Helium, 2 General properties Pronunciation Template:Infobox element/pronunciation Appearance colorless gas, exhibiting a red-orange glow when placed in an electric field 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)Helium in the periodic table Template:Infobox element/periodic table Atomic number (Z ) 2 Group Template:Infobox element/group Period period 1 Block s-block Element category Template:Infobox element/category format Electron configuration 1s2 Electrons per shell
2 Physical properties Phase at STP gas Melting point 0.95 K (−272.20 °C, −457.96 °F) (at 2.5 MPa) Boiling point 4.222 K (−268.928 °C, −452.070 °F) Density (at STP) 0.1786 g/L when liquid (at m.p. ) 0.145 g/cm3 when liquid (at b.p. ) 0.125 g/cm3 Triple point 2.177 K, 5.043 kPa Critical point 5.1953 K, 0.22746 MPa Heat of fusion 0.0138 kJ/mol [[Enthalpy of vaporization|Heat of Template:Engvar ]] 0.0829 kJ/mol Molar heat capacity 20.78 J/(mol·K)[1] [[Vapor pressure|Template:Engvar pressureITS-90 ) Atomic properties Oxidation states Template:Infobox element/symbol-to-oxidation-state Electronegativity Pauling scale: no data [[Ionization energy|Template:Engvar energies]] 1st: 2372.3 kJ/mol 2nd: 5250.5 kJ/mol Covalent radius 28 pm Van der Waals radius 140 pm Template:Infobox element/spectral lines Other properties Crystal structure Template:Infobox element/crystal structure Speed of sound 972 m/s Thermal conductivity 0.1513 W/(m·K) Magnetic ordering diamagnetic [2] Magnetic susceptibility −1.88·10−6 cm3 /mol (298 K)[3] CAS Number 7440-59-7 History Naming after Helios , Greek Titan of the Sun Discovery Pierre Janssen , Norman Lockyer (1868) First isolation William Ramsay , Per Teodor Cleve , Abraham Langlet (1895) Main isotopes of helium Template:Infobox element/isotopes table | references
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