marie Curie: a tribute to the parents of radioactivity. It was announced in 1945 alongside americium. APXS carried it to Mars.
Applications and relevance of curium
Uses of the element
As Cm-244 in sealed sources it excites X-rays in Sojourner, Spirit, and Curiosity to analyze rocks without bringing them home. As a heat source it powers RTGs of early probes and remote beacons.
Uses of its compounds
The ceramic dioxide survives reentry; Cm-248 bombarded with magnesium creates seaborgium. Its solutions calibrate actinide migration in repositories.
Health, safety and environment
Biological role
It has no biological function and is highly radiotoxic: it accumulates in bone and liver and inhalation raises lung risk. Cm-244 is also a neutron source through spontaneous fission.
Safety
It is handled only in hot cells with thick shielding, robots, and monitors. Its heat and neutrons require cooling and additional shielding. It does not exist in consumer products.
Environment
It is part of the high-activity waste that shapes repositories over thousands of years. Research explores transmuting it into shorter-lived isotopes or burning it in fast reactors.
Frequently asked questions about Curium
Why is curium-244 used in the spectrometers of Mars rovers?
Curium-244 emits alpha particles that, when they strike rocks or Martian soil, cause characteristic X-rays to be emitted. By analyzing the energy of those X-rays, the instrument identifies the elements present in the sample. This method, called alpha particle X-ray spectrometry, determines the chemical composition of extraterrestrial materials without having to bring them to Earth.
What was curium-242 used for in the first space missions?
Curium-242 was used as a heat source in radioisotope thermoelectric generators for early space probes. Its decay releases heat that could be converted into electricity to power instruments and communication systems. Although its half-life is short, about 163 days, it was enough for short-duration missions. Curium-244 and other longer-lived isotopes were preferred later.
What role does curium play in nuclear waste research?
Curium is generated as a byproduct in nuclear reactors and is part of the high-activity waste. Its radiotoxicity and its tendency to accumulate in bone complicate its management. Research studies ways to separate it, immobilize it, or transmute it into shorter-lived isotopes. Its presence in waste is one of the reasons deep geological storage is necessary.
What is the difference between curium and plutonium as a space power source?
Plutonium-238 is the isotope most used today in thermoelectric generators for space missions, thanks to its 88-year half-life and lower radiotoxicity. Curium-244 was used in early missions and in spectrometers, but its higher radiotoxicity and lower availability limit its use today. Plutonium offers a more balanced combination of duration, power, and safety for long-duration missions.
What elements can a curium-based APXS spectrometer detect?
The APXS instrument, which uses curium-244 as its alpha particle source, can detect major elements such as magnesium, aluminum, silicon, potassium, calcium, and iron, as well as minor elements such as sodium, phosphorus, sulfur, chlorine, titanium, chromium, and manganese. It is also sensitive to light elements like carbon and oxygen through backscattered alpha particle analysis.
Why is curium-244 better than curium-242 for long missions?
Curium-244 has a half-life of about 18 years, far longer than the 163 days of curium-242. This means it maintains its thermal power for much longer, which is essential for long-duration space missions. In addition, its alpha emission is stable and predictable, which simplifies the design of generators and spectrometers that must work for years without maintenance.
Why is curium associated with Marie and Pierre Curie?
Curium was named in honor of Pierre and Marie Curie, pioneers in the study of radioactivity. Marie Curie discovered polonium and radium, and her work laid the foundations of modern nuclear physics and chemistry. The name recognizes that contribution and links curium with the scientific tradition of radioactive research in the 19th and early 20th centuries.
Main sources consulted: PubChem, Royal Society of Chemistry, NIST, IUPAC. Last revision: November 2026.