Atomic number 96

Curium

Cm
+ phase
− phase

Shells Orbitals 5f 6d 7s

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96 electrons in 7 shells: 2, 8, 18, 32, 25, 9, 2.

Shells are a teaching model: an electron does not travel on a ring at a fixed distance.

Category: Actinide
Period: 7
Block: f
Atomic mass: 247.07035 Da (longest-lived isotope)
Electron configuration: [Rn] 5f⁷ 6d¹ 7s²
Phase at 25 °C: solid
Melting point: 1340 °C
Boiling point: 3100 °C
Density: 13.51 g/cm³
Electronegativity (Pauling): 1.3
Discovered: 1944, Glenn T. Seaborg, Ralph A. James and Albert Ghiorso

Neighbours in the table

Am Bk

Curium was the third element beyond uranium to be discovered, Los Alamos says, although it follows americium in the periodic table. It carries the names of Pierre and Marie Curie, the discoverers of radium and polonium, as IUPAC records. Seaborg, James and Ghiorso first produced it in 1944 by striking plutonium-239 with alpha particles, helium nuclei, sped up in a cyclotron. Werner and Perlman, at the University of California, isolated the first visible sample, 30 µg of curium-242 hydroxide, in 1947. The metal itself was first prepared in 1951, Los Alamos records.

Curium’s configuration, [Rn] 5f⁷ 6d¹ 7s², leaves its 5f subshell exactly half full. Los Alamos attributes the stability of its +3 oxidation state, a bookkeeping charge treating every bond as ionic, to that half-filled shell. Curium resembles gadolinium, the lanthanide directly above it, in some respects, Los Alamos adds. In 1950 Crane, Wallmann and Cunningham found microgram samples of CmF₃ about as magnetically susceptible, or responsive to a magnetic field, as GdF₃. That match gave direct experimental evidence for the electron arrangement of the Cm³⁺ ion.

The metal is malleable and chemically reactive. It takes two crystal forms, Los Alamos says: double hexagonal close-packed and, at high temperature, face-centred cubic close-packed. Its crystal structure is more complex than gadolinium’s, Los Alamos notes.

Where it is found

The Earth’s crust holds no natural curium, IUPAC states. Minute amounts probably form in natural uranium deposits, Los Alamos suggests, where the faint neutron flux of the ore could drive neutron captures and beta decays. Such natural curium has never been detected. Any curium present when the Earth formed vanished long ago, Los Alamos explains, because even curium-247 lives briefly against the planet’s age.

PubChem gives the mass of its longest-lived isotope, curium-247, 247.07035 Da. That isotope halves in 1.56 × 10⁷ years in the International Atomic Energy Agency (IAEA) evaluation, all by alpha emission. It decays to plutonium-243, Jefferson Lab says.

Every sample is therefore made. Curium-244 comes from nuclear reactors, Haz-Map notes. Curium-242 and curium-244 are available by the gram and curium-248 only by the milligram, according to Los Alamos. Only milligrams of curium have been made in all, in Jefferson Lab’s account.

Why it matters

Curium-244 has travelled to Mars. It served as the alpha source inside the alpha particle X-ray spectrometer on the Mars Exploration Rover, IUPAC notes, and the Rosetta/Philae mission carried the same kind of instrument. Curium-242 and curium-244 are both strong alpha emitters. Their alpha emission releases enough heat to make them useful as alpha sources and as heaters in radioisotope thermoelectric generators (RTGs), which turn decay heat into electricity.

Curium-242 gives off about three watts of heat per gram, against half a watt per gram for plutonium-238, according to Los Alamos. Both curium-242 and curium-244 have served as power sources in space and in medicine, Los Alamos adds. Curium-242’s half-life is about 163 days for Jefferson Lab, and 162.8 days for the Atomic Mass Data Center (AMDC). It decays by alpha emission to plutonium-238 or by spontaneous fission, the nucleus splitting unprompted, Jefferson Lab says.

In AMDC’s figures, alpha emission takes 100 per cent of curium-242 decays, spontaneous fission 6.2 × 10⁻⁶ per cent. A far rarer branch ejects a whole silicon-34 nucleus, 1.1 × 10⁻¹⁴ per cent of the time. Curium and its compounds have no commercial applications, according to Jefferson Lab, and serve mainly basic scientific research. Curium might one day power RTGs, Jefferson Lab adds.

Properties

Properties of curium Standard atomic weights from IUPAC/CIAAW (abridged, 2024); other values from PubChem's periodic table, kelvin converted to °C.
Property	Value	Unit	Source
Atomic number	96
Atomic mass	247.07035 Da	Da	PubChem, longest-lived isotope
Melting point	1340 °C	°C	Los Alamos: 1340 °C
Boiling point	3100 °C	°C	Los Alamos: 3100 °C
Density	13.51 g/cm³	g/cm³
Electronegativity (Pauling)	1.3
Electron configuration	1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹⁰ 4p⁶ 5s² 4d¹⁰ 5p⁶ 6s² 4f¹⁴ 5d¹⁰ 6p⁶ 7s² 5f⁷ 6d¹
Phase at 25 °C	solid

This element has no standard atomic weight; the atomic mass shown is that of its longest-lived isotope, as PubChem gives it.

Safety

Curium harms through ionising radiation, and curium-242 and curium-244 are strong alpha emitters. Curium-244 is the radionuclide of most concern in Haz-Map’s summary in PubChem, with the liver as its critical organ and an internal toxicity rated very high. Curium absorbed into the body gathers in the bones, Los Alamos adds, where its radiation destroys the machinery that forms red blood cells. The PubChem Laboratory Chemical Safety Summary is the reference sheet for anyone who handles it.

Questions

Who discovered curium and where?

Glenn T. Seaborg, Ralph A. James and Albert Ghiorso made it in 1944. The reaction, in IUPAC's notation, is ²³⁹Pu(⁴He, n)²⁴²Cm: an alpha particle enters plutonium-239 and a single neutron leaves, giving curium-242. The work took place at the University of California, Berkeley, according to Jefferson Lab and IUPAC. In Los Alamos's account, curium was identified at the wartime metallurgical laboratory of the University of Chicago, with the target bombarded in Berkeley's 60-inch cyclotron.

What colour is curium?

The metal is lustrous and silver, Los Alamos says, and its surface oxidises rapidly in air, forming a thin film that ends as the dioxide, CmO₂. Most compounds and solutions of curium in its +3 state are stable and faintly yellow or yellow-green. Dropped into dilute acid, the metal dissolves quickly to give solutions of Cm³⁺.

What oxidation states does curium have?

Curium's oxidation state is +3 in Jefferson Lab's figures, and Los Alamos lists 6, 4, 3 and 2. The +4 state is only metastable in concentrated fluoride solutions, yet in the solid it holds up well as oxides and fluorides such as CmO₂ and CmF₄. Divalent solids such as CmO have never been seen in bulk. Most characterised curium compounds are trivalent, including CmCl₃, CmBr₃ and CmI₃.

What is the half-life of curium-244?

Curium-244 halves in 18.11 years in the International Atomic Energy Agency (IAEA) evaluation, which assigns 100 per cent of decays to alpha emission and 0.000137 per cent to spontaneous fission. Haz-Map gives 18 years, with alpha decay. Curium-244 captures neutrons far more readily than it fissions, IUPAC notes, and it has been placed in a nuclear reactor to breed heavier curium isotopes.