Selenium, 34 General properties Pronunciation Template:Infobox element/pronunciation Appearance black, red, and gray (not pictured) allotropes Standard atomic weight (A r, standard ) Template:Infobox element/standard atomic weight Mass number Template:Infobox element/symbol-to-most-stable-isotope (most stable isotope)Selenium in the periodic table Template:Infobox element/periodic table Atomic number (Z ) 34 Group Template:Infobox element/group Period period 4 Block p-block Element category Template:Infobox element/category format , sometimes considered a metalloid Electron configuration [Ar ] 3d10 4s2 4p4 Electrons per shell
2, 8, 18, 6 Physical properties Phase at STP solid Melting point 494 K (221 °C, 430 °F) Boiling point 958 K (685 °C, 1265 °F) Density (near r.t. ) gray: 4.81 g/cm3 3 3 when liquid (at m.p. ) 3.99 g/cm3 Critical point 1766 K, 27.2 MPa Heat of fusion gray: 6.69 kJ/mol [[Enthalpy of vaporization|Heat of Template:Engvar ]] 95.48 kJ/mol Molar heat capacity 25.363 J/(mol·K) [[Vapor pressure|Template:Engvar pressure Atomic properties Oxidation states Template:Infobox element/symbol-to-oxidation-state Electronegativity Pauling scale: 2.55 [[Ionization energy|Template:Engvar energies]] 1st: 941.0 kJ/mol 2nd: 2045 kJ/mol 3rd: 2973.7 kJ/mol Atomic radius empirical: 120 pm Covalent radius 120±4 pm Van der Waals radius 190 pm Template:Infobox element/spectral lines Other properties Crystal structure Template:Infobox element/crystal structure Speed of sound thin rod 3350 m/s (at 20 °C) Thermal expansion amorphous: 37 µm/(m·K) (at 25 °C) Thermal conductivity amorphous: 0.519 W/(m·K) Magnetic ordering diamagnetic [1] Magnetic susceptibility −25.0·10−6 cm3 /mol (298 K)[2] Young's modulus 10 GPa Shear modulus 3.7 GPa Bulk modulus 8.3 GPa Poisson ratio 0.33 Mohs hardness 2.0 Brinell hardness 736 MPa CAS Number 7782-49-2 History Naming after Selene , Greek goddess of the moon Discovery and first isolationJöns Jakob Berzelius and Johann Gottlieb Gahn (1817)Main isotopes of selenium Template:Infobox element/isotopes table | references
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