Curium (Cm)

• Element Symbol: Cm
• Atomic Number: 96
• Atomic Mass: 247
• Group # in Periodic Table: n/a
• Group Name: Actinides
• Period in Periodic Table: 7
• Block of Periodic Table: f-block
• Discovered by: Glenn T. Seaborg, Ralph A. James, Albert Ghiorso (1944)

Curium is a silvery, highly radioactive element that is part of the actinide series in the periodic table. The actinides, which are metals with atomic numbers between 89 and 103, also include actinium, thorium, uranium, plutonium, berkelium, californium, einsteinium, fermium, and lawrencium. Some actinide elements, such as uranium and thorium, have isotopes that are naturally abundant within minerals in Earth’s crust. Radioactive decay of these natural isotopes produces other actinides, such as actinium. Some actinides are not found in nature. These actinides must be produced synthetically. Curium is one such synthetic actinide. It can be produced in a particle accelerator or nuclear reactor.

Curium is a transuranium element. This category of elements includes all those with atomic numbers greater than 92, the atomic number of uranium. Transuranium elements are radioactive and unstable. The transuranium actinides were discovered between 1940 and 1961. Curium was the third transuranium element to be discovered.

Curium was discovered in 1944 by a team of American physicists at the University of California, Berkeley, that included Glenn T. Seaborg, Ralph A. James, and Albert Ghiorso. These scientists bombarded a plutonium-239 target with helium ions within a cyclotron particle accelerator. They then sent the target to the Metallurgical Laboratory in Chicago, where it was chemically separated and analyzed for elements. This process yielded an isotope with ninety-six protons. Further neutron bombardments of plutonium in nuclear reactors in Tennessee and Washington State produced isotopes of elements with ninety-six and ninety-five protons.

Although elements 95 and 96 were detected in 1944, their discovery was not officially announced until 1945, when Seaborg presented a paper at the American Chemical Society symposium at Northwestern University in Chicago. However, Seaborg announced the new elements unofficially on the Quiz Kids national radio program just five days before the symposium. When one of the quiz kids asked if any new elements had been discovered at the Metallurgical Laboratory, Seaborg told the boy and the listening audience about elements 95 and 96.

Seaborg’s team took a long time to choose the names of these two elements. Seaborg even asked the listeners of the national radio program Adventures in Science to send in their suggestions. For a while his team referred to these elements as "pandemonium" and "delirium" because they were so tricky to isolate.

Element 95 was ultimately named americium, after the Americas. Element 96 was named curium after the noted nuclear physicists Marie and Pierre Curie. In both cases these names followed the convention of naming the actinides in parallel with their corresponding lanthanides. The lanthanide directly above americium is europium, which is also named after a continent. The lanthanide directly above curium is gadolinium, which is also named after a noted scientist, Finnish chemist Johan Gadolin.

The nuclear research programs at Berkeley and other places in the United States began as part of the Manhattan Project, which funded research on radioactive elements in the United States during World War II. Glenn Seaborg from the Berkeley team was integral in discovering many of these elements. He was also one of the researchers who helped develop the atomic bombs that were used at the end of the war. Seaborg was the first person to head the United States Atomic Energy Commission. After the war he promoted nuclear energy and nuclear medicine.

Physical Properties

Curium is a hard, silvery metal that is solid in its standard state at 298 kelvins (K). Most properties of curium are unknown because it is highly radioactive and can only be produced in small quantities. Curium has a density of 13.51 grams per cubic centimeter (g/cm³). Its melting point is 1345 degrees Celsius (°C). Its boiling point is 3110 °C. The specific heat, thermal conductivity, electrical conductivity, resistivity, and magnetic type of curium are unknown.

Chemical Properties

Curium has a simple hexagonal crystal structure. It oxidizes easily with air. The electron affinity of curium is unknown. This element has four valence electrons. Its electronegativity is 1.3. Its ionization energy is 581 kilojoules per mole (kJ/mol). Curium oxidation states are +3 and +4. The +3 oxidation state is most common and is chemically similar to lanthanides and actinides with +3 oxidation states.

Curium has no stable isotopes. It has twenty-one known radioactive isotopes, with mass numbers ranging from 232 to 252. Curium-242 was the first isotope discovered. Its half-life is 163 days. Curium-244 is commonly used. This isotope has a half-life of 18.1 years. Curium-243 has a half-life of 29.1 years. This element has an electron configuration of [Rn]5f⁷6d¹7s².

Applications

Curium is not found in nature. A number of isotopes have been produced within particle accelerators and nuclear reactors. Four isotopes that have undergone study are curium-242, curium-243, curium-244, and curium-248. These isotopes are generally produced through bombardment of a plutonium or americium target with neutrons. Curium-248 is also a decay product of californium-252.

Curium has very limited uses on Earth. However, it has been used in space probes. The Alpha Particle X-ray Spectrometer (APXS) on the Mars rovers uses curium to bombard Martian rocks with alpha particles and x-rays. This bombardment causes different elements in the rock to produce different x-ray energies, which can be used to identify the types and quantities of elements in Martian rocks. Curium has also been investigated as an alternative to the plutonium that powers radioisotope thermoelectric generators used on spacecraft. These generators convert the heat energy produced through radioactive decay to usable electrical energy.

Curium is intensely radioactive and can be a danger to human health. Only 10–20 milligrams of curium isotope can be studied at one time, even with highly protective barriers in place, because curium undergoes spontaneous fission and releases tremendous amounts of damaging neutrons. Exposure to curium can cause irradiation of bones, which can prevent the formation of red blood cells. Radiation may also cause bone, liver, lung, or stomach cancers.

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