Technical data for Helium

Overview Name Helium Symbol He Atomic Number 2 Atomic Weight 4.002602 Density 0.1785 g/l[note] Melting Point N/A[note] Boiling Point -268.93 °C Thermal properties Phase Gas Melting Point N/A[note] Boiling Point -268.93 °C Absolute Melting Point N/A[note] Absolute Boiling Point 4.22 K Critical Pressure 0.227 MPa (2.24 Atm) Critical Temperature 5.19 K Heat of Fusion 0.02 kJ/mol Heat of Vaporization 0.083 kJ/mol Heat of Combustion N/A Specific Heat 5193.1 J/(kg K)[note] Adiabatic Index 5/3 Neel Point N/A Thermal Conductivity 0.1513 W/(m K) Thermal Expansion N/A Bulk physical properties Density 0.1785 g/l[note] Density (Liquid) N/A Molar Volume 0.022424 Brinell Hardness N/A Mohs Hardness N/A Vickers Hardness N/A Bulk Modulus N/A Shear Modulus N/A Young Modulus N/A Poisson Ratio N/A Refractive Index 1.000035 Speed of Sound 970 m/s Thermal Conductivity 0.1513 W/(m K) Thermal Expansion N/A Reactivity Valence 0 Electronegativity N/A ElectronAffinity 0 kJ/mol Ionization Energies 2372.3, 5250.5 kJ/mol Health and Safety Autoignition Point N/A Flashpoint N/A Heat of Combustion N/A DOT Hazard Class 2.2 DOT Numbers 1963 EU Number N/A NFPA Fire Rating 0 NFPA Hazards N/A NFPA Health Rating 1 NFPA Reactivity Rating 0 RTECS Number RTECSMH6520000 NFPA Label

Classifications Alternate Names None Names of Allotropes None Block p Group 18 Period 1 Electron Configuration 1s2 Color Colorless Discovery 1895 in Sweden and United Kingdom Gas phase Monoatomic CAS Number CAS7440-59-7 CID Number CID23987 Gmelin Number N/A NSC Number N/A RTECS Number RTECSMH6520000 Electrical properties Electrical Type N/A Electrical Conductivity N/A Resistivity N/A Superconducting Point N/A Magnetic properties Magnetic Type Diamagnetic Curie Point N/A Mass Magnetic Susceptibility -5.9×10-9 Molar Magnetic Susceptibility -2.36×10-11 Volume Magnetic Susceptibility -1.05×10-9 Abundances % in Universe 23% % in Sun 23% % in Meteorites N/A % in Earth's Crust 5.5×10-7% % in Oceans 7.2×10-10% % in Humans N/A Atomic dimensions and structure Atomic Radius 31 pm Covalent Radius 32 pm Van der Waals Radius 140 pm Crystal Structure Face Centered Cubic Lattice Angles π/2, π/2, π/2 Lattice Constants 424.2, 424.2, 424.2 pm Space Group Name Fm_ 3m Space Group Number 225 Nuclear Properties Half-Life Stable Lifetime Stable Decay Mode N/A Quantum Numbers 1S0 Neutron Cross Section 0.007 Neutron Mass Absorption 1×10-5 Known Isotopes 3He, 4He, 5He, 6He, 7He, 8He, 9He, 10He Stable Isotopes 3He, 4He Isotopic Abundances 3He 0.000137% 4He 99.999863%