Dy Ho Er Cf Es Fm -- -- -- Element Fact In 1961, the International Union of Pure and Applied Chemistry adopted the isotope carbon-12 as the basis of atomic weight, which has a mass of exactly 12.000 atomic mass unit.

Einsteinium, Es Atomic number: 99 Atomic mass: [252] (no stable nuclide) Natural abundance: - Isotopes: - History and Production Named after Albert Einstein (1879-1955). It was discovered by G.R. Choppin, S.G. Thompson, A. Ghiorso and B.G. Harvey in 1952, in the debris of the first large thermonumclear explosion in the Pacific. The isotope produced was 253Es with a half-life of about 20 days. It can also be prepared in nuclear reactors by irridiating 239Pu with neutron over several years. Of nineteen isotopes being found, 254Es has the longest half-life of 276 days. Physical Data It is a silvery, highly radioactive metal. It is biologically harmful due to its intense radiological behavior. Interatomic distance: - Melting point: 860°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: Electronic configuration: [Rn] 5f11 7s2 Term symbol: 5I15/2 Electron affinity: -   Electronegativity (Pauline): - Ionization energy (first, second, third): 619.436, -, - kJ/mol Chemical properties

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