Helium (chemical symbol He) is a noble gas and the least reactive of all the chemical elements. It is important as a cryogenic coolant in many MRI scanners.
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Chemistry
Physical chemistry
Helium is an odourless colourless non-flammable gas. It has an atomic number 2 with a relative atomic weight of 4.0026. It has a melting point of -272°C and a boiling point of -269°C, which are the lowest of any element. It is the least reactive of all the chemical elements 1.
Radiochemistry
More than 99.999% of helium on earth is of the helium-4 form. The nucleus of helium-4 is identical to an alpha particle. Most of the remaining terrestrial hydrogen is helium-3, which is the only other stable isotope 1,2.
The other isotopes are all radioactive, helium-5 to helium-10 have been generated artificially but they all have very short half-lives 2.
Toxicity
Like any gas, helium can produce death by asphyxiation, if it prevents the inhalation of sufficient oxygen. Due to its low density, helium rapidly replaces oxygen in the lungs when it is inhaled in a closed atmosphere and because there is no accumulation of carbon dioxide, no breathing reflex supervenes.
The initial symptoms and signs of hypoxia are seen when oxygen levels decrease to 12-16% (cf. normal atmospheric concentration of 21%):
breathlessness
tachycardia
lethargy
Loss of consciousness rapidly ensues when oxygen concentrations become even lower (~6-10%), with convulsions and agonal breathing as hypoxic-ischaemic brain injury occurs 3.
Radiological importance
hyperpolarised helium-3 pulmonary functional MRI 4
helium is important as a cryogenic coolant for MRI systems
History and etymology
During the solar eclipse of the summer of 1868, the French astronomer Jules Janssen identified a yellow spectral line which he assumed was sodium. Later that year, in October, Norman Lockyer also saw this line. He realised that it could not be sodium as the line was located between the two known sodium lines and therefore represented a new element. Consequently the name helium was derived from “Helios”, the Greek sun god, by Lockyer and his colleague, the English chemist Edward Frankland. The discovery of helium is now credited to Janssen and Lockyer 1.
Helium was not discovered on Earth itself until 1895 when the Swedish chemists Per Teodor Cleve and Nils Abraham Langlet found a novel gas being produced from the ore cleveite. William Ramsay, a Scottish chemist, found a gas after he treated cleveite with acid which displayed the same spectral line as Lockyer and Janssen had found, and this was later confirmed by Lockyer 1.
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