Curium · element 96
A silvery metal that glows purple in the dark and has probed Martian rocks for NASA
Key facts
Curium is intensely radioactive. A sample of it glows faintly purple in a dark room, not because it is luminous in the usual sense but because its own alpha particles excite the air and the surface around it. It generates enough heat that a few grams would feel warm through a glove. The element was created in 1944, named after Marie and Pierre Curie, and has since travelled to Mars and to a comet.
It was the third transuranium element to be discovered, though its announcement came together with americium, and it marks a key point in the actinide series where the 5f electron shell is exactly half full.
Where it is found
No curium exists in nature except possibly a few atoms from past nuclear tests. It forms in reactors when plutonium and americium capture neutrons, so it accumulates in spent fuel over time, though at much lower levels than plutonium. Extracting it requires chemical processing of highly radioactive material.
Dedicated production uses high-flux reactors such as the High Flux Isotope Reactor at Oak Ridge in the United States and the SM reactor at Dimitrovgrad in Russia. Targets of plutonium or americium are irradiated for months, then dissolved and separated. The output is measured in grams per year, and curium-244 is the most common product because its 18-year half-life is convenient to work with.
How it was discovered
Glenn Seaborg, Ralph James and Albert Ghiorso made curium in the summer of 1944. They bombarded plutonium-239 with alpha particles in the 60-inch cyclotron at Berkeley, then shipped the target to the Metallurgical Laboratory in Chicago for analysis. The product was curium-242, which they identified by its alpha decay. Because the chemistry of the new element was so similar to that of the lighter actinides, separating it took weeks of effort.
The name honours the Curies, following the pattern of gadolinium above it in the table, which was named after the chemist Johan Gadolin. The announcement was delayed until after the war ended in 1945.
What we use it for
Curium-244 is a compact source of alpha particles, and that gives it a niche in space science. Alpha particle X-ray spectrometers on NASA's Sojourner, Spirit, Opportunity and Curiosity rovers fired alpha particles from curium at rocks and read the X-rays that came back, revealing which elements were present. The Philae lander that touched down on comet 67P in 2014 carried the same kind of instrument. A curium source needs no power and works for years.
Curium has been studied as a heat source for radioisotope generators because curium-244 gives out far more heat per gram than plutonium-238, but its neutron emission makes shielding difficult. Curium-248 is a valuable target for making superheavy elements: bombarding it with calcium-48 produced the first atoms of livermorium at Dubna.
Reading the numbers
Curium melts at about 1,340 °C, one of the highest melting points among the actinides, and boils near 3,110 °C. Its density of 13.5 grams per cubic centimetre is a little above lead and about one and three quarter times iron. The configuration [Rn] 5f7 6d1 7s2 is the direct parallel to gadolinium's [Xe] 4f7 5d1 6s2. A half-filled f shell is unusually stable, so the seven 5f electrons stay out of bonding and the single 6d electron plus two 7s electrons do the work. That explains why curium is almost always found in the +3 oxidation state, why it is so hard to separate from americium, and why its melting point is higher than that of its neighbours. Its electronegativity of 1.28 places it among the reactive metals.
Safety and the environment
Curium is one of the most radiotoxic elements that exists in weighable amounts. Curium-244 emits alpha particles, and its spontaneous fission produces neutrons, which are penetrating and require heavy shielding. Inside the body curium concentrates in bone, liver and lungs. It is handled only in hot cells with remote manipulators. Environmental releases are limited to traces from weapons testing and accidents. In spent fuel, curium's contribution to heat output is significant in the first decades but fades as curium-244 decays away.
Did you know?
- Curium glows purple in the dark because its intense alpha radiation excites the surrounding air and its own surface.
- Curium-244 produces roughly 2.8 watts of heat per gram, several times more than plutonium-238.
- The instrument that identified elements in Martian rocks on four NASA rovers used a curium alpha source.
- Curium-247 has a half-life of about 15.6 million years, long by human standards but far too short to have survived from the formation of the Earth.
- Curium was discovered in mid 1944, months before americium, but both were revealed to the public together in November 1945.
More in the actinides
The nearest neighbours of curium in the family, by atomic number.
See all 15 actinides on one page, or browse all 118 elements.
Frequently asked questions
Why does curium glow?
Its radioactivity is so intense that alpha particles excite atoms in the air and on its surface, which release the energy as a faint purple light. The effect is visible only in the dark.
What is curium used for?
Mainly as an alpha particle source in scientific instruments, notably the spectrometers on Mars rovers. It is also used as a target for creating heavier elements and has been studied as a heat source.
Is curium found in nature?
No. Any curium on Earth was made in reactors or by nuclear weapons. Its longest-lived isotope could not have survived from the formation of the planet.
Who is curium named after?
Marie and Pierre Curie, the pioneers of radioactivity research who discovered polonium and radium. The name mirrors gadolinium, the element above it, which was named after a chemist.
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Written by Anthony. Updated September 2026. Numerical data from Periodic-Table-JSON (CC BY-SA), cross-checked against the Curium reference article. Spotted an error? Tell us.