Helium Facts – Atomic Number 2 or He 1

Element cell for Helium

Periodic Table cell for the element Helium.

Basic Helium Facts

Name: Helium

Atomic Number: 2

Element Symbol: He

Group: 18

Period: 1

Block: s

Element Family: Noble Gas

Atomic Mass: 4.002602(2)

Electron Configuration: 1s2

Discovery: First identified: Pierre Janssen in 1868. First Isolated: Sir William Ramsay in 1895.
Janssen was a French astronomer who observed a new yellow spectral line while observing a total solar eclipse in 1868. He assumed it was part of the sodium spectrum. English astronomer Norman Lockyer later observed the same line but could not link it to sodium. Together with English chemist Edward Frankland, they decided it was associated with a new element. In 1985, Scottish chemist William Ramsay isolated a gas from a sample of a mineral cleveite which contained the yellow spectral line. Swedish chemists Per Teodor Cleve and Abraham Langlet independently made the same discovery the same year in Sweden.

Name Origin: Helios, Greek god of the  Sun. Lockyer and Frankland named their unknown element after the location it was first discovered, the Sun.

Common Isotopes:

Helium-3 is a stable isotope of helium containing 2 protons and  1 neutron. He-3 accounts for 1.37 x 10-4% of helium found in the atmosphere

Helium-4 is also stable with 2 protons and 2 neutrons. It is the most common form of helium found. It’s abundance in the atmosphere is 99.999863% of all helium found.


Helium in a gas-discharge tube. (pslawinski, metal-halide.net)

Physical Data

Density: 0.000164 g/cm3

Melting Point: 0.95K (-272.20 ºC or -457.96 ºF) at 2.5 MPa

Boiling Point: 4.222 K (-2683928 ºC or -452.070 ºF)

Triple Point: 2.177 K at 5.043 kPa

Critical Point: 5.1953 K at 227.45 kPa

State at 20ºC: Gas

Heat of Fusion: 0.0138 kJ/mol

Heat of Vaporization: 0.0829 kJ/mol

Molar Heat Capacity: 20.78 J/mol·K

Helium Atom

Electron shell configuration of a helium atom.

Atomic Data

Atomic Radius: 1.40 Å

Covalent Radius: 0.37 Å

Van der Waals Radius:  1.40 Å

Electron Affinity: not stable

Electronegativity: unknown

1st Ionization Energy: 2372.3 kJ/mol

2nd Ionization Energy: 5250.5 kJ/mol

Common Oxidation States: 0

Fun Facts

  • Helium is the lightest and least dense of the noble gases.
  • Helium was the first element to be discovered outside of Earth. The first evidence of the existence of helium was from spectroscopy data from the Sun.
  • Even though helium is relatively rare on Earth, it is the second most abundant element. Helium accounts for roughly 23% of all elemental mass of the universe.
  • Helium has the lowest melting and boiling points of all the elements. Creating solid and liquid helium takes extreme pressures and low temperatures.
  • Helium gas is colorless, tasteless, odorless, non-toxic and inert.
  • Most helium gas is extracted from natural gas deposits.
  • Radioactive emissions from α decay are helium nuclei.
  • Helium is the second least reactive noble gas after neon.
  • Inhaling helium gas can raise the pitch of a person’s voice.
  • Even though helium was discovered by its yellow spectral line, ionized helium gas has a reddish orange glow.

Learn more about elements on the periodic table.