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Helium,  2He
General properties
Name, symbol Helium, He
Appearance colorless gas (gaseous phase); colorless liquid or superfluid (liquid phase); exhibits a red-orange glow when placed in a high-voltage electric field
Helium in the periodic table


Atomic number 2
Standard atomic weight (Ar) 4.002602(2)
Group, block Noble gasses; s-block
Period period 1
Electron configuration 1s2
per shell
Physical properties
Phase Gas
Melting point 0.95 K ​(−272.20 °C, ​​−457.96 °F) (at 2.5 MPa)
Boiling point 4.222 K ​(−268.928 °C, ​​−452.07 °F)
Density at  (0 °C and 101.325 kPa) 0.1786 g/L
when liquid, at  0.145 g/cm3 (m.p.)
when liquid, at  0.125 g/cm3 (b.p.)
Triple point 2.177 K, ​​5.043 kPa
Heat of fusion 0.0138 kJ/mol
Heat of 0.0829 kJ/mol
Molar heat capacity 20.78 J/(mol·K)
Atomic properties
Oxidation states 0
Electronegativity Pauling scale: No data
energies 1st: 2372.3 kJ/mol
2nd: 5250.5 kJ/mol
Covalent radius 28 pm
Van der Waals radius 140 pm
Crystal structure
Speed of sound 972 m/s
Thermal conductivity 0.1513 W/(m·K)
∞ W/(m·K) (superfluid, < 2.2 K)
Magnetic ordering Diamagnetic
CAS Registry Number 7440-59-7
Naming after Helios, Greek god of the Sun
Discovery Pierre Janssen, Norman Lockyer (1868)
· references

Helium is a monoatomic gaseous chemical element with symbol He and atomic number 2. Although its position in the periodic table is still debated, it is generally considered to be part of the noble gas group in the periodic table. As its boiling and melting points are extremely close to absolute zero, helium presents several rare and unique properties extremely useful in physics, such as superfluidity.



Helium is the most inert chemical and does not react with any chemical element or compound at any temperature. Van der Waals compounds and excimers of helium however can be formed at cryogenic temperatures, such as LiHe, He2, HeNe, etc. Helium hydride ion can be formed through the radioactive decay of tritium. This helionium cation HeH+ is also known from physics; in chemistry, it is counted among the strongest acids, but does not exist for any measurable time for chemical reactions, instantly protonating anything it comes in contact with. Helium has also been put inside the hollow carbon cage molecules (fullerenes) by heating under high pressure. These structures are considered to be the smallest helium balloons.

None of these however have any importance to amateur chemistry as they require expensive equipment to investigate.


Helium is a colorless, odorless, non-toxic inert gas,with melting point of 4.222 K ​(−268.928 °C, ​−452.070 °F) and melting point of 0.95 K(-268.928 °C, ​−452.070 °F). It can only become solid at pressures higher than 25 atm, due to the high zero point energy of its atomic motions. It has a density of 0.1786 g/L at 0 °C and standard pressure, making it twice as dense as hydrogen. Being lighter than air, it will quickly rise.


Helium is sold in compressed gas cylinders, which can be purchased from welding supply places. "Balloon gas" is typically a mixture of helium and air.


Industrially, helium is isolated from the fractional distillation of natural gas, but due to helium's properties, this process is not feasible for the amateur chemist.


  • Helium balloons
  • Funny voice
  • Air-free atmosphere
  • Demonstration of superconductivity of various chemical elements and compounds (cryogenic liquid helium)



Helium gas in non-toxic, but if inhaled in large quantities it may replace the oxygen from lungs, leading to asphyxiation.

Liquid helium may cause frostbites on contact with bare skin.


Helium cylinders must be stored away from any heat source and inspected periodically.

Helium dewars should be stored similar.


Helium can be safely released in air.


Relevant Sciencemadness threads