Template:Infobox helium

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2 hydrogen ← helium → lithium
-
↑
He
↓
Ne
General
Name, symbol, number helium, He, 2
Chemical series noble gases
Group, period, block 18, 1, s
Appearance colorless
Standard atomic weight 4.002602(2) g·mol−1
Electron configuration 1s2
Electrons per shell 2
Physical properties
Phase gas
Density (0 °C, 101.325 kPa)
0.1786 g/L
Melting point (at 2.5 MPa) 0.95 K
(−272.2 °C, −458.0 °F)
Boiling point 4.22 K
(−268.93 °C, −452.07 °F)
Critical point 5.19 K, 0.227 MPa
Heat of fusion 0.0138 kJ·mol−1
Heat of vaporization 0.0829 kJ·mol−1
Specific heat capacity (25 °C) 20.786 J·mol−1·K−1
Vapor pressure (defined by ITS-90)
P/Pa 1 10 100 1 k 10 k 100 k
at T/K     1.23 1.67 2.48 4.21
Atomic properties
Crystal structure hexagonal close-packed
Electronegativity no data (Pauling scale)
Ionization energies 1st: 2372.3 kJ·mol−1
2nd: 5250.5 kJ·mol−1
Atomic radius (calc.) 31 pm
Covalent radius 32 pm
Van der Waals radius 140 pm
Miscellaneous
Thermal conductivity (300 K) 0.1513  W·m−1·K−1
Thermal expansion (25 °C) { µm·m−1·K−1
CAS registry number 7440-59-7
Selected isotopes
Main article: Isotopes of helium
iso NA half-life DM DE (MeV) DP
3He 0.000137%* 3He is stable with 1 neutrons
4He 99.999863%* 4He is stable with 2 neutrons
*Atmospheric value, abundance may differ elsewhere.
References
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