Tb Dy Ho Bk Cf Es -- -- -- Element Fact Rhenium was the last naturally occurring element to be identified, in 1925. It was so rare that only 1 g of the metal was extracted out of 660 kg of molybdenite ore.

Californium, Cf Atomic number: 98 Atomic mass: [251] (no stable nuclide) Natural abundance: - Isotopes: - History and Production Derived from the state California. It was first produced in 1950 by S.G. Thompson, K. Street, A. Ghiorso and G.T. Seaborg, by bombarding microgram quantities of 242Cm with high energy helium ions. To date, production of the element has not been accomplished, although few compounds in very small amounts have been prepared. The 252Cf is a strong neutron emitter and has been used as a neutron source for gold and silvery discovery, in determining water and oil-bearing layers in oil explorations. A small amount of the isotope is used for cancer research. Physical Data It is a radioactive metal, probably silvery in appearance. It is a biological hazard, due to its strong neutron emission. Interatomic distance: - Melting point: 900°C Boiling point: n/a Thermal conductivity/Wm-1K-1: 10 est. (27°C) Density/kgm-3: n/a Standard Thermodynamic Data (atomic gas) Enthalpy of formation: - Gibbs free energy of formation: - Entropy: - Heat capacity: - Electronic data Electronic configuration: [Rn] 5f10 7s2 Term symbol: 5I8 Electron affinity: -   Electronegativity (Pauline): - Ionization energy (first, second, third): 606.092, -, - kJ/mol Chemical properties

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