Curium

Curium, 96Cm
Curium.jpg
Curium
Pronunciationm/ (KEWR-ee-əm)
Appearancesilvery metallic, glows purple in the dark
Mass number[247]
Curium in the periodic table
HydrogenHelium
LithiumBerylliumBoronCarbonNitrogenOxygenFluorineNeon
SodiumMagnesiumAluminiumSiliconPhosphorusSulfurChlorineArgon
PotassiumCalciumScandiumTitaniumVanadiumChromiumManganeseIronCobaltNickelCopperZincGalliumGermaniumArsenicSeleniumBromineKrypton
RubidiumStrontiumYttriumZirconiumNiobiumMolybdenumTechnetiumRutheniumRhodiumPalladiumSilverCadmiumIndiumTinAntimonyTelluriumIodineXenon
CaesiumBariumLanthanumCeriumPraseodymiumNeodymiumPromethiumSamariumEuropiumGadoliniumTerbiumDysprosiumHolmiumErbiumThuliumYtterbiumLutetiumHafniumTantalumTungstenRheniumOsmiumIridiumPlatinumGoldMercury (element)ThalliumLeadBismuthPoloniumAstatineRadon
FranciumRadiumActiniumThoriumProtactiniumUraniumNeptuniumPlutoniumAmericiumCuriumBerkeliumCaliforniumEinsteiniumFermiumMendeleviumNobeliumLawrenciumRutherfordiumDubniumSeaborgiumBohriumHassiumMeitneriumDarmstadtiumRoentgeniumCoperniciumNihoniumFleroviumMoscoviumLivermoriumTennessineOganesson
Gd

Cm

(Upn)
americiumcuriumberkelium
Atomic number (Z)96
Groupgroup n/a
Periodperiod 7
Blockf-block
Element category  Actinide
Electron configuration[Rn] 5f7 6d1 7s2
Electrons per shell2, 8, 18, 32, 25, 9, 2
Physical properties
Phase at STPsolid
Melting point1613 K ​(1340 °C, ​2444 °F)
Boiling point3383 K ​(3110 °C, ​5630 °F)
Density (near r.t.)13.51 g/cm3
Heat of fusion13.85 kJ/mol
Vapor pressure
P (Pa)1101001 k10 k100 k
at T (K)17881982
Atomic properties
Oxidation states+2, +3, +4, +5,[1] +6[2] (an amphoteric oxide)
ElectronegativityPauling scale: 1.3
Ionization energies
  • 1st: 581 kJ/mol
Atomic radiusempirical: 174 pm
Covalent radius169±3 pm
Color lines in a spectral range
Spectral lines of curium
Other properties
Natural occurrencesynthetic
Crystal structuredouble hexagonal close-packed (dhcp)
Double hexagonal close packed crystal structure for curium
Electrical resistivity1.25 µΩ·m[3]
Magnetic orderingantiferromagnetic-paramagnetic transition at 52 K[3]
CAS Number7440-51-9
History
Namingnamed after Marie Skłodowska-Curie and Pierre Curie
DiscoveryGlenn T. Seaborg, Ralph A. James, Albert Ghiorso (1944)
Main isotopes of curium
Iso­topeAbun­danceHalf-life (t1/2)Decay modePro­duct
242Cmsyn160 dSF
α238Pu
243Cmsyn29.1 yα239Pu
ε243Am
SF
244Cmsyn18.1 ySF
α240Pu
245Cmsyn8500 ySF
α241Pu
246Cmsyn4730 yα242Pu
SF
247Cmsyn1.56×107 yα243Pu
248Cmsyn3.40×105 yα244Pu
SF
250Cmsyn9000 ySF
α246Pu
β250Bk
| references

Curium is a transuranic radioactive chemical element with the 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. The discovery was kept secret and only released to the public in November 1947. Most curium is produced by bombarding uranium or plutonium with neutrons in nuclear reactors – one tonne of spent nuclear fuel contains about 20 grams of curium.

Curium is a hard, dense, silvery metal with a relatively high melting point and boiling point for an actinide. Whereas it is paramagnetic at ambient conditions, it becomes antiferromagnetic upon cooling, and other magnetic transitions are also observed for many curium compounds. In compounds, curium usually exhibits valence +3 and sometimes +4, and the +3 valence is predominant in solutions. Curium readily oxidizes, and its oxides are a dominant form of this element. It forms strongly fluorescent complexes with various organic compounds, but there is no evidence of its incorporation into bacteria and archaea. When introduced into the human body, curium accumulates in the bones, lungs and liver, where it promotes cancer.

All known isotopes of curium are radioactive and have a small critical mass for a sustained nuclear chain reaction. They predominantly emit α-particles, and the heat released in this process can serve as a heat source in radioisotope thermoelectric generators, but this application is hindered by the scarcity and high cost of curium isotopes. Curium is used in production of heavier actinides and of the 238Pu radionuclide for power sources in artificial pacemakers. It served as the α-source in the alpha particle X-ray spectrometers installed on several space probes, including the Sojourner, Spirit, Opportunity and Curiosity Mars rovers and the Philae lander on comet 67P/Churyumov–Gerasimenko, to analyze the composition and structure of the surface.

History

The 60-inch (150 cm) cyclotron at the Lawrence Radiation Laboratory, University of California, Berkeley, in August 1939.

Although curium had likely been produced in previous nuclear experiments, it was first intentionally synthesized, isolated and identified in 1944, at the University of California, Berkeley, by Glenn T. Seaborg, Ralph A. James, and Albert Ghiorso. In their experiments, they used a 60-inch (150 cm) cyclotron.[4]

Curium was chemically identified at the Metallurgical Laboratory (now Argonne National Laboratory) at the University of Chicago. It was the third transuranium element to be discovered even though it is the fourth in the series – the lighter element americium was unknown at the time.[5][6]

The sample was prepared as follows: first plutonium nitrate solution was coated on a platinum foil of about 0.5 cm2 area, the solution was evaporated and the residue was converted into plutonium(IV) oxide (PuO2) by annealing. Following cyclotron irradiation of the oxide, the coating was dissolved with nitric acid and then precipitated as the hydroxide using concentrated aqueous ammonia solution. The residue was dissolved in perchloric acid, and further separation was carried out by ion exchange to yield a certain isotope of curium. The separation of curium and americium was so painstaking that the Berkeley group initially called those elements pandemonium (from Greek for all demons or hell) and delirium (from Latin for madness).[7][8][9][10]

The curium-242 isotope was produced in July–August 1944 by bombarding 239Pu with α-particles to produce curium with the release of a neutron:

Curium-242 was unambiguously identified by the characteristic energy of the α-particles emitted during the decay:

The half-life of this alpha decay was first measured as 150 days and then corrected to 162.8 days.[11]

Another isotope 240Cm was produced in a similar reaction in March 1945:

The half-life of the 240Cm α-decay was correctly determined as 26.7 days.[11]

The discovery of curium, as well as americium, in 1944 was closely related to the Manhattan Project, so the results were confidential and declassified only in 1945. Seaborg leaked the synthesis of the elements 95 and 96 on the U.S. radio show for children, the Quiz Kids, five days before the official presentation at an American Chemical Society meeting on November 11, 1945, when one of the listeners asked whether any new transuranium element beside plutonium and neptunium had been discovered during the war.[7] The discovery of curium (242Cm and 240Cm), its production, and its compounds was later patented listing only Seaborg as the inventor.[12]

The new element was named after Marie Skłodowska-Curie and her husband Pierre Curie who are noted for discovering radium and for their work in radioactivity. It followed the example of gadolinium, a lanthanide element above curium in the periodic table, which was named after the explorer of the rare earth elements Johan Gadolin:[13]

"As the name for the element of atomic number 96 we should like to propose "curium", with symbol Cm. The evidence indicates that element 96 contains seven 5f electrons and is thus analogous to the element gadolinium with its seven 4f electrons in the regular rare earth series. On this base element 96 is named after the Curies in a manner analogous to the naming of gadolinium, in which the chemist Gadolin was honored."[5]

The first curium samples were barely visible, and were identified by their radioactivity. Louis Werner and Isadore Perlman created the first substantial sample of 30 µg curium-242 hydroxide at the University of California, Berkeley in 1947 by bombarding americium-241 with neutrons.[14][15][16] Macroscopic amounts of curium(III) fluoride were obtained in 1950 by W. W. T. Crane, J. C. Wallmann and B. B. Cunningham. Its magnetic susceptibility was very close to that of GdF3 providing the first experimental evidence for the +3 valence of curium in its compounds.[14] Curium metal was produced only in 1951 by reduction of CmF3 with barium.[17][18]