Key information
Atomic number: 
2
Atomic weight: 
4
Density: 
126 kg/m³
Boiling point: 
4.26° K
Melting point: 
3.46° K
Heat vapour: 
0.08 kJ/mol
Heat fusion: 
0.02 kJ/mol
Group: 
Noble Gases

Helium is very light and unreactive, qualities which make it safe enough for filling airships. It is a member of a group of the periodic table with the description Noble Gases, the name referring to their non-reactivity with other elements.

Helium is the most second abundant element in the universe. Just about all the helium on Earth is as the result of radioactive decay. Commercially exploitable reserves of helium occur as a byproduct of production from natural gas fields in the USA.

The existence of helium was first predicted from the analysis of the solar spectrum in 1868. Dark absorption lines in the solar spectrum (the first observation of spectral lines) were discovered by William Wollaston (1766-1828) in 1801, and further investigated by Joseph von Fraunhofer. Fraunhofer developed the spectroscope and used this to study the dark lines in the spectrum subsequently referred to as Fraunhofer lines.

The French astronomer Pierre Jules Janssen (1824-1907), was in India observing a solar eclipse, when he noticed a bright yellow line with a wavelength of 587.49 nm. This was a previously unknown spectral line in the Sun. In the same year Joseph Norman Lockyer confirmed Janssen''s discovery with the observation that this was a previously unknown element giving it the name Helium. It was the first element which exists on Earth, to be originally predicted by solar observation.

Electron Shell: 
Amazon Books
Image of The Last Sorcerers: The Path from Alchemy to the Periodic Table
Author: National Academy of Sciences, Richard Morris
Publisher: Henry (Joseph) Press (2003)
Binding: Hardcover, 294 pages