Helium,  ₂He

General properties
Pronunciation	/ˈhiːliəm/ ​(HEE-lee-əm)
Appearance	colorless gas, exhibiting a red-orange glow when placed in an electric field
Standard atomic weight (Ar, standard)	4.002602(2)
Helium in the periodic table
Hydrogen Helium Lithium Beryllium Boron Carbon Nitrogen Oxygen Fluorine Neon Sodium Magnesium Aluminium Silicon Phosphorus Sulfur Chlorine Argon Potassium Calcium Scandium Titanium Vanadium Chromium Manganese Iron Cobalt Nickel Copper Zinc Gallium Germanium Arsenic Selenium Bromine Krypton Rubidium Strontium Yttrium Zirconium Niobium Molybdenum Technetium Ruthenium Rhodium Palladium Silver Cadmium Indium Tin Antimony Tellurium Iodine Xenon Caesium Barium Lanthanum Cerium Praseodymium Neodymium Promethium Samarium Europium Gadolinium Terbium Dysprosium Holmium Erbium Thulium Ytterbium Lutetium Hafnium Tantalum Tungsten Rhenium Osmium Iridium Platinum Gold Mercury (element) Thallium Lead Bismuth Polonium Astatine Radon Francium Radium Actinium Thorium Protactinium Uranium Neptunium Plutonium Americium Curium Berkelium Californium Einsteinium Fermium Mendelevium Nobelium Lawrencium Rutherfordium Dubnium Seaborgium Bohrium Hassium Meitnerium Darmstadtium Roentgenium Copernicium Nihonium Flerovium Moscovium Livermorium Tennessine Oganesson – ↑ He ↓ Ne hydrogen ← helium → lithium
Atomic number (Z)	2
Group	group 18 (noble gases)
Period	period 1
Block	s-block
Element category	noble gas
Electron configuration	1s2
Electrons per shell	2
Physical properties
Phase at STP	He: 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
Heat of vaporization	0.0829 kJ/mol
Molar heat capacity	20.78 J/(mol·K)
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
Oxidation states	0
Electronegativity	Pauling scale: no data
Ionization energies	1st: 2372.3 kJ/mol 2nd: 5250.5 kJ/mol
Covalent radius	28 pm
Van der Waals radius	140 pm
Spectral lines of helium
Other properties
Natural occurrence	He: Primordial
Crystal structure	​hexagonal close-packed (hcp)
Speed of sound	972 m/s
Thermal conductivity	0.1513 W/(m·K)
Magnetic ordering	diamagnetic
Magnetic susceptibility	−1.88·10−6 cm3/mol (298 K)
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
Iso­tope Abun­dance Half-life (1/2) Decay mode Pro­duct 3He 0.0002% stable 4He 99.9998% stable
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Because it is very light, helium is the gas of choice to fill airships such as the Goodyear blimp

Helium is a chemical element. It has the chemical symbol He, atomic number 2, and atomic weight of about 4.002602. There are 9 isotopes of helium, only two of which are stable. These are ³He and ⁴He. ⁴He is by far the most common isotope.