Helium is the lightest noble gas and the second lightest element. The chemical symbol of helium is He. Helium is a non-toxic, tasteless, non-flammable, odorless, colorless, tasteless, inert, monatomic gas. The stable isotopes of helium are helium-3 and helium-4. The isotopic abundances of helium-3 is 0.000137%. Helium-3 is composed of 2 protons and 1 neutron. Helium-4 is composed of 2 protons and 2 neutrons. The isotopic abundances of helium-4 is 99.999863%. Helium has the following unique features:

  • Its boiling point of 4.22 K (-268.93 °C) is substantially lower than other known elements.
  • Helium does not solidify at normal atmospheric pressure, so it has not a melting point. Other known elements have higher melting points. Helium is solid at the pressure of 25 atmospheres and the temperature of 0.95 K (-272.2 °C). As the pressure increases, the temperature at solid helium occurs, increases likewise. Solid helium occurs at room temperature, when the pressure is 114000 atmospheres.
  • It is completely inert, so it does not form stable molecules.
  • Helium is seven times lighter than air. It is so light that the gravity force of the Earth is not strong enough to hold on to it. Helium escapes into space, when helium is released into the  atmosphere.
  • Helium has a relatively high thermal conductivity of 0.1513 W/(m·K). The thermal conductivity of helium is five times higher than the thermal conductivity of air.
  • Helium has a sound velocity of 970 m/s, so helium conducts sound three times faster than air.
  • Helium does not become radioactive under irradiation.
  • Helium has a relatively high ionization potential due to the fact that only two electrons completely fill the lowest energy shell.
  • Helium has an extremely low index of refraction of 1.000036.
  • Helium has a low viscosity of 0.0186 centipoise.
  • Helium becomes a superfluid at temperatures close to absolute zero. A superfluid is a fluid with a viscosity of zero, so the superfluid flows up and over the walls of containers. The state of superfluid is called Helium II. Other known elements do not become superfluid at temperatures close to absolute zero.

The physical and chemical properties of helium are presented below.

Physical properties
Atomic Number: 2 Atomic Weight: 4.002602
Density: 0.1785 g/l at 0 °C Liquid Density: 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
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
Electrical properties
Electrical Conductivity: N/A Electrical Type: N/A
Superconducting Point: N/A Resistivity: N/A
Valence: 0 Electronegativity: N/A
Electron Affinity: 0 kJ/mol Ionization Energies: 2372.3, 5250.5 kJ/mol
Melting Point: N/A Absolute Melting Point: N/A
Boiling Point: 4.22 K (-268.93 °C) Absolute Boiling Point: 4.22 K (-268.93 °C)
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 in gas phase: 5193.1 J/(kg K)
Adiabatic Index: 5/3 Point: N/A
Thermal Conductivity: 0.1513 W/(m K) Thermal Expansion: N/A
Valence: 0 Electronegativity: N/A
Electron Affinity: 0 kJ/mol Ionization Energies: 2372.3, 5250.5 kJ/mol
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
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 Known Isotopes: 7He, 8He, 9He,10He
Stable Isotopes: 3He, 4He
% 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
Alternate Names: None Names of Allotropes: None
Block: p Group: 18
Period: 1 Electron Configuration: 1s2
Color: Colorless Phase: Gas
Gas phase: Monoatomic CAS Number: CAS7440-59-7
CID Number: CID23987 Gmelin Number: N/A
NSC Number: N/A
Health and Safety
Autoignition Point: N/A Flashpoint: N/A
DOT Hazard Class: 2.2 DOT Numbers: 1963
EU Number: N/A RTECS Number: RTECSMH6520000
NFPA Fire Rating: 0 NFPA Hazards: N/A
NFPA Health Rating: 1 NFPA Reactivity Rating: 0