Curium – facts and properties

Position in the periodic table

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Occurrence and extraction

Curium is not found in nature. The longest-lived isotope, curium-247, has a half-life of about 16 million years, far less than the age of the Earth. Other isotopes decay faster: curium-244 halves in about 18 years and curium-242 in a little over five months. Curium can arise only through nuclear reactions. In reactors tiny amounts form when plutonium and americium climb to heavier nuclei through many neutron captures and beta decays.

In 1944 Seaborg, James and Ghiorso bombarded plutonium-239 with alpha particles and made curium-242. The amount was too small to see. The new element was recognised by its radiation and its chemical behaviour. Only in 1947 did Louis Werner and Isadore Perlman manage to separate a visible amount, as curium hydroxide. Marie and Pierre Curie had studied radioactivity and discovered polonium and radium, and the name recalls this.

Properties and reactions

Curium is in period 7 and is the eighth actinoid, in the actinoid row of the table between americium on its left and berkelium on its right. Above it, in the lanthanoid row, stands gadolinium. Like gadolinium, curium has a half-filled f shell and one d electron, and its electron configuration is [Rn] 5f⁷ 6d¹ 7s². Its electronegativity is 1.3, the density is 13.51 g/cm³ and the melting point is 1345 °C. The boiling point of 3127 °C rests on estimates only. Its atomic mass is given as 247 u, the mass number of the longest-lived isotope, curium-247.

Curium is a silvery, lustrous metal that tarnishes in air. Almost always it is trivalent, just like gadolinium. The ion Cm³⁺ has the especially stable, half-filled 5f shell. Tetravalent curium is known only from a few compounds, such as the oxide and the fluoride.

All curium isotopes are strongly radioactive. The alpha radiation releases a lot of heat, and curium-242 decays in this way to plutonium-238. Curium-244 also undergoes spontaneous fission, which releases neutrons. If the element enters the body, it settles in bone and liver and irradiates them from within. It is therefore handled only by specialists in shielded, sealed chambers.

Research and synthesis

Curium has no everyday use. Its best-known use is in space exploration: curium-244 serves as the alpha source in alpha particle X-ray spectrometers, or APXS for short. The alpha particles strike a sample of rock or soil. They make its atoms emit characteristic X-rays, and some of the particles are scattered back. From both signals one can read which elements the sample contains and in what proportions. Such instruments work on the Mars rovers Spirit, Opportunity and Curiosity. As early as the 1960s the Surveyor probes examined the lunar soil with a curium-242 source.

Because curium-242 and curium-244 give off so much heat, they have also been studied as heat sources for radioisotope batteries. Plutonium-238 won out for this purpose.

For research, curium is made from plutonium and americium in high-flux reactors. Curium-248 serves as target material for making superheavy elements, for example livermorium. The chemistry of curium is a field of research in itself, since it provides comparison values for the heaviest actinoids.

Quick question

What is the atomic number of curium (Cm)?

FAQ