Template:infobox_helium

2 hydrogen ← helium → lithium - ↑ He ↓ Ne Periodic table - Extended periodic table
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
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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