Stable isotope Helium (He)

Isotope Z(p) N(n) Atomic Mass Natural Abundance Enrichment Level Chemical Form
>He-3 2 1 3.016029309 0.000134% ≥99.80% Gas

Properties of Helium

Name Helium
Symbol He
Atomic number 2
Atomic weight 4.0026
Standard state Gas
CAS Registry ID 7440-59-7
Group in periodic table 2
Group name Noble gas
Period in periodic table 1
Block in periodic table s-block
Color Colorless
Classification Noble gas
Melting point -272.2 °C
Boiling point -268.93 °C
Thermal conductivity 0.1513 W/(m·K)
Heat of vaporization 0.083 kJ·mol-1
Heat of fusion 0.02 kJ·mol-1
Density of gas 0.1785 g/L at 0 ºC and 1 atm
Density of liquid No data available
Electron configuration 1s2
Atomic radius 0.33 Å
Oxidation state 0
Critical temperature -267.96 ºC
Critical pressure 2.24 atm
Critical volume 57 cm3/mo
Refractive index 1.000036 at 0 ºC and 1 atm
Solubility in water 0.0285 mg/L (calculated) at 25 ºC or 0.174 mL/L at NTP