What is the atomic number of He


The Interactive Periodic Table of the Elements (PSE)

Helium: characteristics

Helium (official name according to IUPAC)
18 (old nomenclature: Group VIII A)
4.0026 u (rounded to the five significant digits)
4.002602 u +/- 2 digit (s) [1]
4,20 · 10-7 % (percentage by mass of the earth's shell, i.e. the earth's crust / oceans up to 16 km depth)

Helium: atomic properties

kJ / moleV
1.2372.3 kJ / mol24.587 eV
2.5250.5 kJ / mol54.418 eV
Number of unstable isotopes:
  • 2 electrons in the outermost occupied shell
  • 0 electrons in the second to last occupied shell
  • 0 electrons in the occupied shell below

Helium: substance properties

gaseous (under normal conditions)
Specific heat of fusion:
Specific heat of vaporization:
Thermal conductivity:
Specific heat capacity:
0.0001785 g / cm3
(= 0.1785 g / l, under normal conditions)
447.1 nm
667.8 nm
402.6 nm
493.3 nm
587.8 nm
706.5 nm
728.1 nm

  1. 1 digit = least significant digit, i.e. 2.435 +/- 3 digits means 2.432 ... 2.438
  2. The "CAS Registry Number" is that of the element helium from Chemical Abstracts Service Assigned key number, which makes it easier to find specialist articles about this element in all publications and databases structured according to the CAS system.
    Legal notice: CAS registry number is a registered trademark of the American Chemical Society.